Quest for Fusion Energy - Why? - What? - How? - Challenges?
Dr. Art Molvik

Retired,
Lawrence Livermoor National Laboratory.

Monday, September 30, 3:00-4:00pm, South Engineering 208

Refreshments at 2:30, South Engineering 208 (online visit)

Art Molvik will discuss: Why we need new energy sources? What fusion is and how we attempt to make it work on earth, including “magnetic bottle” fusion and inertial “micro-bomb” fusion? What “bricks” did Art add to the edifice of fusion?  And finally, what are the challenges remaining after fusion has been demonstrated in the laboratory?

Bio:

Art Molvik became enthralled with the idea of fusion energy, his senior year of high school, in Grand Forks, ND, after reading the book, Project Sherwood. It described the just-declassified world-wide program to achieve fusion. He prepared by majoring in physics at Concordia College, Moorhead, MN and selecting a graduate school with a fusion energy program, the University of Wisconsin, Madison, WI. Here he earned a Ph.D., and met and married Kathy. Joining Lawrence Livermore National Laboratory, he contributed to magnetic mirror and other “magnetic bottle” fusion, and to heavy-ion inertial fusion energy. He currently consults with a startup fusion energy company in Stockholm, Sweden.

Persistent luminescence in rare-earth doped materials
Dr. Khang Hoang

Executive Director | Center for Computationally Assisted Science and Technology
Adjunct Professor  | Department of Physics
NDSU.

Monday, September 23, 3:00-4:00pm, South Engineering 208

Refreshments at 2:30, South Engineering 216

On March 12, 1993, after two years of trial and error, Murayama, Takeuchi, Aoki, and Matsuzawa of Nemoto & Co., Ltd. (Japan) successfully developed a new material, SrAl2O4:Eu2+,Dy3+, that re-emits extremely bright green light over many hours in the dark. Their discovery gave birth to an exciting and rapidly growing era of scientific research on persistent luminescence. Three decades later, despite great progress in understanding the phenomenon and in discovering new materials, details of the underlying mechanism for the persistent luminescence observed in the Nemoto phosphor and similar materials are still under debate, and the search for new or improved persistent phosphors is still largely trial and error. In this talk, I offer a detailed understanding of the material and identify specific atomic-scale imperfections responsible for the material’s interesting properties and superior performance. The talk is expected to be light on technical details and more on the discovery process and insights for materials design. I will also briefly discuss computational studies of rare-earth doped materials for solid-state lighting, scintillation, and quantum information applications.

The presentation will include a live demonstration of the phosphor.

Bio: Khang Hoang received his Ph.D. in Physics from Michigan State University. He is currently the Executive Director of the NDSU Center for Computationally Assisted Science and Technology (CCAST) and an adjunct faculty in the Department of Physics. Before joining NDSU, he was a postdoc scholar at the University of California–Santa Barbara and then a postdoc fellow at the U.S. Naval Research Laboratory. His research interests include computational modeling, theory, and design of materials for energy storage and conversion, optoelectronics, and quantum applications. At CCAST, he leads a diverse team of talented professionals responsible for developing, managing, and delivering efficient, high-performance compute and research data storage resources and services to researchers at NDSU and beyond

Electrostatics with short-range truncated Coulomb Potential
Dr. Sylvio May

Professor,
Department of Physics,
NDSU.

Monday, April 22, 3:00-4:00pm, South Engineering 208

Refreshments at 2:30, South Engineering 216

I will discuss an effort of doing electrostatics with a Coulomb potential that is truncated below a cutoff distance. That is, the potential is constant up to the cutoff distance and only then decays 1/r. I will argue that this potential may be useful, for example, when incorporating charge discreteness or short-range charge-charge correlations into the framework of continuum electrostatics. How does Poisson’s equation look like? Can it be solved, and what do we learn? Everyone interested in a different perspective, perhaps even a deeper understanding, of Poisson’s equation in electrostatics is encouraged to attend.

Multiscale modeling of liquid-like phase-separated chromatin in the cell nucleus
Dr. Rabia Laghmach

Post Doctoral Researcher,
Department of Chemistry,
Iowa State.

Monday, May 6, 3:00-4:00pm, South Engineering 208

Refreshments at 2:30, South Engineering 216

Eukaryotic nucleus confines nearly two-meter-long polymeric chromatin folding chains within its micron-sized membrane bounds while remaining dynamic and accessible for gene regulation machinery. The nucleus is compartmentalized through microphase separation of epigenetically decorated chromatin types into transcriptionally active euchromatin and silent heterochromatin compartments. The spatial arrangement of chromatin compartments is cell type-specific, which evolves along the lifetime of the cell. The three-dimensional structures of chromatin organization play an important role in regulating cellular function, and its disruption leads to diseases. Investigating the principles underlying the genome architecture and its dynamics could pave the way to identifying the relationship between chromatin structures, gene expression, and gene-regulatory machinery. The chromatin capture and imaging techniques have revealed a much more dynamic and stochastic nature of chromatin structures, which is reminiscent of multiphase fluid behavior. A natural question that emerges from the experimental observations is how much of large-scale chromatin ordering and dynamics could be explained away just by the liquid-like phase separation of chromatin. In this talk, I will present recent results on mesoscale nuclear dynamics of chromatin phase separation based on our developed multiphase liquid model of the nucleus called MELON-4D, which can resolve nuclear geometry, chromosomal territories, compartments, and nuclear lamina. The model enables rapid hypothesis-driven prototyping of nuclear dynamics in 4D, which facilitates comparison with whole nucleus imaging experiments. As an application, we model the Drosophila nucleus spanning the interphase and map phase diagram of nuclear morphologies. We shed light on the interplay of adhesive and cohesive interactions within the nucleus, giving rise to distinct radial organization seen in conventional, inverted, and senescent nuclear architectures.

Bio:Dr. Rabia Laghmach is a postdoctoral research associate at the Department of Chemistry at Iowa State University. He studied theoretical physics and received a Master of Science in Physics from the University of Lyon, and a Ph.D. in Condensed Matter Physics and Materials Science from the National Institute of Applied Sciences INSA of Lyon, France. His research interests focus on investigating the emergent physics of complex systems exhibiting internal moving and interacting interface problems, including self-assembly, dynamics of chromatin organization, biomolecular condensation, polymer phase separation, and dynamics of soft, active fluids at mesoscopic scales, by using physics-based computational modeling approaches.

Transfer Kinetics of Cargo Items among Mobile Nanocarriers
Md. Faruk Hossain

Department of Physics,
NDSU.

Monday, May 6, 12:00-1:00pm, South Engineering 208

Refreshments at 2:30, South Engineering 216

This seminar is also a part of Faruk’s comprehensive examination.

Nanocarriers, including liposomes, micelles, dendrimers, microgels, and various nanoparticles, can be employed as delivery vehicles. These nanoscale vehicles can be designed to transport cargo, typically therapeutic agents, between different entities and to ensure delivery to specific target sites.  The transfer mechanism of cargo items among nanocarriers proceeds either through a collision or diffusion mechanism. The work we present aims at a foundational understanding of the transfer kinetics of cargo items among the members of an ensemble of mobile nanocarriers. Upon carrier-carrier collisions, cargo items are transferred with a certain probability from one to another carrier. Given the nanocarriers host a sufficiently large number of non-interacting cargo items, the initial distribution evolves toward a Gaussian at equilibrium. The distribution of the cargo items among the carriers is determined by a partial differential equation, a Fokker-Planck equation, that we solve analytically. The consideration of an arbitrary number of chemically distinct carrier types leads to a far more complex system, which constitutes the main focus of the present work. Its mathematical analysis involves the eigenvalues of a matrix known as rate matrix and the verification of conservation relations for the numbers of carriers and cargo. We derive solutions of the rate equations, a set of parametrically coupled Fokker-Planck equations, which form the basis to address multiple related problems in the future, including spatial variations analogous to diffusion-reaction phenomena, effects of cargo aggregation inside carriers, and more complex carrier types. The detailed formulation of a kinetic model for the cargo distribution among the mobile nanocarriers has, to the best of our knowledge, not been addressed previously. In the future, we intend to develop advanced models derived from the insights gained through this current research.

Bio:Md Faruk Hossain is originally from Bangladesh. He obtained his master's degree in physics from University of Rajshahi, Bangladesh. He is currently pursuing his PhD in Physics under the supervision of Professor Sylvio May in the Department of Physics at North Dakota State University.

Hybrid Continuum-Particle Coupling for multi-scale Simulation of Blood Flows
Dr. Trung Bao Le

Associate Professor,
North Dakota Water Resources Institute Fellow,
Computational Fluids Laboratory
Department of Civil and Environmental Engineering,
NDSU.

Monday, April 29, 3:00-4:00pm, South Engineering 208

Refreshments at 2:30, South Engineering 216

Cardiovascular diseases and cancer metastasis are the leading causes of death worldwide. In this talk, recent developments of numerical methods for blood flows from the perspective of multi-scale simulation is reviewed to demonstrate the feasibility of applying simulation for clinical applications and treatments. We have developed a hybrid continuum-particle approach for simulating blood flow at cellular scales with the combination of the immersed boundary method and the Dissipative Particle Dynamics method to perform Fluid-Structure Interaction simulations for blood cells and cancer cells. High resolution simulation results are compared with the measurements from laboratory experiments and in-vivo data. The simulation data show that blood flow dynamics are more complicated than previously thought. In these cases, transient phenomena of blood cells such as parachuting or rolling are ubiquitous. Quantifying such phenomena could lead to new ways of early diagnosing as well as monitoring the diseases. This work is funded by the National Science Foundation ND ACES project (Award # 1946202 -https://www.nsf.gov/awardsearch/showAward?AWD_ID=1946202) and the Department of Energy (Lawrence Berkeley National Laboratory).

Bio:Dr. Trung Bao Le is an Assistant Professor at the Department of Civil, Construction, and Environmental Engineering at North Dakota State University, United States. He is the recipient of national and international awards including the Gallery of Fluid Motion (American Physical Society - Division of Fluid Dynamics) and the 2023 NSF CAREER Award.

Fun With Sticky Sheets
Dr. Andrew B. Croll

Professor,
Department of Physics,
NDSU.

Monday, April 22, 3:00-4:00pm, South Engineering 208

Refreshments at 2:30, South Engineering 216

Elastic sheets are easily bent into a variety of interesting shapes, but when released typically recover their initially flat homogeneous shape.  To lock in structure, it is more common to use an elasto-plastic material such as paper because sharply deformed regions (folds) will not completely relax when released.  In this work, we introduce Kuttsukigami design, a system in which designs are sculpted from thin sheets using only adhesion (stickiness) to lock in form.  We find this new approach opens up a world of new materials and geometries for thin sheet design.  Specifically, we examine several geometries created with thin sticky sheets and examine how they are held together by a balance of adhesion and elasticity.  This perspective enables a system of metrology for thin elastic structures.  We go on to discuss the geometry created by sticky sheets encapsulating a cargo (such as a ravioli).  Finally, we show how kuttsukugami enables the creation of reusable and reconfigurable designs which we demonstrate with a reconfigurable sticky-logic gate circuit.

Bio:Andrew Croll is a Physics Faculty at NDSU, which he joined in 2010. Before that he did a postdoc at Amherst University. He received a PhD in Physics from McMaster University. He received a Batchelor in Physics (with a minor in Computer Science) from the University of Waterloo.

Single-Molecule Techniques for Studying Molecular Machines
Dr. Paul Pease,

Sr. Business Development Scientist
LUMICKS

Monday, April 15, 3:00-4:00pm, 221 South Engineering

Refreshments at 2:30

At the heart of biological processes are complex molecular machines that operate at some of the smallest scales of distance, force, and time. In this seminar I will discuss some of the challenges and limitations of traditional approaches to understanding these molecular machines and review some of the single-molecule techniques that have enabled an exciting array of discoveries. In particular, I will focus on optical tweezers and the mechanistic insights that can be gained from correlative measurements of forces and single-molecule imaging. Specific examples will include the role of DNA topology in CRSPR gene editing, direct measurements of cytoskeleton motor proteins under external loads, measurements of cellular membrane tension propagation, and the physical properties of biomolecular condensates.

How does adhesion work?
Dr. Andrew B. Croll

Professor,
Department of Physics,
North Dakota State University

Monday, March 25, 3:00-4:00pm, 208 South Engineering

Refreshments at 2:30

In this talk I will attempt to show how non-specific atom-scale forces result in macroscopic-scale outcomes such as adhesion.  I will begin with a review of the basic continuum ideas of surface science which lead to simple results like spherical-cap droplets on surfaces.  Next, I will show how an understanding of mechanics (fracture, really) is necessary to connect the microscopic details of surfaces to the macroscopic world that might try to separate them.  Finally, I will show practical results from our recent work examining the adhesion of silicone elastomers.  Specifically, I will show how viscoelasticity is a critical issue for understanding the adhesion of elastomers, compare our results to some recent theoretical models and point some classical polymer physics along the way.

INVESTIGATION OF 2D POLYMER THIN FILMS IN 3D STRUCTURES INVOLVING CRUMPLING, ORIGAMI, KIRIGAMI AND ADHESION

Wathsala M. A. Jayawardana

Candidate for Ph.D.
Department of Physics,
North Dakota State University

Wednesday, Mar. 27, 3:00-4:00pm, 221 South Engineering. <Zoom link available upon request>

This dissertation delves into the fascinating world of thin films and their ability to transform into three-dimensional (3D) structures. One can simply crush a sheet of paper into a ball to make a 3D object faster, yet complex structure or by following a more organized folding technique (Origami) to create 3D objects with predictable structures. This dissertation mainly focuses on such 3D structures and introduces cuts (Kirigami) and adhesion to those 3D structures to understand more of their mechanical behavior.

 The investigation of polymer crumple mechanics by introducing the Kirigami approach, exploring how the sheet's topology influences crumpling, is discussed in this dissertation, which employs confocal microscopy, force experiments, and molecular dynamics simulations to investigate the effects of cutting on the behavior of crumpled sheets. The findings reveal that cutting does not significantly alter the compressive behavior; force scales according to a power law regardless of cuts, with only minor reductions in magnitudes.

The second chapter of this dissertation studies Origami-inspired adhesive capable of securely holding objects on a wall yet easily removable without damage, which should withstand significant forces when attached, then swiftly transition to a low-adhesion state for removal. Bi-stability of the proposed Origami designs with different compliance to achieve noticeable switching ratios has been investigated. These devices demonstrate moderate switching ratios and scalability, offering potential for arrayed applications through repetition of the Origami pattern.

Crumples combined with adhesion create stable 3D structures made from elastic thin sheets. The last chapter discusses the impact mitigation of such crumpling systems by observing sticky crumpled matter subjected to simple ball drop tests. These findings highlight the potential utility of sticky crumples as replacements for intricate engineered structures in protective layers.

Applying Theoretical Frameworks from Cognitive Psychology to Assess Faculty Professional

Alistair McIrney

Candidate for Ph.D.
Department of Physics,
North Dakota State University

Monday, Mar. 25, 1:00-2:00pm, <ONLINE ONLY, SEE EMAIL FOR LINK>

Refreshments at 2:30

Understanding human behavior and reasoning is essential for developing successful instruction.  Discipline-based education researchers have examined how students learn, informing the development of successful instructional strategies.  Research has also identified barriers to the successful implementation of such strategies and the need for professional development.  This work utilizes two theoretical frameworks from psychology to examine: (1) efforts to enact instructional change and (2) instructional approaches designed to improve students' reasoning in physics.  The Theory of Planned Behavior (TPB) is used to assess professional development to support the successful implementation of evidence-based instructional strategies.  The Dual-Process Theories of Reasoning (DPToR) are used to model human reasoning and explain persistent inconsistencies in student responses.  Guided by the TPB, an assessment instrument was created, validated, and implemented to evaluate instructor’s beliefs and intentions about active-learning methodologies.  The inconsistencies in student reasoning are examined using the DPToR, which describes reasoning in terms of two processes. Process 1 is quick but frequently inaccurate.  Process 2 is analytical but time-consuming and effortful. Three different types of DPToR-informed instructional interventions are implemented to help students develop skills to navigate reasoning hazards. Data from introductory calculus-based physics classes will be presented and implications for instruction will be discussed.

Equitable Grading Research & Practice in Physics & other STEM Courses
Dr. Cassandra Paul

Associate Professor,
Physics & Astronomy/Science Education Program
San José State University

Tuesday, March 19th, 2:00-3:00pm, 116 South Engineering

In higher education, the role of grades is paramount. The grade that students receive in each course communicates to the institution the degree to which the student was successful. Passing grades indicate that students are able to continue on to more advanced courses in the same topical area. Enough poor grades can cause a student to fail a course and, as a function of the individual university, this can in turn affect the student’s time-to-degree, their retention in a major, or even in their retention in college itself. However, despite its importance, many faculty receive little to no instruction on how to assign grades. I will discuss four different grading strategies that have been shown to decrease equity gaps in physics and other STEM courses:revise/retake options; minimum grading/4.0 scale; ungrading; and course component weighting. I will review some research behind these options, their effect on different students, and some practical ideas for how to implement them in your classroom.

Combining the Bethe-Salpeter Equation, Kadanoff-Baym-Keldysh Technique, and Redfield Theory for an Improved Description of Semiconductor Nanocrystal Observables

Hadassah Griffin

Candidate for Ph.D.
Department of Physics,
North Dakota State University

Monday, Mar. 18, 12:00-1:00pm, 120 South Engineering.

Refreshments at 2:30

Semiconductor nanocrystals (NCs) are used in devices such as solar cells. The physical behavior of NCs can be described by different ab initio methods. For my doctoral research, I am combining the Bethe-Salpeter Equation (BSE), Kadanoff-Baym-Keldysh (KBK) technique, and Redfield Theory to describe NCs. The BSE describes exciton effects which are necessary for an accurate description of NCs. The KBK technique can be used to construct the Boltzmann Transport equation for different relaxation channels in a photo-excited nonequilibrium systems including exciton effects. Redfield Theory describes quantum system dynamics for a system weakly coupled to an external thermal bath. By combining aspects of these methods, my goal is to improve descriptions of NC observables. In this work, I present a review of these methods, preliminary results for Si, Pb, and Janus quantum dot NCs which were calculated by combining BSE output with Redfield Theory, and future plans for my doctoral study.

Using Coarse-Grained Simulations in the Design of Polymer-Based Nanocarriers
Dr. Mariano E. Brito

Post Doctoral Researcher,
Institute for Computational Physics
University of Stuttgart

Physics Seminar:  Monday, March 11, 3-4pm, South Engineering 208

Refreshments at 2:30pm in SE 216

A variety of nanoassemblies can be conveniently achieved by fine-tuning the strength of the hydrophobic interactions of block copolymers in selective solvents, giving rise to assemblies with various interesting topologies. In particular, block copolymer micelles have attracted great attention in the area of drug delivery and imaging in biomedicine due to their easy-to-tune properties and straightforward large-scale production. In the present work, we have investigated the micellization process of linear–dendritic block copolymers in order to elucidate the effect of branching on the micellar properties. We focus on block copolymers formed by linear hydrophobic blocks attached to either dendritic neutral or charged hydrophilic blocks. We have implemented a simple protocol for determining the equilibrium micellar size, which permits the study of linear–dendritic block copolymers in a wide range of block morphologies in an efficient and parallelizable manner. We have explored the impact of different topological and charge properties of the hydrophilic blocks on the equilibrium micellar properties and compared them to predictions from self-consistent field theory and scaling theory. We have found that, at higher degrees of branching in the corona and for short polymer chains, excluded volume interactions strongly influence the micellar aggregation as well as their effective charge.

<CANCELLED>Some Investigations on Stress Localization in Thin Elastic Sheets

Marcelo A. Dias

Reader in Structural Engineering,
Institute for Infrastructure and Environment,School of Engineering
The University of Edinburgh

Tuesday, Feb. 27, 3:00-4:00pm, Hidatsa Room, Memorial Union.

Kirigami, an ancient art of paper cutting, serves as a profound source of inspiration for innovating the morphology and mechanics of thin elastic sheets, offering promising avenues for reconfigurable structures such as large deployable structures and microstructures like stretchable electronics. Through manipulation of cut patterns, these sheets exhibit nonlinear responses at the macroscopic level, stemming from localized instabilities. This talk delves into the phenomenon of stress localization in Kirigami, particularly focusing on the deformation of thin sheets bearing a solitary cut—an elemental geometric unit in Kirigami design. In this exploration, we delve into the intricacies of stress localization, encompassing the formation of e-cones, which is pivotal in understanding how pattern formation manifest within these structures.

This seminar consists of three March Meeting practice talks:

Including Excitonic Effects in Phonon-Mediated Relaxation in Semiconductor Nanocrystals

Hadassah Griffin
Ph.D. Candidate
Department of Physics
NDSU

Accounting for the electron-hole bound state effects is necessary for an accurate description of dynamics of photo-excited semiconductor nanomaterials. Here, we use density function theory (DFT) simulation output to solve the Bethe-Salpeter equation (BSE) for singlet excitons and incorporate the BSE results for exciton states into the existing technique for phonon-mediated relaxation in nanomaterials based on reduced density matrix and Redfield theory. This results in a method that includes excitonic effects. As an application, we compute photoluminescence quantum yield (PLQY) spectrum in several semiconductor nanocrystals.

Bio:Hadassah Griffin is a graduate student at the North Dakota State University Department of Physics. Her research advisor is Dr. Andrei Kryjevski. She started her B. S. Physics degree at Brigham Young University in Provo and completed it at Brigham Young University---Idaho in 2021.

Monte Carlo Lattice Gas for Fluctuating Fluids

Noah Seekins
Ph.D. Candidate
Department of Physics
NDSU

For small fluid systems (or close to a critical point) fluctuations are important in fluids. The reason for fluctuations can be traced back to the fact that nature is discrete. However, the current mainstream fluid simulation models do not have an ideal answer for fluctuations. The lattice Boltzmann, for example, needs fluctuations to be added externally using a Langevin approach.

Lattice gas methods are inherently noisy, and a lot of work analyzing fluctuating systems was pe rformed with them. Their non-noisy limit, the lattice Boltzmann method, eventually replaced these hydrodynamic lattice gas methods because certain artifacts of the original Boolean lattice gases could be overcome. We have been able to show that integer lattice gases can be constructed that are equivalent to the lattice Boltzmann method. These lattice gases offer both inherent fluctuations and unconditional stability that lattice Boltzmann lacks.

We present here an efficient integer lattice gas method based on distribution sampling for the hydrodynamic regime, allowing hydrodynamic systems to now be simulated with inherent fluctuations at a numerical cost comparable to lattice Boltzmann methods.

Bio:Noah is a PhD. student at North Dakota State University studying computational fluid simulation methods with Dr. Alexander Wagner. Noah has earned a B.S. in Physics and Mathematics from NDSU previously.

Derivation of lattice Boltzmann from coarse graining Molecular Dynamics and lattice gases

Alexander Wagner
Associate Professor
Department of Physics
NDSU

Fluids can be modelled at different scales, on the macroscopic scale through conservation equations for mass, momentum, and energy (i.e. Navier Stokes) and on the microscopic scale through Newtonian mechanics of molecules (or even more rigorous quantum descriptions). These descriptions have to match (at least in principle) and a bridging of those scales is accomplished using kinetic theory through Liouville's equation, the equivalent BBGKY hirarchy, its lowest order approximation the Boltzmann equation and from this, by taking the hydrodynamic limit, the macroscopic conservation equations.

Numerical methods span a similar scale from Molecular Dynamics over lattice gases and lattice Boltzmann to tradtional CFD discretizations of the macroscopic equations. We are focusing in this talk on deriving lattice gas and lattice Boltzmann methods directly from Molecular Dynamics simulations. Such an approach is a promising alternative to deriving lattice Boltzmann methods from demanding a match to macroscopic equations, and this approach suggests interesting alternative implementations. In particular this approach suggested novel integer lattice gases that can recover modern lattice Boltzmann methods as their hydrodynamic limit. We wil report on some recent progress in this area.

Bio:Alexander Wagner is originally from Germany where he obtained an undergraduate degree in Physics from the University of Bielefeld. He then obtained a D.Phil from Oxford University, UK in theoretical Physics and worked as a postdoc at MIT, USA and the University of Edinburgh before accepting a faculty position in the Department of Physics at NDSU. He is a co-chair of the DSFD conference (DSFD.org) and an editor for PRE.

Novel Numerical Approach to simulate Photochemical Processes

Landon Johnson

Candidate for Ph.D.
Department of Chemistry,
North Dakota State University

Monday, Feb. 12, 3:00-4:00pm, 208 South Engineering.

Refreshments at 2:30

Photochemical processes, e.g. laser chemical vapor deposition and photocatalysis, have greatly expanded the kinds of materials that can be synthesized and improved the precision of fabrication processes. However, accurately modeling the behavior in such systems at the atomic scale remains an open challenge due to the complexity of the coupled quantum mechanical electronic and photonic degrees of freedom. This presentation will introduce software that is being developed in the Kilin group to address this challenge. The software is still in an early stage of validation, and as such, its framework and theoretical underpinnings will be the primary focus. An initial batch of promising results will also be discussed.

Bio:Landon Johnson is a native of Fargo that obtained a B.S. with a dual major in physics and mathematics and a minor in computer science from NDSU in 2019. After graduating, he continued working as a tradesman in various capacities as a career until returning to NDSU for graduate school in 2022. He is now a PhD student in the materials and nanotechnology program studying computational quantum chemistry under Dr. Dmitri Kilin. Since returning to graduate school, he has spent his summers working on the machine learning side of developing an accelerated nuclear fuel qualification process under Dr. Galen Craven's mentorship at Los Alamos National Laboratory.

2023 Nobel Prizes in Chemistry & Physics: Controlling Electrons in Quantum Dots and Atoms
Dr. Erik Hobbie

Professor,
Department of Physics and Department of Coatings and Polymeric Materials
North Dakota State University

Monday, February 5, 3:00-4:00pm, 221 South Engineering

Refreshments at 2:30

2023 Nobel Prizes in Chemistry & Physics: Controlling Electrons in Quantum Dots and Atoms

A brief overview of the 2023 Nobel prizes in Chemistry and Physics is presented, the former from a physics perspective and the latter from a chemistry perspective. Quantum dots and quantum confinement are described using the Heisenberg uncertainty principle, Bloch waves and semiconductors, while attophysics is addressed through the periodic table of elements. The significance of the award to L'Huillier and her work will also be considered.

Bio:Erik K Hobbie is a Professor of Physics and Coatings & Polymeric Materials and directs the graduate program in Materials & Nanotechnology at NDSU. His research is focused on the synthesis, characterization, and application of colloidal semiconductor nanocrystals.

2023 Nobel Prizes in Chemistry & Physics: Controlling Electrons in Quantum Dots and Atoms

A brief overview of the 2023 Nobel prizes in Chemistry and Physics is presented, the former from a physics perspective and the latter from a chemistry perspective. Quantum dots and quantum confinement are described using the Heisenberg uncertainty principle, Bloch waves and semiconductors, while attophysics is addressed through the periodic table of elements. The significance of the award to L'Huillier and her work will also be considered.

Bio:Erik K Hobbie is a Professor of Physics and Coatings & Polymeric Materials and directs the graduate program in Materials & Nanotechnology at NDSU. His research is focused on the synthesis, characterization, and application of colloidal semiconductor nanocrystals.

Population Dynamics and Chaos: effects of spatial diffusion
Dr. Alexander Wagner

Associate Professor,
Department of Physics,
North Dakota State University

Monday, January 29, 3:00-4:00pm, 221 South Engineering

Refreshments at 2:30

While teaching our Computational Physics course I revisited an old problem: the dynamics of a simple population dynamics where growth is related to a birth rate b and a death rate that is proportional to the number of individuals (usually because a carrying capacity is reached). This is simply written as               N(t+1) = [1+b-d*N(t)] N(t)For small birth rates b, the dynamics is initially an exponential growth, but then the growth rate declines until it vanishes when N(t)=b/d. But for larger growth rates this stable point can be overshot leading initially to an oscillatory behavior and for even larger values of b to a much more complex behavior. When we plot the number of individuals $N(t)$ for large t (say for 100 times) and look how this changes with the birth rate b (leaving d=0.01 fixed) we get a bifurcation diagram.

When we now also allow different populations at different lattice sites and allow individuals to migrate the behavior can create rather spectacular patterns. I will show some results for these new (or at least new to me) phenomena and invite you to play with this very simple algorithm that can create fascinating patterns.  I will also present some preliminary result on other fascinating aspects of this model and some of the unexpected behavior we observed. This is a “research in progress” talk, and I hope that it will stimulate lively discussions.

Bio:Alexander Wagner is originally from Germany where he obtained an undergraduate degree in Physics from the University of Bielefeld. He then obtained a D.Phil from Oxford University, UK in theoretical Physics and worked as a postdoc at MIT, USA and the University of Edinburgh before accepting a faculty position in the Department of Physics at NDSU. He is a co-chair of the DSFD conference (DSFD.org) and an editor for PRE.

Probing students' reasoning with multi-variable expressions in the context of potential difference

Safana Ismael

Candidate for Ph.D.
Department of Physics,
North Dakota State University

Monday, Dec. 4, 3:00-4:00pm, 208 South Engineering.

Refreshments at 2:30

Research suggests that some reasoning difficulties persist even after targeted instruction. One such instance is the student’s incorrect reasoning with multi-variable expressions in the context of Electromagnetism in the calculus-based introductory physics course. In this talk, I will focus on student reasoning with Potential Difference, DV=-WEF/qtest. Specifically, after relevant instruction, students struggle to recognize that if the value of a test charge moving between two points is changed, the potential difference between the two points remains the same.  We designed instructional intervention (a blend of web-based assignments and classroom instruction) to prob students' reasoning with multivariable expressions in the context of (1) math problems and (2) analogous problems in the context of physics. The results suggest that student performance on math problems does not predict their performance on analogous problems in the context of physics.

Viscoelasticity and the Persson-Brener Model

Kurt VanDonselaar

Candidate for Ph.D.
Department of Physics,
North Dakota State University

Monday, Oct. 29, 3:00-4:00pm, 208 South Engineering.

Refreshments at 2:30

Polymeric coatings are widely produced by industry and used to create barriers between structures and the elements. Recently, effort has focused on soft coatings that prevent ice and other unwanted foulants from adhering to surfaces. While low surface energies of soft materials, such as polydimethylsiloxanes (PDMS), promise easy removal of attached foulants and ice, the simple physical limit based on surface energy has not been achieved in practice. It is largely believed that the failure is due to viscoelastic losses in the soft coating materials. To better understand the viscoelastic losses in soft adherent PDMS materials, we perform JKR adhesion experiments on several elastomers, at different temperatures and over a set of speeds that spans several orders of magnitude. Each elastomer also undergoes DMA experiments to characterize the dynamic mechanical modulus from the glassy to rubbery regime. We demonstrate that the adhesion tests are qualitatively related to the dynamic moduli and use a more direct comparison (the Persson-Brener model of crack propagation [1]) to show a quantitative relationship between adhesion and dynamic moduli at low speeds.

[1]  B. N. J. Persson and E. A. Brener, “Crack propagation in viscoelastic solids,” PHYSICAL REVIEW E 71, 036123 (2005).

Bio:Kurt VanDonselaar is a Ph.D. student at North Dakota State University. He is studying polymeric materials in the Croll lab. He earned a B.S. in physics from Winona State University and a M.S. in physics from the University of Minnesota Duluth.

Quantum Mechanics and the measurement problem
Dr. Alexander Wagner

Associate Professor,
Department of Physics,
North Dakota State University

Monday, October 9, 3:00-4:00pm, 221 South Engineering

Refreshments at 2:30

Quantum Mechanics describes the continuous evolution of a physical system following the Schrödinger equation. This continuous evolution  is interrupted when a “measurement” occurs. This form of the evolution equation is unlike anything else that has been suggested for Physical systems. Despite the enormous success of using this theory to make predictions this formalism has baffled researchers and philosophers, and more research into these fundamentals of Quantum Mechanics has long been discouraged. However, teaching Quantum Mechanics this term I have started thinking about this issue again.

In this talk I intend to give a brief overview of the issues involved, including a brief overview of hidden variable and many worlds ideas.

References:
[1] Griffith: Quantum Mechanics
[2] Sean Carroll: Something Deeply Hidden

Switching Adhesion With Origami

Wathsala Amadoru Jayawardana

Candidate for Ph.D.
Department of Physics,
North Dakota State University

Monday, Sept. 25, 3:00-4:00pm, 208 South Engineering.

Refreshments at 2:30

One of the most interesting features of origami, is its ability to form both mechanically stable (or stiff) and unstable (or soft) configurations.  Here we show how the changes in stiffness of origami structures relate to their adhesion to surfaces.   Specifically, we designed three bi-stable origami designs made of polycarbonate, cut with a Cricut cutter, and supplemented with 3D printed parts to enable testing of adhesive force.    We use the well-understood polydimethylsiloxane(PDMS) elastomers as adhesive pads in order to carefully explore adhesion as the designs shift their mechanical state.  Ultimately, we show how the ‘stiff’ state breaks adhesive contact similar to a bulk solid but the soft states tend to fail by initiating ‘peel’ modes.  Moderate switching ratios (Fon/Foff ~50) were achieved and many options for scaling designs up or down in adhesive response were identified.

Some relevant References

[1] Jayawardana, W.M., Liao, Y., Li, Z., Xia, W. and Croll, A.B., 2023. Crumpled Kirigami. Soft Matter, 19(6), pp.1081-1091.

[2] Croll, A.B., Liao, Y., Li, Z., Jayawardana, W.M., Elder, T. and Xia, W., 2022. Sticky crumpled matter. Matter, 5(6), pp.1792-1805.

Bio: Wathsala  Jayawardana is a graduate student in the Department of Physics at North Dakota State University and is working with her adviser Andrew Croll.

Deriving lattice Boltzmann from Molecular Dynamics

Alexander Wagner

Professor
Department of Physics,
North Dakota State University

Monday, Sept 11, 3:00-4:00pm, 208 South Engineering (contact Alexander Wagner for zoom link).

Refreshments at 2:30

Lattice Boltzmann Methods are a relatively new approach to simulating many complex fluid systems. A key motivation behind the development of these methods was that they were derivable from simply underlying Statistical Mechanics models, i.e. hydrodynamic lattice gases. Lattice Boltzmann was originally derived as a Boltzmann limit of such lattice gases, but subsequent improvements of these Methods severed the link to such underlying methods.

While the improvements were significant, they also came with drawbacks: while lattice gases are unconditionally stable, modern lattice Boltzmann Methods are not. Lattice gases obeyed an H-Theorem, but most modern lattice Boltzmann Methods do not. Fluctuations were included in lattice gases, but they turn out to be less trivial to re-introduce into lattice Boltzmann Methods.

All those observations are reasons to examine what novel lattice gases might look like that include the advantages of modern lattice Boltzmann Methods and retain the advantages of lattice gases. In this talk we will give a brief overview of deriving lattice gases that have Boltzmann limits that are very close to current lattice Boltzmann Methods and a coarse-graining approach the maps Molecular Dynamics simulations onto lattice gas (and lattice Boltzmann) Methods and what we can learn about the underlying Physics from those approaches.

Below I give a brief bibliography of our recent papers on this subject:

[1] Lattice gas with molecular dynamics collision operatorMR Parsa, AJ WagnerPhysical Review E 96 (1), 013314 (2017)

[2] Integer lattice gas with Monte Carlo collision operator recovers the lattice Boltzmann method with Poisson-distributed fluctuationsT Blommel, AJ WagnerPhysical Review E 97 (2), 023310 (2018)

[3] Validity of the molecular-dynamics-lattice-gas global equilibrium distribution functionMR Parsa, A Pachalieva, AJ WagnerInternational Journal of Modern Physics C 30 (10), 1941007 (2019)

[4] Non-Gaussian distribution of displacements for Lennard-Jones particles in equilibriumA Pachalieva, AJ WagnerPhysical Review E 102 (5), 053310 (2020)

[5] Large fluctuations in nonideal coarse-grained systemsMR Parsa, AJ WagnerPhysical Review Letters 124 (23), 234501 (2020)

[6] Nonuniqueness of fluctuating momentum in coarse-grained systemsMR Parsa, C Kim, AJ WagnerPhysical Review E 104 (1), 015304 (2021)

[7] Molecular dynamics lattice gas equilibrium distribution function for Lennard–Jones particlesA Pachalieva, AJ WagnerPhilosophical Transactions of the Royal Society A 379 (2208), 20200404 (2021)

[8] Connecting lattice Boltzmann methods to physical reality by coarse-graining Molecular Dynamics simulationsA Pachalieva, AJ WagnerarXiv preprint arXiv:2109.05009 (2021)

[9] Integer lattice gas with a sampling collision operator for the fluctuating diffusion equationN Seekins, AJ WagnerPhysical Review E 105 (3), 035303 (2022)

[10] Overrelaxation in a diffusive integer lattice gasKyle Strand and Alexander J. WagnerPhys. Rev. E 105, L06330 (2022)

Bio: Alexander Wagner is a Professor in the Department of Physics at North Dakota State University. He received his D.Phil. in theoretical Physics from Oxford University working under Julia Yeomans on lattice Boltzmann Methods for multi-phase and multi-component mixtures. He then moved as a postdoc to the Department of Materials Science and Engineering at MIT where he worked with Chris Scott and developed lattice Boltzmann Methods for viscoelastic fluids. From there he moved as a postdoc in the Department of Physics at the University of Edinburgh where he worked with Mike Cates on sheared phase-separation and scaling issues in Phase ordering.

He is involved in the Discrete Simulations of Fluid Dynamics conference, which he chaired from 2008-2012 and he remains active in the international committee. Since 2012 he has been a remote editor for Physical Review E, taking over the responsibility for the Computational Physics section. His current research interests focus on the fundamental underpinnings of discrete fluid simulation methods.

Measuring More than Content: The development and testing of a diagnostic instrument designed to assess student physics mindware and reasoning consistency

Brianna Santangelo

Candidate for Ph.D.
Department of Physics,
North Dakota State University

Thursday, April 6, 3:00-4:00pm, 208 South Engineering (contact Alexander Wagner for zoom link).

Refreshments at 2:30

One of the goals of physics instruction is to help students improve their reasoning. Research from cognitive psychology suggests that to reason productively, students need to possess content knowledge (i.e., mindware) and the reasoning skills necessary to apply this knowledge productively. We build on the research from cognitive psychology to develop a multi-tier assessment instrument designed to disentangle and measure multiple aspects of student thinking such as mindware and reasoning skills. In this talk, I will discuss Dual-process Theories of Reasoning (DPToR) that informed our investigations of student thinking, the screening-target methodology used to develop specific assessment items, and the examination of the evidence for the instrument validity. Data from students' written responses and interviews and from instrument administration will be presented.  Implications for future research on student reasoning in physics and physics instruction will also be discussed.  

Bio: Brianna Santangelo received her B.S. in Physics and Education from The College of New Jersey in 2017 before coming to North Dakota State University to study Physics Education. In 2019, Brianna won the NSF Graduate Research Fellowship which helped fund her project on utilizing Dual Process Theories of Reasoning to develop physics assessment designed to disentangle student content knowledge and their reasoning consistency with that content. She received her M.S. in Physics from NDSU in 2021 and has won awards such as the Darrell and Carol Strobel Graduate Research Award, the Darrell and Carol Strobel Graduate Excellence Award, and the GIS Doctoral Dissertation Fellowship.

Graduate Student,
Department of Physics,
North Dakota State University

This seminar constitutes the public part of Kurt’s comprehensive exam to advance to PhD candidate status.

Monday, November 27, 3:00-4:00pm, 208 South Engineering <In Person>
Online for Special Cases Only: contact Alexander Wagner  for details

Coatings that are anti-fouling, anti-icing, durable, and non-toxic are currently in demand by industries and governments around the world. Current trends in coating design have focused on the use of soft, elastomeric materials. Elastomers are easily deformed, often behave viscoelastically, and adhere to surfaces. In this presentation the fundamentals of fracture mechanics and adhesion are reviewed. We develop methods to quasi-statically measure the effective elastic modulus (Young's modulus) and the energy release rate, which is a quantity that describes the "adhesiveness" of a material and is related to the contact area, applied force, and effective elastic modulus. We also performed dynamic experiments that relate the energy release rate to the crack velocity. We perform these measurements for three different model PDMS elastomers at different temperatures. Additionally, two interfaces are utilized; a glass lens probe against a rectangular sample or a polymeric lens against ice grown on a glass slide.

  We layout methods for exploring the connection between speed and adhesion using an empirical model [1] and a theoretical model by Persson and Brener [2]. It is shown that the quasi-static energy release rate values are dependent on the temperature, but the velocity dependence is independent of temperature and whether the interface is glass-polymer or ice-polymer.

[1] D Maugis and M Barquins. Fracture mechanics and adherence of viscoelastic solids. In

     Adhesion and adsorption of polymers, pages 203–277. Springer, 1980.

[2] BNJ Persson and EA Brener. Crack propagation in viscoelastic solids. Physical Review

     E, 71(3):036123, 2005.

Bio: Kurt VanDonselaar is a Ph.D. student at North Dakota State University. He is studying polymeric materials in the Croll lab. He earned a B.S. in physics from Winona State University and a M.S. in physics from the University of Minnesota Duluth.

Associate Professor,
Department of Physics,
North Dakota State University

Monday, November 14, 3:00-4:00pm, 208 South Engineering <In Person>
Online for Special Cases Only: contact Alexander Wagner  for details

The fermion sign problem, when severe, prevents the computation of physical quantities of a system of interacting fermions via stochastic evaluation of its path integral defined on discretized space-time due to the oscillatory nature of the integrand exp(-S), where S is the imaginary-time action. However, in the Kohn-Sham orbital basis, which is the output of a Density Functional Theory simulation, the path integral lattice field theory approach for electrons in a semiconductor nanoparticle may have only a mild fermion sign problem and is amenable to evaluation by standard stochastic methods. This is evidenced by our simulations of silicon hydrogen-passivated nanocrystals (NCs), such as Si₃₅H_36, Si₈₇H₇₆, Si₁₄₇H₁₀₀ and Si₂₉₃H₁₇₂, which range in size 1.0 - 2.4 nm and contain 176 to 1344 valence electrons, and to a 1.8 nm hetero-structured (Janus-type) NC Cd₃₇Pb₃₁Se₆₈ with 1582 valence electrons. We find that approximating the fermion action by its leading order polarization term results in a positive-definite integrand, and is a very good approximation of the full action. We compute imaginary-time electron propagators and extract the energies of low-lying electron and hole levels. Our quasiparticle gap predictions agree with the results of previous G₀W₀ calculations. This formalism naturally allows calculations of more complex excited states, such as excitons and trions, for which we present some results.

Bio: Andrei Kryjevski an Associate Professor in the Department of Physics at North Dakota State University. He received his Ph.D. in Physics from the University of Washington in 2004. He did a Postdoc at the Department of Electrical Engineering of the University of Washington until 2005 when he moved to Indiana University. In 2007 he moved to Washington University in Saint Louis and accepted a faculty position at NDSU in 2008.

Professor,
Department of Physics,
North Dakota State University

Monday, November 7, 3:00-4:00pm, 208 South Engineering <In Person>
Online for Special Cases Only: contact Alexander Wagner  for details

Lattice Boltzmann Methods are a relatively new approach to simulating many complex fluid systems. A key motivation behind the development of these methods was that they were derivable from simply underlying Statistical Mechanics models, i.e. hydrodynamic lattice gases. Lattice Boltzmann was originally derived as a Boltzmann limit of such lattice gases, but subsequent improvements of these Methods severed the link to such underlying methods.

While the improvements were significant, they also came with drawbacks: while lattice gases are unconditionally stable, modern lattice Boltzmann Methods are not. Lattice gases obeyed an H-Theorem, but most modern lattice Boltzmann Methods do not. Fluctuations were included in lattice gases, but they turn out to be less trivial to re-introduce into lattice Boltzmann Methods.

All those observations are reasons to examine what novel lattice gases might look like that include the advantages of modern lattice Boltzmann Methods and retain the advantages of lattice gases. In this talk we will give a brief overview of deriving lattice gases that have Boltzmann limits that are very close to current lattice Boltzmann Methods and a coarse-graining approach the maps Molecular Dynamics simulations onto lattice gas (and lattice Boltzmann) Methods and what we can learn about the underlying Physics from those approaches.

Below I give a brief bibliography of our recent papers on this subject:

[1] Lattice gas with molecular dynamics collision operator

MR Parsa, AJ Wagner

Physical Review E 96 (1), 013314 (2017)

[2] Integer lattice gas with Monte Carlo collision operator recovers the lattice Boltzmann method with Poisson-distributed fluctuations

T Blommel, AJ Wagner

Physical Review E 97 (2), 023310 (2018)

[3] Validity of the molecular-dynamics-lattice-gas global equilibrium distribution function

MR Parsa, A Pachalieva, AJ Wagner

International Journal of Modern Physics C 30 (10), 1941007 (2019)

[4] Non-Gaussian distribution of displacements for Lennard-Jones particles in equilibrium

A Pachalieva, AJ Wagner

Physical Review E 102 (5), 053310 (2020)

[5] Large fluctuations in nonideal coarse-grained systems

MR Parsa, AJ Wagner

Physical Review Letters 124 (23), 234501 (2020)

[6] Nonuniqueness of fluctuating momentum in coarse-grained systems

MR Parsa, C Kim, AJ Wagner

Physical Review E 104 (1), 015304 (2021)

[7] Molecular dynamics lattice gas equilibrium distribution function for Lennard–Jones particles

A Pachalieva, AJ Wagner

Philosophical Transactions of the Royal Society A 379 (2208), 20200404 (2021)

[8] Connecting lattice Boltzmann methods to physical reality by coarse-graining Molecular Dynamics simulations

A Pachalieva, AJ Wagner

arXiv preprint arXiv:2109.05009 (2021)

[9] Integer lattice gas with a sampling collision operator for the fluctuating diffusion equation

N Seekins, AJ Wagner

Physical Review E 105 (3), 035303 (2022)

[10] Overrelaxation in a diffusive integer lattice gas

Kyle Strand and Alexander J. Wagner

Phys. Rev. E 105, L06330 (2022)

Bio: Alexander Wagner is a Professor in the Department of Physics at North Dakota State University. He received his D.Phil. in theoretical Physics from Oxford University working under Julia Yeomans on lattice Boltzmann Methods for multi-phase and multi-component mixtures. He then moved as a postdoc to the Department of Materials Science and Engineering at MIT where he worked with Chris Scott and developed lattice Boltzmann Methods for viscoelastic fluids. From there he moved as a postdoc in the Department of Physics at the University of Edinburgh where he worked with Mike Cates on sheared phase-separation and scaling issues in Phase ordering. 

He is involved in the Discrete Simulations of Fluid Dynamics conference, which he chaired from 2008-2012 and he remains active in the international committee. Since 2012 he has been a remote editor for Physical Review E, taking over the responsibility for the Computational Physics section. His current research interests focus on the fundamental underpinnings of discrete fluid simulation methods.

Northern Cass High School,
Hunter, ND

Monday, October 31, 3:00-4:00pm, 208 South Engineering <In Person>
Online for Special Cases Only: contact Alexander Wagner  for details

There are not many analytical solutions of the lattice Boltzmann equation. Those that exist are powerful since they allow a close analysis of the method itself. We sought an analytical solution for the lattice Boltzmann equation describing a steady state, the imcompressible Couette flow. To do so, we used a Galilean transformation Ansatz to relate the two neighboring lattice Boltzmann densities. Verified using computer simulations using certain boundary conditions, the solution works for a quadratic equilibrium distribution, but not for an entropic equilibrium distribution. We compare the results to a previous one presented in the literature, which appears to have been incorrect. Extensions of the Ansatz regarding transient flows, non-coquette flows, and more are discussed.

Bio: Jordan Larson is a senior at Northern Cass High School working with Professor Wagner’s research group. Last year, I reached out to Professor May inquiring about internship opportunities. After some interviews I choose Wagner’s work. This year, I have the support of my school’s Studies course to give me more resources to work with Prof. Wagner, compared to last year’s situation. Learning more about the lattice Boltzmann Method, the last year has been eye opening about what research is actually like as a physicist. Confidently I can say I want to pursue research as a career path.

Professor,
Department of Physics,
Emory University

Monday, October 24, 3:00-4:00pm, 208 South Engineering
Online: contact Alexander Wagner  for details

Surface bound chains such as grafted and adsorbed polymers are widely used for modifying interfacial interactions in polymer materials and nanocomposites.  However, the underlying mechanisms by which changes to the material properties are conferred to the system are not well understood.  To isolate the physics behind local property changes, we focus on a single interface and use localized fluorescence to probe how end-grafted chains increase the local glass transition temperature Tg(z) next to an interface with varying grafting density, tethered chain length, and end functional group.  We also begin to address the more complicated system of adsorbed chains where non-specific interactions adhere polymer chains to surfaces forming “bound layers”.  This work builds on our group's recent work that demonstrated a local Tg(z) increase of 50 K in polystyrene (PS) matrices with PS end-grafted chains at low grafting densities [Huang, Roth, ACS Macro Letters 2018, 7, 269].  Possible underlying physical mechanisms for this behavior will be discussed.

NDSU Physics-Chemistry Seminar
Thursday, October 13, 2022, 4:00 PM
Sugihara Hall 252 and on Zoom:

https://ndsu.zoom.us/j/95375486917?pwd=QnRrbXlFNFJBSWJNbGpWc0FhK0tEZz09

Braden M. Weight¹ and Pengfei Huo^2,3

¹Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, U.S.A
²Department of Chemistry, University of Rochester, Rochester, New York 14627, USA
³The Institute of Optics, Hajim School of Engineering, University of Rochester, Rochester, New York 14627, USA

Polaritonic chemistry has become the leading direction to control a multitude of processes, such as charge transfer, selective bond breaking, and excited state dynamics. An exciton-polariton is an entangled state of light and matter in which the native excitonic and photonic degrees of freedom hybridize to form new states. These new states can be tuned in various ways to modify and produce unique properties, such as the potential energy landscape or the emission efficiency of materials. However, much is still unknown about how these new hybrid states can modify such chemical properties. For example, the nature of the exciton is lost and becomes effectively mixed with all other molecular excitations, and so the shape and distribution of resulting exciton-polariton wavefunctions will be dramatically different from the uncoupled excitonic picture. These changes will dictate all the resulting properties, such as the absorption/emission spectra though the polaritonic transition dipole moment and the excited state dynamics through modification of the potential energy surfaces. In this work, we explore a set of real, atomistic molecules via density functional theory and couple their electronic structure to a single-mode cavity in order to explore the resulting properties of the coupled system.

Associate Professor,
Department of Physics,
University of North Dakota
 

Monday, October 10, 3:00-4:00pm, 208 South Engineering
Online: contact Alexander Wagner  for details

The discovery of ferromagnetism in single-layer CrI3 in 2017 spurred a search for magnetism in other families of two-dimensional materials, including transition metal chalcogenides and MXenes [1,2].  First-principles methods such as spin-polarized DFT can be used to predict magnetic ground states, but to obtain finite-temperature properties such as critical temperatures, one usually has to fit DFT data using an effective model, and then simulate that model using Monte Carlo simulations.  Using CrI3 as a case study, I will illustrate the power (and pitfalls!) of this DFT+MC "multiscale modeling" approach.

[1] "Strain–Spintronics: Modulating Electronic and Magnetic Properties of Hf2MnC2O2 MXene by Uniaxial Strain," E. M. D. Siriwardane, P. Karki, Y. L. Loh, and Deniz Çakır, J. Phys. Chem. C 2019, 123, 19, 12451–12459

[2] "Engineering magnetic anisotropy and exchange couplings in double transition metal MXenes via surface defects," E. M. D. Siriwardane, P. Karki, Y. L. Loh, and Deniz Çakır, J. Phys.: Condens. Matter 33 035801

Maynard, Cooper & Gale, P.C.
Shareholder
Registered Patent Attorney

Monday, October 3, 3:00-4:00pm, 208 South Engineering
Online: contact Alexander Wagner  for details

The Bayh-Dole Act of 1980 amended the patent laws of the U.S. to provide amechanism by which universities, research foundations and small businessesmay elect to own inventions that they made with funding from the federalgovernment. The Act provides the framework in which the number of patentsobtained by universities, technology transfer licensing, and commercializationof such inventions through “start-up” businesses have increased by orders ofmagnitude. This presentation will describe how this mechanism works,background information about ownership of inventions in the form of patentsand know-how, why the Act was passed and current proposals for exercisingthe government’s “march-in” rights.

Monday, Sept. 26, 2022, 3:00-4:00pm, Room: SE 208

Online: contact Alexander Wagner for details

In recent years, origami design has been used by the engineering community to solve many problems and has also inspired much fundamental study of mechanisms, limits, and geometry.  While not often discussed, origami suffers from several drawbacks such as not being easily reconfigurable (as creases or hinges require memory in the sheet) and not easily creating curved or closed structures.  In this work, we show how sticky sheets (kuttsukugami) can not only create identical structures as in traditional origami but can be used to solve many of the problems.  We show how sticky sheets can easily create stable curved structures, how they create a truly reusable and reconfigurable systems, how they open up the use of materials that cannot be used in traditional origami and how they can be used to create measurement and encapsulation schemes.

Monday, Sept. 12, 2022, 3:00-4:00pm, Room: SE 208

Online: contact Alexander Wagner for details

Recent analyses of scientific databases indicate that the volume of publications is growing at a rate exceeding 4% per year, doubling about every 15 years [1, 2]. How to make sense of it all? In this informal discussion, I will first offer some suggestions, then host a brainstorming session, on best practices for efficiently and responsibly processing the mountain of information out there in the exponentially growing body of scientific literature. All are welcome to join the discussion and offer tips and wisdom.

[1] Bornmann, L., Haunschild, R., Mutz, R. Growth rates of modern science: a latent piecewise growth curve approach to model publication numbers from established and new literature databases. Humanit Soc Sci Commun 8, 224 (2021). doi.org/10.1057/s41599-021-00903-w

[2] Fortunato, S., Bergstrom, C. T., Börner, K., Evans, J. A., Helbing, D., Milojević, S., Barabási, A.-L. Science of science. Science 359, 6379 (2018). doi.org/10.1126/science.aao0185

Monday, Aug. 29, 2022, 3:00-4:00pm, Room: SE 208

Online: contact Alexander Wagner for details

The cause for the relatively high compressive strength of a crumpled sheet has long fascinated many researchers. When a sheet is confined in a spherical cavity it slowly bends and collides with itself until it becomestrapped in a complicated random state. Much research has focused on the geometry of the complex constructin the hopes that the structure coupled with the physics of ‘building block’ substructures (bends, folds, d-cones and ridges) will lead to the creation of a fundamental model. Our approachto the crumpled matter problem focuses on intentionally simple experiments aimed at the macroscopic measurables of the crumpledstate. For example, we use both plastic and elastic sheets to test how crumpling isaffected by plasticity, we alter levels ofinter-sheet adhesion to see if crumpled matter ischanged as it become more immobile, and we add cuts to disturb the arrangement of bending networks within the crumple.In this talk, we will focus on some of our more recent work with sticky and cut sheets where we couple our simple experiments with a coarse-grained molecular dynamics modelfor added insight. If time permits, we will also show how ‘sticky crumpled matter’can find application as a unique roughness-tolerant pressure sensitive adhesive.

Monday, April 25, 3:00-4:00pm, 221 South Engineering and on Zoom (Contact Alexander Wagner for link).

Refreshments at 2:30

Nuclear fusion, where deuterium (D) and tritium (T) fuse to make alpha particles (helium ions), neutrons and a great amount of excess energy is the physical process which powers the sun and if harnessed in the lab, has the potential to provide a clean and abundant worldwide energy resource. One way to achieve this is by using lasers to rapidly compress and heat a DT target to the conditions necessary for fusion - a process referred to as inertial confinement fusion (ICF). Significant efforts towards making ICF a viable energy resource began in the late 1970s, but it wasn't until 2021 when a burning plasma (a state in which most of the heating of the target comes from fusion reactions within the target) was achieved in an experiment at the National Ignition Facility. This experiment set a new record for the energy produced by an ICF implosion - 1.3 MJ, which is 70% of the energy delivered by the laser and 25 times the previous record set in 2018. In this talk I will discuss some of the physics behind these recent exciting results and in particular, the important role of first-principles simulation of matter at extremely high temperatures and densities where currently the most successful tool is molecular dynamics driven by density functional theory.

Monday, April 11, 3:00-4:00pm, This will be a  hybrid seminar in room 208 in South Engineering and on zoom (contact Alexander Wagner for link)

Rare-earth (RE) doped semiconductors have long been of interest for optoelectronics and spintronics. More recently, they have also been considered for quantum applications (e.g., quantum computing, quantum memories, and quantum communication). Whether a RE dopant is being harnessed for traditional optical applications or novel quantum technologies, having a fundamental understanding of the interaction between the dopant and the semiconductor host is key to realizing its potential. In this talk, I present recent studies of the interaction between gallium nitride (GaN) and lanthanide dopants using state-of-the-art first-principles defect calculations [1]. I will then discuss its implications on defect↔band and intra-f optical transitions and prospects of RE-doped semiconductors for quantum applications.

 [1] Hoang, Phys. Rev. Mater. 5, 034601 (2021); Phys. Rev. Mater. (2022), in press, arXiv.2201.03651.

Monday, April 4, 2:00-3:00pm, <NOTE SPECIAL TIME>South Engineering 208 and on Zoom (contact Alexander Wagner for link)

Please note the earlier time of 2pm

The lattice Boltzmann method (LBM) became a household name in the simulation of nearly-incompressible flows. In this presentation, I shall first review a success of LBM in its traditional domain and demonstrate ways to develop compressible flow LBM for transonic and mildly supersonic flow using a recent extended formulation. After that, I will present arguments on why  the conventional LBM formulations become difficult for hypersonic flows and describe an ongoing research on the Particle-on-Demand model for extreme compressible flow simulations.

Monte-Carlo Lattice Gases: Increasing the Efficiency of Integer Lattice Gases via a Sampling Collision Operator
Noah Seekins

Ph.D. Candidate,
Department of Physics,
North Dakota State University

Monday, March 21, 3:00-4:00pm, South Engineering 208 and on Zoom (contact Alexander Wagner for link)

As lattice Boltzmann simulations have been the primary lattice-based fluid dynamics simulation method for the past several decades, lattice gases have fallen by the wayside due to lack of exposure and research, as well as the wide gap in computational efficiency between the two methods. However, since lattice gases, specifically integer lattice gases, which have been discussed most recently by Thomas Blommel, have several advantages over the lattice Boltzmann method, a way to close that efficiency gap would be needed for further research to be warranted.

We have created, by utilizing a sampling collision operator, the Monte Carlo Lattice Gas, a lattice gas method that is relatively comparable to the lattice Boltzmann method, being within an order of magnitude in efficiency, for the diffusive case (non-momentum conserving) [1] and have begun to test a similar method for hydrodynamic (momentum conserving) systems.

[1] Noah Seekins and Alexander J. Wagner, Phys. Rev. E 105, 035303 – Published 18 March 2022

Applications of time-dependent excited-state molecular dynamics
Dmitri Kilin

Department of Chemistry and Biochemistry,
North Dakota State University,

Monday, Feb. 7, 2022, 3:00-4:00pm, 208 South Engineering HyFlex (contact Alexander Wagner for zoom information)

Refreshments at 2:30 PM in SE 216

Open quantum system concept is used for practical exploration of excited state dynamics in molecules and nanostructures in the limits of electromagnetic radiation of low- and mid- intensity. Modeling of low-intensity excitations predicts radiative and nonradiative dissipation of electronic excitation energy and allows to assess efficiency of photoluminescence and charge transfer in semiconductor nanostructures. In contrast, modeling of mid-intensity laser field opens an opportunity to facilitate additional processes in molecular systems, such as chemical reactions with substantial activation energy, not available in conventional chemistry. A DFT based time-dependent excited-state molecular dynamics (TDESMD) approach offers a compromise in numeric expense / accuracy balance in modeling photoinduced chemical reactions. The TDESMD combines concepts of Rabi oscillations and trajectory surface hopping. The leading order process is represented by the molecule undergoing cyclic excitations and de-excitations. During excitation cycles, the nuclear kinetic energy is accumulated to overcome the reaction barriers. The dynamical formation of multiple products is observed in TDESMD trajectories. A recent update to TDESMD approach considers intermediates and transition states in non-singlet configurations, for the pathways including fragments and radicals. Applications of TDESMD include photofragmentation of gas-phase metal-organic complexes and small organic molecules as well as photopolymerization of inorganic monomers and photo-adsorption of functional groups to single wall carbon nanotubes, leading to formation of IR-emitting covalent defects. Similar methods were used to explore photo-desorption of ligands from surface of lead-halide perovskite quantum dots.

Supported by NSF CHE-1944921 and CHE-2004197

Physics of Soft Colloids: From Shape-Shifting Polymers to Magical Swelling Microgels
Prof. Alan Denton

Interim Chair of Physics,
North Dakota State University,

Monday, Jan. 31, 2022, 3:00-4:00pm, 208 South Engineering HyFlex (contact Alexander Wagner for zoom information)

Refreshments at 2:30 PM in SE 216

Colloidal dispersions comprise tiny particles dispersed in a fluid, the particles so small as not to sediment out, yet large enough to exhibit diffusive (Brownian) motion, visible under a light microscope. Ranging in size from nanometers to  microns, colloidal particles also vary in softness, from flexible polymer coils to rigid glassy beads. The capacity to change size and shape gives soft colloids unusual thermal, mechanical, and optical properties, enabling many applications in the biomedical, food, pharmaceutical, and consumer care industries. Biopolymers (e.g., DNA, RNA, proteins), with their ability to readily change conformation and encode genetic information, provide the foundation of life. Stiffer but still soft colloids include polymer microgels, surfactant micelles, and lipid vesicles. Microgels are microscopic elastic networks of cross-linked polymers with the remarkable ability to swell and deswell in response to changes in their environment. With their capacity to encapsulate cargo (e.g., drug or dye molecules), microgels have important applications to drug delivery, biosensing, and filtration. Forces between soft colloids depend on elastic and other properties of the particles. When colloids are mixed with polymers, depletion forces can come into play. Effective attractive forces between pairs of colloids, induced by depletion of nonadsorbing polymers, can drive demixing into colloid-rich and polymer-rich phases, with practical relevance for water purification, stability of consumer products, and macromolecular crowding in biological cells. In this talk, I will unpack the zoology of soft colloids and describe how molecular simulation methods can help us understand connections between single-particle properties and collective properties of soft materials, such as thermodynamic phase behavior.

The James Webb Space Telescope: What’s the Big Deal?
Prof. Alan Denton

Interim Chair of Physics,
North Dakota State University,

Monday, Jan. 24, 2022, 3:00-4:00pm, 208 South Engineering HyFlex (Zoom Meeting ID: 964 4434 9875   Passcode: 492288 )

Refreshments at 2:30 PM in SE 216

On December 25, 2021, the James Webb Space Telescope (JWST), an international collaboration among NASA, the European Space Agency, and the Canadian Space Agency, launched aboard an Ariane 5 rocket from the Guiana Space Centre in French Guiana. A successor to the Hubble Space Telescope, the JWST is NASA’s new flagship mission for astrophysics and cosmology. The telescope carries a 6.5-meter diameter (unfolded) segmented mirror and a multitude of powerful instruments with high resolution at infrared wavelengths, allowing a broad range of observations of distant (thus old) objects, including the most ancient stars and galaxies and the atmospheres of extrasolar planets, with implications for the origin of life. In this presentation, aimed at a general audience, I will summarize the history, present, and future of the JWST, show some pretty pictures, and host a discussion of what it all means.

Fundamental studies of interfacial forces acting on thin films

Timothy Twohig

Ph.D. Candidate,
Department of Physics,
North Dakota State University

Thursday, October 28, 9:00-10:00am, Via Zoom.  Contact Andrew Croll for zoom link.

Please note the special time and date!

The seminar is part of Timothy's PhD thesis defense.

The ancient art of origami uses sheets of paper and precise folding to create complex, three-dimensional shapes out of flat, quasi two-dimensional sheets.  Lately, origami has emerged as a unique way to solve many problems in engineering and science, and as technology and devices are scaled to smaller sizes many attempts have been made to scale origami methods down too.  In pursuit of a more detailed understanding of origami on microscales, studies were conducted on many aspects of thin film assembly involved in the creation of complex and stable structures at a microscopic size.  First inspired by ‘capillary origami methods’, the microscopic details of the interaction of a capillary drop with a thin film were explored in relation to macroscopic observations.  Next, the effect of the adhesion of a film to its substrate was explored.  This interaction enabled the creation and guiding of peel fronts which resulted in precisely controlled placement of folds even in purely elastic materials. Research to explore the stability of ‘elastic not plastic’ folds and their application in origami design was conducted, giving guidelines for which films could be used in specific applications.  Finally, new structures were designed and built from thin films using adhesion rather than plastic folding in origami inspired design.  The new method allowed traditional origami designs to be created, but more importantly, it enabled new structures that cannot be created with traditional origami methods.  We finish by showing how this new ‘adhesion origami’ is already present in many everyday situations.

Some thoughts on entropy and the arrow of time: why we remember the past and not the future

Alexander Wagner

Professor,
Department of Physics,
North Dakota State University

Monday, October 4, 1:00-2:00pm, Via Zoom.  Contact Alexander Wagner for zoom link.

This will be not your typical reseach seminar, but rather a presentation that I hope will prove entertaining and stimulate discussions about a fundamental topic.

This work is inspired both by  Christopher Sorensen's capstone project on entropy on lattice gas and lattice Boltzmann applications this summer, and my lecture on entropy while teaching the Fundamentals of Physics course this semester. Both of these encounters with entropy raised some fundamental questions in my understanding of the second law of theromodynamics.

Hawking's framed the problem beautifully when he raised the question "why do we remember the past and not the future?". This is intimately related to the existence of the second law of thermodynamics which is the only Physical Law that implies a direction of time. In particular I will be discussing the difficulty of reconciling the second law of thermodynamics with the time-reversibility of Newton's equations.

I will argue that you can simulate macroscopically irreversible processes (like diffusion) with the help of Molecular Dynamics simulations that simply solve Newton's equations for a (large) number of particles. However, if the full information of the physical state is retained, there is no increase in entropy in the microscopic representation, even though it simulates the same situation that is represented by an irreversible hydrodynamic description that does show an increase in entropy.

I hope that this apparent contradiction will stimulate a lively discussion with the audience.

I will end with a (potentially outlandish) speculations as to a possible resolution.

Biophysical Characterization of Living Cells and Membrane Receptors by Atomic Force Spectroscopy

Lina Alhalhooly

Ph.D. Candidate,
Department of Physics,
North Dakota State University

Friday, October 1, 1:00-2:00pm, Via Zoom.  Contact Alexander Wagner for zoom link.

Please note the special time and date!

The seminar is part of Lina's PhD thesis defense.

Cellular biomechanics such as membrane deformability play an important role in controlling the cell development and maintaining cellular functions. By exploiting atomic force spectroscopy technique, we have studied dynamic cellular biomechanics in response to the environmental changes and interactions between membrane receptors and their ligands. First, the biomechanical and biophysical properties of various cancer cells were monitored after chemotherapeutic drug exposure in low and high oxygen condition. The cellular elasticity and morphological changes were measured in a time-dependent manner before and after treatment. Our results show direct evidence of the drug-induced changes of the cytoskeletal components, as well as the effect of a low-oxygen environment on enhancing the cancer cell resistance against the chemotherapeutic treatment. Second, single and multiple interactions between membrane receptors (integrins) and peptide ligands (RGD) were examined. The unbinding force of the single receptor-ligand bond and force-based receptor distributions on the cell surface were quantified and compared between several cells. By varying loading rates, the dissociation off rates of the ligand-receptor bond and the changes in Gibbs free energy between a ligand-receptor bound and transition state in the presence and absence of the external force were examined. Furthermore, multivalent RGD-integrin interactions along with the thermodynamic parameters and a free energy landscape alteration during synchronous and asynchronous unbinding processes of multiple RGD-integrins bonds were also examined.

Developing and testing a diagnostic instrument designed to disentangle student reasoning from conceptual understanding
Brianna Santangelo

Ph.D. Candidate
Department of Physics,
North Dakota State University

Monday, April 26, 3:00-4:00 pm,

Online: Contact Alexander Wagner for details.

Many instructors have experienced a situation where students demonstrate content competency on one task but fail to do so on an analogous task that requires the application of the same content understanding and reasoning skills. Observed inconsistencies in student performance could be interpreted in several ways. For example, in some cases, it could be argued that students may simply not possess the formal knowledge and skills necessary to arrive at a correct answer. In other cases, however, students may switch their cognitive mode, seeming to abandon the formal knowledge and skills in favor of (perhaps more appealing, but incorrect) intuitive ideas. In order to address the identified patterns of incorrect and inconsistent responses, it is critical to have a diagnostic instrument that allows for (1) disentangling reasoning from conceptual understanding and (2) pinpointing specific aspects of student thinking (conceptual understanding, reasoning strategies, or both) that should be targeted during instruction. Already existing assessment instrument in physics tend to measure improvements in overall student performance rather than documenting growth along both dimensions (i.e., conceptual understanding and reasoning). In this presentation, I will discuss current efforts directed toward the development of the Dimensions of Conceptual Understanding and Reasoning Instrument and will outline future work that focuses on multiple data streams used for establishing validity and reliability.

Lattice Stochastic Methods for Strongly Interacting Systems
Dr. Tom Luu

Institute for Advanced Simulation,
Institut für Kernphysik,
Jülich Center for Hadron Physics,
Forschungszentrum Jülich GmbH
Germany

Monday, April 12, 10:00-11:00am,

Online: Contact Alexander Wagner for details.

I discuss how lattice stochastic methods provide a synergistic interplay between the fields of particle physics and condensed matter/solid-state physics.  I describe how such synergy has enabled simulations of the largest lattices to date used to determine the quantum phase transition of the Hubbard model on the 2D honeycomb lattice.  I also provide an example of how recent algorithmic advances in tackling the sign problem in simulations of solid-state systems can potentially benefit particle physics.

Fargo, my 2nd hometown
Dr. Ping He

Director of Engineering/R&D,
J.A. Woollam Co., Inc.
Lincon, NE

Monday, March 9, 3:00-4:00pm, 221 South Engineering

Online: Contact Alexander Wagner for details.

Ping came to Fargo in 1985 as a foreign student from Beijing, China.  He studied at the department of physics and astronomy for MS with the guidance of Prof. Sinha from 1985 to 1988.  As the first few students from China, Ping will share the experience studying and living in the city of Fargo that did not exist in most of the US maps back in the 80’s in China.

After graduated from UNL in 1993, Ping was employed at J. A. Woollam Co, a startup specializing an optical technology called ellipsometry.  A brief introduction of this technology and its applications will be discussed.  Ping will share his work experience and the role by a physicist in the industry.

We will also hear brief accounts from three current students with somewhat comparable experiences, Wathsala Amadoru Jayawardana, Lina Alhalhooly, and Jose Agudelo about what it is like to discover the US by coming to NDSU over 40 years later.

Electric Double Layers and the Development of More Efficient Batteries
Dr. Guilherme Volpa Bossa

Department of Physics,
São Paulo State University (UNESP),
Institute of Biosciences, Humanities and Exact Sciences,
São José do Rio Preto, SP 15054-000, Brazil

Monday, February 8, 3:00-4:00pm, 221 South Engineering

Online: Contact Alexander Wagner for details.

Close to an electrode in an electrolyte, an electric double layer (EDL) forms. EDLs have continuously been studied due to their broad range of applications in Biophysics, Chemistry, polymer and material sciences. In recent years, the continuous demand for more efficient energy and environmentally-friendly storage devices has motivated several companies to explore the ability of EDLs to store electrochemical energy. In line with this, supercapacitors can be used as short-term energy storage and power management devices, where their functioning relies on employing electrostatic double-layer capacitance over a large specific surface area. Additionally, previous works have shown that the capacitance of an EDL is influenced not only by the electrode chemical and structural properties, but also by the type of electrolyte used. In this talk we present some of the state of the art approaches that can help us to elucidate how different contributions  affect the capacitance and, hence, develop  batteries characterized by longer life-cycle, safety, and shorter charge/discharge time.

Sticky Crumpled Matter
Dr. Andrew B. Croll

Associate Professor,
Department of Physics,
NDSU

Monday, February 1, 3:00-4:00pm, 221 South Engineering

Online: Contact Alexander Wagner for details.

There has recently been a surge of interest aimed towards understanding the load bearing ability of randomly confined thin films (crumpled matter).  Advances have been made; notably, the discovery that adhesion plays a major role in setting the magnitude of the compressive strength of a crumple.  In this work, we explore the details of this relation in greater depth through confocal microscopy and compression testing of thin, sticky, crumpled polymer films.  Specifically, we measure the work done in compression cycles of polydimethylsiloxane (PDMS) elastomer films of varying modulus and adhesive strength (as characterized by an energy release rate, Gc).  Remarkably, we find the order-of-magnitude increase in stiffness of crumpled matter caused by adhesive interactions is independent of the value of Gc while at the same time the total work of a cycle is highly correlated with Gc.

Tape Loop Adhesion
Dr. Andrew B. Croll

Associate Professor,
Department of Physics,
NDSU

Monday, January 25, 3:00-4:00pm, 221 South Engineering

Online: Contact Alexander Wagner for details.

A tape loop is a very common method used to adhere two parallel objects, while maintaining an easy release through peeling.  Despite the prevalence of the tape loop, some interesting questions arise when the adhesion and removal of the loop are scrutinized.  Consider, for example, that a tape loop placed on a surface will have a cylindrical shape, and remain undeformed indefinitely regardless of the adhesive or surface on which it sits.  If a plate which is parallel to the substrate is used to compress the loop, it deforms significantly becoming almost 2 dimensional when completely flattened.  If the compressive force is removed, the loop will stay in an elongated shape indefinitely.  Interestingly, this suggests that the system is not in equilibrium during the process. In order to more deeply understand the basic physics of the tape-loop system, we conduct compression and retractions cycles of primarily elastic materials (polydimethylsiloxane, PDMS, and polycarbonate, PC) in semi-cylindrical geometry, approximating half a tape loop.  Measuring forces, as well as the geometry of the loop at all stages of the cycle allows us to clarify the basic mechanics.  We make use of the ‘sticky elastica’ to model the cycle, and identify the role of adhesion in the process.  Remarkably, we find that adhesion makes zero contribution to the process during compression.  Guided by our model, we have developed a second experiment in which we show how to measure the energy release rate of any thin material, using nothing more than a ruler.

Crumpled Kirigami
Wathsala Mayurika Amadoru Jayawardana

Candidate for Ph.D.,
Department of Physics,
North Dakota State University

Friday, December 4, 2020, 12:00-1:00pm,

Online: contact Alexander Wagner for details

When a thin sheet is confined to a volume much smaller than its length (or width), it forms a complex state of sharp bends, point-like d-cones and extended ridges known as crumpled matter. What is most interesting about this state, is its high resistance to compression given its light weight. While the origins of this strength still remain a matter of debate, much has been learned through simple experiments and models. One of the prevalent theories suggests that ridges, objects created between two adjacent point defects (d-cones) are dominant, as they store significant energy in sheet stretching (rather low energy bending). In this work, we couple confocal microscopy with simple force experiments and explore how adding cuts to a sheet alters its behavior in the crumpled state. Our results show some consistency with an alternate model in which the dominant energy is the extreme bending in the d-cone cores.

This seminar is part of Wathsala Jayawardana's comprehensive exam for the degree of Doctor of Philosophy at NDSU.

Bright Silicon Nanocrystals from a Liquid Precursor: Quasi-Direct Recombination with High Quantum Yield
Reed Peterson

Candidate for M.Sc.,
Department of Physics,
North Dakota State University

Monday, November 16, 2020, 3:00-4:00pm,

Online: contact Alexander Wagner for details

We have studied bandgap photoluminescent and quantum confinement properties of silicon nanocrystals which are of current
 interest for many applications, to wit, solar windows and biomedical markers. In this seminar, I will present the liquid precursor cyclohexasilane as used in the nonthermal plasma synthesis of silicon nanocrystals with core emission. Through size separation
 by density gradient ultracentrifugation to form fractions, we achieve photoluminescence quantum yields exceeding 65%. Time-resolved photoluminescence spectra and lifetime measurements of these fractions elucidate a zero-phonon radiative channel that anticorrelates
 the quantum yield. I will detail these findings by presenting the experiments of the recent paper [1]

Reference:
[1] High Quantum Yield and Quasi-Direct Recombination Dynamics Originating from Silicon Nanocrystals made with a Liquid Precursor (Pringle
 et al., ACS Nano. 2020).

Osmotic Swelling Behavior of Ionic Microgels
Mohammed Alziyadi

Candidate for Ph.D.,
Department of Physics,
North Dakota State University

Monday, November 9, 2020, 1:00-2:00pm,

Online: contact Alexander Wagner for details

We have studied thermodynamic and structural properties of aqueous dispersions of ionic microgels — soft colloidal particles composed of cross-linked polymer gels that swell in a good solvent. Starting from a coarse-grained model of microgel particles, we perform theoretical calculations and computer simulations using two complementary implementations of Poisson-Boltzmann (PB) theory. Within the framework of a cell model, the nonlinear PB equation is exactly solved and used to compute counterion distributions and osmotic pressures. By varying the free energy with respect to microgel size, we obtain exact statistical mechanical relations for the electrostatic component of the single-particle osmotic pressure. Explicit results are presented for equilibrium swelling ratios of charged microcapsules and of charged cylindrical and spherical microgels with fixed charge uniformly distributed over the surface or volume of the particle. Molecular dynamics simulations validate the theoretical findings. In the second method, within a one-component model framework, based on a linear-response approximation for effective electrostatic interactions, we develop Monte Carlo (MC) simulations to compute the equilibrium swelling ratio, bulk osmotic pressure, radial distribution function, and static structure factor.
 
Results presented demonstrate that swelling of ionic microgels increases with increasing microgel charge and decreases with increasing concentration of salt and microgels. In addition, results demonstrate that the microion distributions and osmotic pressure determine equilibrium swelling of microgels. Cell model predictions for bulk osmotic pressure agree well with data from MC simulations of the one-component model. The MC simulations also provide access to structural properties and to swelling behavior of microgels in highly concentrated suspensions. Taken together, results obtained in this work provide insight into factors of importance for practical use of microgels as drug delivery systems, in tissue engineering, and for other biomedical applications.

This seminar is part of Mohammed Alziyadi's final exam for the degree of Doctor of Philosophy at NDSU.

Electrostatic Interactions Inside a Lipid Membrane
Sylvio May

Chair and Professor,
Department of Physics,
North Dakota State University

Monday, November 2, 2020, 3:00-4:00pm,

Online: contact Alexander Wagner for details

Interactions between charges and dipoles inside a lipid membrane are partially screened. The screening arises both from the polarization of water and from the structure of the electric double layer formed by the salt ions outside the membrane. Assuming that the membrane can be represented as a dielectric slab of low dielectric constant sandwiched by an aqueous solution containing mobile ions, a theoretical model is developed to quantify the strength of electrostatic interactions inside a lipid membrane that is valid in the linear limit of Poisson–Boltzmann theory.

Analysis of multilayer coatings using lattice Boltzmann
Aaron Feickert

Ph.D.,
Alumnus of the Department of Physics
and the
Department of Coatings and Polymeric materials,
North Dakota State University

Monday, May 4, 2020, 3:00-4:00pm,

Online: contact Alexander Wagner for details

We present a one-dimensional lattice Boltzmann simulation which provides an accurate and precise model for Fickian diffusion [1]. Simple analytical solutions for diffusion exist for reservoir-substrate systems, and have been used to model test cycling [2]. We extend this work to simulate multi-layer coatings. Typically multi-layer coatings have a protective part to avoid corrosion and additional layers to impart additional coating properties. We show here  that for some cases there can be two-layer coatings that at the same thickness have a better corosion protection than either of the layers making up the whole thickness. This unexpected result was obtained using our novel modeling framework.

Taken together, this modeling framework provides useful data on the construction of coating application stacks for use in automotive, aerospace, and other critical industry and infrastructure applications to avoid degradation and prolong service life.

References:
[1] Kyle T. Strand, Aaron J. Feickert, and Alexander J. Wagner, Phys. Rev. E 95, 063311 (2017)
[2] Aaron J. Feickert and Alexander J. Wagner, Phys. Rev. Materials 1, 033804 (2017)

A Density Functional Theory and Many Body Perturbation Theory Based Study of Charge Separation in Doped Silicon Nanowires
Nathan Walker

M.Sc. Candidate,
Department of Physics,
North Dakota State University

Monday, April 27, 2020, 3:00-4:00pm,

Online: contact Alexander Wagner for details

We analyze a toy model for p-n junction photovoltaic devices by simulating photoexcited state dynamics in silicon nanowires. One nanowire is  of diameter 1.17 nm.  The other  has an approximately rhombic cross-section with d1 = 1.16 nm and d2 = 1.71 nm.  Both nanowires have been doped with aluminum and phosphorus atoms and capped with gold leads. We use Boltzmann transport equation (BE) that includes phonon emission, carrier multiplication (CM), and exciton transfer. BE rates are computed using non-equilibrium finite-temperature many-body perturbation theory (MBPT) based on Density Functional Theory (DFT) simulations, including excitonic effects from Bethe-Salpeter Equation. We compute total charge transfer amount generated from the initial photoexcitation and find an enhancement when CM is included.

Multivalent effects on interactions between the ligands and cell-surface receptors probed  by a binding force spectroscopy
Lina Alhalhooly

Ph.D. Candidate,
Department of Physics,
North Dakota State University

Monday, April 20, 2020, 3:00-4:00pm,

Online: contact Alexander Wagner for details

Monovalent and multivalent binding and unbinding interactions between peptide-ligands and integrin-receptors on living cell membrane were measured and quantified using an atomic force microscopy (AFM). Initially, we performed single-molecule force spectroscopy measurements with peptide-functionalized AFM probes, targeting specific integrin receptors, and identified and quantified the binding strength as well as receptors’ distribution on the cell membrane. Next, bivalent, trivalent, and multivalent binding interactions were examined and compared by varying the retracting and approaching speed of the AFM probes. Here, we discuss multivalent effects on the binding sensitivity, selectivity, and strength and their dependence on parameters including receptor concentrations of non-cancerous and cancerous cell-lines.

Enhanced Optical Properties of Single-Walled Carbon Nanotubes via SP3-Hybridization Defects from Many-Body Perturbation Theory Based on Density Functional Theory Calculations
Braden Weight

M.Sc. Candidate,
Department of Physics,
North Dakota State University

Monday, March 30, 2020, 3:00-4:00pm,

Online: contact Alexander Wagner for details

The optical consequences of functionalized carbon nanotubes (CNTs) (via a pair of SP3-hybridized functional groups attached to a carbon ring) have been explored in great depth due to their promise of superior electronic properties for tunable emission in infrared energies. These studies have relied on time-dependent density functional theory (TD-DFT) calculations to model the excited states of these particles, but very little work has been completed on multiple exciton generation (MEG) processes within these systems. Here we employ a novel method based in non-equilibrium, finite-temperature, many-body perturbation theory (MBPT) calculations that utilize output from density functional theory (DFT) to accurately model excited states of these systems. We solve the Boltzmann transport equation (BE), including phonon absorption/emission and biexciton formation/recombination terms [1,2]. With this approach we compute an array of CNTs of varying chirality and functionalization scheme. We see that SP3-defect functionalization of pristine CNTs that had high-energy biexciton MEG thresholds (E ≈ 2.4Eg) can be reduced to 2Eg, which drastically increases their value as efficient multiple exciton sources.

Excited State Dynamics in 1D Thermoelectric Materials
Kevin Gima

M.Sc. Candidate,
Department of Physics,
North Dakota State University

Monday, March 23, 2020, 3:00-4:00pm,

Online: contact Alexander Wagner for details

A fundamental postulate of quantum mechanics is if an isolated quantum system is excited optically or thermally from the ground state to an excited eigenstate, it will remain there for an infinitely long time. However, in an open system an excited state eventually returns to the ground state via nonadiabatic coupling. Therefore, nonadiabatic coupling computations are critical in understanding photo-induced charge transfer, photocatalysis, and thermally activated charge transfer. Nonadiabatic calculations are in increasing demand for time-dependent and nonequilibrium phenomena (1)
Here, nonadiabatic computations are used to study the thermoelectric effect and evaluate electron relaxation rates in lead telluride nanowires. Ke = 1/τel is defined as the electron relaxation rate. It is directly connected to the thermoelectric figure of merit in a material. This work provides computational evidence in support of the proceeding hypothesis. The hypothesis is the electron relaxation rates will comply with the following band gap law: Ke = Aexp(-αΔE), where Ke is the electronic relaxation rate, A and α are constants, and ΔE is the energy difference between the initial and final states. This work reports results on PbTe (lead telluride) atomistic models doped with sodium and iodine that contain approximately 300 atoms in simulation cells with periodic boundary conditions. The calculations are performed in the basis of ground state DFT with PBE functional, under the VASP software.(2) The transitions between states are modeled in terms of Redfield equation of motion(3) parameterized by on-the-fly nonadiabatic couplings along thermalized molecular dynamics trajectory.(4) The initial excited states are approximated by promotion of an electron from occupied to unoccupied Kohn-Sham orbital. An index of occupied orbital is e=referred by a label iH representing HOMO-iH+1 for electron hole and by label iE representing LUMO+iE-1 for electron. Four orbital transitions were studied: iH1-iE8, iH5-iE10, iH3-iE20, and iH12-iE15. In -6each scenario, the change of energy and position with respect to time was graphically calculated (5)(6). More importantly, the electron and hole relaxation rates were calculated. Thereafter the decay constants were extrapolated.

References:
(1) Dedi, P.-C. Lee, et al., Stress-induced growth of single-crystalline lead telluride nanowires and their thermoelectric transport properties, Applied Physics Letters. 103 (2013) 023115.
(2) G. Kresse and J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 54, 11169 (1996).
(3) Redfield A. G. , The Theory of Relaxation Processes. Advances in Magnetic and Optical Resonance. 1: 1–32, (1965)
(4) Hammes-Schiffer, S. and Tully, J.C., Proton transfer in solution: Molecular dynamics with quantum transitions, J. Chem. Phys. 101, 4657 (1994).
(5) Kilin, D. S., and Micha, D. A., Relaxation of Photoexcited Electrons at a Nanostructured Si(111) Surface, J. Phys. Chem. Lett. 2010, 1, 7, 1073-1077
(6) Fatima et. al., Photoexcited Electron Lifetimes Influenced by Momentum Dispersion in Silicon Nanowires, J. Phys. Chem. C 2019, 123, 12, 7457-7466.

Beyond Regular Microgels: Exotic Architectures in Crowded Environments
Dr. Andrea Scotti

Institute of Physical Chemistry,
RWTH Aachen University

Monday, March 9, 3:00-4:00pm, 221 South Engineering

Refreshments at 2:30

Colloids have provided important insights into phase transitions in condensed matter, e.g., crystallization and glass formation, since they offer a versatile model for studying fundamental processes in atomic systems on length and time scales accessible with conventional techniques (confocal microscopy or dynamic light scattering). A particular class of colloids is represented by solutions of soft, deformable microgels, polymeric crosslinked networks swollen in a good solvent that respond to external stimuli, e.g., changes in temperature or pH. Typical microgels have dimensions between tens and hundreds of nanometers and present a core with high polymer density surrounded by a less dense, softer fuzzy shell. In the last decades, new synthetic protocols have been developed enriching the possible internal structures of microgels. Microgels with very poorly crosslinked polymeric networks are called ultra-low crosslinked and, depending on the dimensionality, behave either as particle or as flexible polymer. Another important class of new microgels are the so-called hollow microgels that possess a solvent-filled cavity in their center. Recently, we have also successfully synthesized hollow elliptical microgels. In my talk, I will present some of the most recent findings on the phase behavior and flow properties of these new families of microgels investigated by scattering techniques, rheology, atomic force and transmission electron microscopy, and Langmuir-Blodgett trough techniques.

Deriving LBM Collision Operator using the Coarse-Graining MDLG Approach
Aleksandra Pachalieva

Technical University of Munich;
and
Theoretical Division and CNLS,
Los Alamos National Laboratory

Monday, January 27, 3:00-4:00pm, 221 South Engineering

Refreshments at 2:30

We introduce the Molecular-Dynamics-Lattice-Gas (MDLG) [1] method that establishes a direct link between a lattice gas method and the coarse-graining of a Molecular Dynamics (MD) approach. Due to its connection to MD, the MDLG rigorously recovers the hydrodynamics and allows to validate the behavior of the Lattice Gas (LG) or Lattice Boltzmann Methods (LBM) directly without using
the standard kinetic theory approach. The MDLG analysis is a first principles approach that verify and examine the properties of the LG and LBM methods. Aspects that can be examined include fluctuating, thermal, multi-phase and
multi component systems.

We have developed a correction of the analytical solution of the global equilibrium distribution function [1] in order to derive a universal LBM collision operator from an underlying MD simulation. Such collision operator can give significant insight how to construct more stable and robust LBM models. Until now over-relaxation has been seen as a useful numerical trick without a fundamental physics basis. However, preliminary results from the MDLG analysis show that we can measure actual over-relaxation from an underlying MD coarsening using the MDLG approach, which means that over-relaxation is indeed a physical effect.

References
[1] Parsa, M. Reza and Wagner, Alexander J., Phys. Rev. E 96, 013314 (2017).

The progress toward developing an instrument to measure student reasoning
Brianna Santangelo

Ph. D. Candidate
Dept. of Physics
North Dakota State University

Monday, Dec. 9, 3:00-4:00pm, 221 South Engineering

Refreshments at 2:30

One of the goals of physics instruction is to help students develop reasoning skills in the context of physics. However, it is challenging to design instruments capable of measuring student reasoning in order to make claims about improvements.  The challenges stem from two aspects. First, it is difficult to disentangle conceptual understanding from reasoning.  Second, to reason productively, a certain level of conceptual understanding is required.  As such, a traditional pre- and post-test methodology is not appropriate for documenting changes in reasoning.  To address the challenges, we have been developing sequences of screening-target questions: screening questions probe conceptual understanding, while target questions require students to apply this understanding in situations that present reasoning challenges.  The level of consistency in student performance on screening and target questions is used to make inferences about reasoning skills.

Osmotic Swelling Behavior of Ionic Microgels
Mohammed Alziyadi

Ph.D. Candidate
Dept. of Physics
North Dakota State University

Monday, Dec. 2, 3:00-4:00pm, 221 South Engineering

Refreshments at 2:30

During the past few years, ionic microgels of various geometries, e.g., capsules and cylinders, have attracted much attention due to their unique properties, particularly, their stimulus-sensitive swelling behavior. The use of ionic microgels in biotechnologies and medicine is increasingly attractive for applications, such as controlled drug delivery, water filtration, biosensing, and tissue engineering. The ability to model and predict the swelling behavior of microgel particles and variation of osmotic pressure with respect to changes in external environmental conditions is very important in such applications. The focus of this research is on developing a model for the influence of microions (counterion/coions) on the osmotic pressure and swelling of ionic microgels. To model swelling of microgels, we developed a procedure based on a thermodynamic framework for swollen gel subject to constraints. Starting from the primitive model, where the solvent is a dielectric continuum of uniform dielectric constant, we derived some exact statistical mechanical relations for the electrostatic contribution to the osmotic pressure of an ionic microgel in the cell model [1]. Next, we tested our exact results numerically by solving the Poisson-Boltzmann equation and computing ion densities and electrostatic osmotic pressure. Furthermore, we performed molecular dynamics simulations to validate the numerical predictions of our theory. Combining our theory of the electrostatic osmotic pressure with Flory-Rehner theory of polymer networks, we predict equilibrium swelling ratios of ionic microgels as a function of concentration. Our results can help guide the design of smart, responsive particles.[1] A. R. Denton and M. O. Alziyadi, J. Chem. Phys. 151, 074903 (2019).

50 Years After Becoming A Bison, I Work On Dragons
or
Quantum Dragon Nanodevices:
Zero Electrical Resistance in Disordered Systems
Mark A. Novotny

Professor and Head
Dept. of Physics and Astronomy
William L. Giles Distinguished Professor
Mississippi State University

Monday, Oct 21, 3:00-4:00pm, 221 South Engineering

Refreshments at 2:30

I started as a Physics major at NDSU in the fall of 1969.  Fifty years later, I perform research on quantum computing and quantum dragon nanodevices.  Some ‘wisdom’ from my time at NDSU and after will pepper my ‘proper’ presentation on quantum dragons.

Quantum effects in nanodevices can lead to unexpected physical properties.  Quantum dragon nanodevices are introduced.  They can have very strange shapes, be very disordered, be very tatty, and still have complete electron transmission when attached appropriately to uniform leads.  Complete electron transmission induces zero electrical resistance in four probe measurements.  Furthermore, even though the quantum dragon devices have arbitrarily strong locally-correlated disorder, they exhibit ‘order amidst disorder’.  A number of quantum dragon devices will be described, as well as instances where small deviations cause nanodevices to be almost quantum dragons.

HJ-Aggregate Theory Applied to Interacting SP3­­-hybridization
 Defects in Carbon Nanotubes
Braden M. Weight

Graduate Student,
Department of Physics,
Department of Chemistry and Biochemistry,
North Dakota State University

Monday, Oct 7, 3:00-4:00pm, 221 South Engineering

Single-walled carbon nanotubes (SWCNTs) have been recently studied in greater depth due to their promise of superior electronic properties for tunable emission in the infrared. Optical features of functionalized (via SP3-hybridization defects) CNTs have been narrowed to only a few main parameters: (I) chirality, (II) defect configuration, and (III) defect electronegativity. Previous theoretical studies have been directed at single-defect pairs attached to the CNT surface, and, until recently, no literature has discussed the effects of defect concentration on CNTs in any depth [1,2]. In this work, we aim to model the interactions between nearby defects using density functional theory (DFT) and, extending to excited states, with time-dependent DFT (TD-DFT) in order to fit these interactions to a well-known descriptor of analogous systems known as HJ-aggregate theory.[1] Nat. Comm. 2019, 10, 1, 2041-1723[2] ACS Nano 2019, 13, 7, 8222-8228I

The effect of domain size polydispersity on the structural and dynamical properties of lipid monolayers
Elena Rufeil Fiori

Assistant Professor,
Facultad de Matmàtica, Astronomìa y Fìsica,
Ciudad Universitaria, Argentina

Monday, Sept 16, 3:00-4:00pm, 221 South Engineering

In lipid monolayers with phase coexistence, domains of the liquid-condensed phase always present size polydispersity. Because of the difference in surface densities, domains have excess dipolar density with respect to the surrounding liquid expanded phase, originating a dipolar inter-domain interaction. This interaction depends on the domain area, and hence the presence of a domain size distribution is associated with interaction polydispersity. By means of Brownian dynamics simulations, we study the radial distribution function (RDF) and the average time-dependent self-diffusion coefficient of lipid monolayers with normally distributed size domains. For this purpose, we vary the relevant system parameters, polydispersity and interaction strength, within a range of experimental interest. We also analyze the consequences of using a monodisperse model to determine the interaction strength from an experimental RDF.

Numerical simulation of various problems with the Lattice Boltzmann Method
Luben Cabezas Gómez

Associate Professor,
Department of Mechanical Engineering
São Carlos School of Engineering
University of São Paulo

Monday, May 20, 3:00-4:00pm, 221 South Engineering

Refreshments at 2:30

Prof. Gomez will present numerical simulation results obtained with the Lattice Boltzmann Method for various problems. Starting with single-phase thermal flows in 2D he will show how to extend the model to obtain a simple simulation of nucleate boiling problems. From these results he will propose extensions of this approach to by using the pseudopential LB method.

Deswelling Effect on Structural and Dynamic Properties of Ionic Microgel Suspensions
Mariano Brito

Graduate Student,
Institute of Complex Systems,
Forschungszentrum Jülich, Germany

Monday, April 29, 3:00-4:00pm, 221 South Engineering

Refreshments at 2:30

Microgels are solvent-containing, cross-linked polymer networks of colloidal size that can reversibly swell or deswell in response to external stimuli. Ionic microgels, in particular, are highly sensitive to changes in environmental conditions such as temperature, solvent quality, polymer cross-linking, suspension ionic strength and particle concentration, which allows for controlling their size and effective interaction. In this work, we theoretically study the effects of concentration-dependent deswelling of weakly-crosslinked ionic microgels on dynamic and structural suspension properties [1]. We use and compare two different theoretical approaches to calculate the equilibrium microgel size, namely the Denton-Tang method based on Poisson-Boltzmann cell model [2], and a multiparticle-based thermodynamic perturbation method [3]. In combination with an effective interaction potential for spherical ionic microgels [4], we compute static pair correlation functions and structure factors. These are used as input in our calculations of dynamic suspension properties including the hydrodynamic function, collective diffusion coefficient, and low- and high-frequency viscosities. As a consequence of the concentration-dependent deswelling, we show how the collective diffusion is enhanced while the viscosity is lowered in suspensions of ionic microgels.

[1] M. Brito, A. R. Denton and G. Nägele, to be submitted.
[2] A. R. Denton and Qiyun Tang, J. Chem. Phys. 145, 164901 (2016).
[3] T. J. Weyer and A. R. Denton, Soft Matter 14, 4530 (2018).
[4] A. R. Denton, Phys. Rev. E 67, 011804 (2003).

Computational Modeling of Photo-Physics and Excited-State Dynamics in Inorganic Lead Halide Perovskite Nanocrystals: Using the Polaron as an Efficient Infrared Emission
Aaron Forde

Department of Materials Science and Nanotechnology,
& Department of Chemistry and Biochemistry
North Dakota State University

Monday, April 8, 3:00-4:00pm, 221 South Engineering

Fully-inorganic CsPbX3 (X=I,Br,Cl) lead-halide perovskite (LHP) nanocrystals (NCs) have become promising materials for opto-electronic applications such as light-emitting diodes (LEDs) and photovoltaics (PVs). In part, this is due to broadly tunable visible photoluminescence (PL) across the visible spectrum and high PL quantum yields, originating from synergy of intrinsic properties of LHP materials and surface passivation by organic ligands.1 The NCs demonstrate signatures of the elusive ‘phonon-bottleneck’ which is predicted to occur in confined nanomaterials2 and ‘spin-forbidden’ PL from Mn2+ doped CsPb1-xMnxBr3 NC atomistic model.3 In addition, these polar semiconducting LHP NCCs materials are also novel in part due to their unique excited-state properties, such as polaron formation. Interestingly, polaronic optical transitions occur in the infrared (IR) spectrum and could be utilized as an IR emission source. In the spirit of the hydrogenic Hamiltonian model used to describe impurity states in conventional semiconductors,4 the polarons electronic structure can be approximated using a mean-field Coulomb interaction from the nuclei interacting with a localized photo-excited charge carrier. To model the polaron electronic structure we use two-component spinor Kohn-Sham orbitals (SKSOs) as a basis, which include relativistic corrections and the spin-orbit coupling (SOC) interaction. Optical transitions between polaronic states are found from transition dipoles in the independent orbital approximation. Efficiency of IR emission is determined from rates of non-radiative recombination (k_NR) and radiative recombination (k_R) as k_R/(k_R+k_NR). k_R is found from ensemble averaged oscillator strengths along a molecular dynamics trajectory which are used to compute the Einstein coefficient of spontaneous emission. k_NR is found from propagating the excited-state density matrix for electronic degrees of freedom, in terms of Redfield theory, which is parameterized from non-adiabatic couplings between electronic and nuclear degrees of freedom. Implications of this work could provide a framework for utilizing APbX3 materials as IR emitters/receivers for telecommunications and wireless devices or as qubits for quantum computation1. Forde, A.; Inerbaev, T.; Kilin, D., Role of Cation-Anion Organic Ligands for Optical Properties of Fully Inorganic Perovskite Quantum Dots. MRS Advances 2018, 3, 3255-3261.2. Forde, A.; Inerbaev, T.; Hobbie, E. K.; Kilin, D. S., Excited-State Dynamics of a Cspbbr3 Nanocrystal Terminated with Binary Ligands: Sparse Density of States with Giant Spin–Orbit Coupling Suppresses Carrier Cooling. J. Am. Chem. Soc. 2019, 141, 4388-4397.3. Forde, A.; Inerbaev, T.; Kilin, D., Spinor Dynamics in Pristine and Mn2+-Doped Cspbbr3 Nc: Role of Spin–Orbit Coupling in Ground- and Excited-State Dynamics. The Journal of Physical Chemistry C 2018, 122, 26196-26213.4. Kohn, W., Shallow Impurity States in Silicon and Germanium. Solid State Physics-Advances in Research and Applications 1957, 5, 257-320.

The Role of Ion-Ion Correlations for the Differential Capacitance of Ionic Liquids
Rachel Downing

Graduate Student,
Department of Physics,
NDSU

Monday, March 18, 3:00-4:00pm, 221 South Engineering

Refreshments at 2:30

We employ the quasi-chemical approximation to incorporate ion-ion correlations into a lattice gas model for the electric double layer formed by a compact, size-symmetric ionic liquid. The resulting differential capacitance transitions from a bell-shape to a camel-shape profile for increasing correlation strength. No such transition is present if the quasi-chemical approximation is replaced by a random mixing approximation. If Coulomb interactions dominate on molecular length scales, the differential capacitance has a tendency to adopt a camel-shape profile. Hence, our model offers a physical interpretation for the observed camel shape of the differential capacitance in many ionic liquids.

The relationship between cognitive reflection and performance in physics
Cody Gette

Graduate Student,
Department of Physics,
NDSU

Monday, February 25, 3:00-4:00pm, 221 South Engineering

Refreshments at 2:30

Dual-process theories of cognition suggest that many inconsistencies in student reasoning in introductory physics may stem from a fast, automatic, and intuitive process interfering with slow and analytical thinking. The Cognitive Reflection Test (CRT) has been developed in psychology to gauge the tendency of a reasoner to engage analytical thinking to evaluate (and possibly override) these initial intuitive ideas. In our ongoing, multi-institutional project, we have been exploring the applicability of the CRT to probe the relationship between cognitive reflection and performance in physics (as measured, for example, by the Force and Motion Conceptual Evaluation, or FMCE). Results from our investigation as well as implications for instruction will be discussed.

Transition to turbulence in blood flows: Insights from numerical simulation
Trung Bao Le

Assistant Professor,
Civil and Environmental Engineering,
NDSU.

Monday, February 11, 3:00-4:00pm, 221 South Engineering

Refreshments at 2:30

As human heart beats, blood is driven toward the aorta  with a velocity in an order of 1 m/s within 100ms. Such a high velocity region resembles a pulsed jet through a nonaxisymmetric orifice with a dynamically changing area. As a result, three-dimensional vortex rings with intricate topology emerge that interact with the complex cardiac anatomy and give rise to shear layers, regions of recirculation, and flow instabilities that could ultimately lead to transition to turbulence. In this talk, a summary of recent advances in in-vivo/in-vitro and computational techniques to study the transient nature of cardiovascular flows. A short discussion will be introduced on the vortex formation principles and their relations to instabilities, vortex breakdown and transition to turbulence in hemodynamics. Our experimental and computational works point to the conclusion that high-resolution both spatially and temporally is required to capture these types of dynamics. Finally, we propose new parameters/bio-markers and methodologies to quantify hemodynamics in pathological conditions using in-vivo data.

Uncovering the Mysteries of Disease Deep Inside the Living Brain
Melanie Martin

Director, MRM Centre
Co-Director, The University of Winnipeg Brain Imaging and Metabolic Research Lab
Professor, Physics, University of Winnipeg

<Special Time!>Friday, February 8, 10:00-10:50pm, 221 South Engineering

Magnetic Resonance Imaging (MRI) provides a non-invasive means of looking deep inside a living being. We will discuss how MRI works and why it is used in hospitals. My research focuses on developing high-resolution (100 um)^3 MRI for mouse models of human diseases to understand the changes associated with disease. I will present recent advances in inferring micron-sized cell diameters and how physics fits into the research.

How can molecular simulation help us understand the onset of order during the nucleation process?
Caroline Desgranges

Independent Researcher,
Grand Forks, ND

Monday, February 4, 3:00-4:00pm, 221 South Engineering

Refreshments at 2:30

Crystallization has remained an incredibly complex and mysterious process. This is, for instance, the case on the nanoscale where our ability to control nanoparticle properties is limited by our lack of understanding of how nanoparticles nucleate and grow. Similarly, on the macroscopic scale, it is crucial to control the outcome of the crystallization process, since different crystal structures (polymorphs) often have different physical properties. In this talk, I will discuss how molecular simulation can shed light on the molecular mechanisms underlying crystal nucleation. Since the formation of a nucleus of a critical size is a rare event, advanced (non-Boltzmann) sampling techniques need to be applied to study this process. Here, the nucleation step is simulated using a combination of molecular dynamics simulations with the umbrella sampling technique. This sampling method, used together with a reaction coordinate (or order parameter) characteristic of crystalline order, is particularly well suited to study nucleation, since it allows the system to overcome the free energy barrier of nucleation and provides a direct access to the nucleation mechanism. More specifically, I will present recent results obtained on model systems, metals, alloys as well as in semiconductors.

A Physicist's Journey to a Career in Software Engineering.
Wyatt Davis

AUTOSAR Reuse Software Engineer
DISTek Integration, Inc.

<Special Time!>Monday, January 28, 4:00-5:00pm, 221 South Engineering

Refreshments at 3:30

Ninety five percent of U.S. physics Bachelor’s, and 90% of physics Master’s, recipients who enter the private sector do not have physicist/astronomer in their job title. Most physics degree recipients, like myself, enter into engineering, IT, other STEM, or non-STEM jobs. In May of 2018 I received my Master’s in physics from NDSU. Within a couple of weeks of graduating I had accepted an offer from DISTek Integration Inc. to work on embedded enterprise reuse software for John Deere. Stick around to the end of this presentation and you’ll learn a bit about my computational physics research background, my current engineering role, and some (hopefully) useful advice for those looking to pursue private sector careers with a physics degree in hand.

Crumpled Matter
Andrew Croll

Associate Professor
Department of Physics,
North Dakota State University

Monday, December 3, 3:00-4:00pm, 221 South Engineering

A sheet that is confined to a space much smaller than its longest dimensions, will collapse into a random structure know as a crumple.  Crumpled objects are interesting materials with properties similar to those of foams, however are not commonly used because there are no trustworthy structure-property relations for crumples.  Simply put, crumples cannot be trusted.  In this work we describe a simple empirical relation which accurately describes the amount of force need to crush a crumpled object, and prove its experimental validity over fifty orders of magnitude.  We explore several recent models of crumpling which are rationally based on the dominance of different ‘building block’ substructures, but conclude that none accurately describe the process.  To explore the reason for this disagreement between theory and experiment, we more closely examine several substructures.  Ultimately, we show that the stretching ridge is consistent with the crumples behaviour, however is only indirectly responsible for the resistance of the crumple to compression.  We go on to show bending is in fact the dominant underlying structure of both crumples and ridges.  Our work will ultimately enable the use of this new state of crumpled matter.

Understanding Glass Transition Through Interfacial Properties
Zahra Fakhraai

Associate Professor
Department of Chemistry,
University of Pennsylvania

Monday, November 26, 3:00-4:00pm, 221 South Engineering

Refreshments at 3:30

Free surfaces and interfaces can affect properties of glassy systems over length scales that can be much longer than the intermolecular interaction potential. A fundamental understanding of the magnitude and length scale of these effects can allow us to understand the glass transition phenomenon and engineer nano-scaled materials with unique properties. In this presentation I show two examples of such effects and their use in producing thermally and kinetically stable glass materials.

Extensive research in the past two decades has shown that the free surface of glasses, in particular for polymeric and organic glasses, have dramatically faster dynamics, resulting in strong reduction in their glass transition temperature, Tg, in ultra-thin films. We have recently shown that the surface dynamics can be faster by as much as eight orders of magnitude resulting in an apparent glass to liquid transition in molecular glass films as thick as 30 nm. The details of the thickness-dependent relaxation dynamics in thin films can elucidate properties of bulk glass that are key in verifying glass transition models. The enhanced mobility over such a large length scale can also help produce stable glasses with unique molecular packing upon physical vapor deposition.

In another example, we demonstrated that polymers’ thermal stability can be significantly improved in highly confined geometries. Capillary rise infiltration (CaRI) is used to load randomly closed-packed films of nanoparticles with various polymers. By changing the NP diameter, polymer can be confined in length scales as small as 2-3 nm. Under these conditions, entropic and enthalpic effects induced by interfaces play the dominant role in stabilizing the polymer, leading to higher Tg, higher viscosity, and improved resistance to burning and thermal degradation. We discuss the role of various parameters in achieving these thermally stable states.

MesoscopicElectronic structure of semiconductor nanoparticles from stochastic evaluation of imaginary-time path integral: nonrelativistic U(1) lattice gauge theory in the Kohn-Sham basis
Andrei Kryjevski

Assistant Professor
Department of Physics,
North Dakota State University

Monday, November 19, 3:00-4:00pm, 221 South Engineering

In the Kohn-Sham orbital basis imaginary-time path integral for electrons in a semiconductor nanoparticle has a sufficiently mild fermion sign problem
to be amenable to evaluation by the standard stochastic methods. Utilizing output from the density functional theory simulations with
Perdew, Burke and Ernzerhof exchange-correlation functional we compute imaginary-time electron propagators in several silicon hydrogen-passivated
nanocrystals, such as Si35H36 and Si87H76, and extract energies of low-lying electron and hole levels. Our quasiparticle gap predictions are in good agreement with the results of recent G0W0 calculations by the Galli's group.

Mesoscopic simulation of transport phenomena in fibrous porous media
Ulf Schiller

Assistant Professor
Materials Science and Engineering
Clemson University

Monday, November 5, 3:00-4:00pm, 221 South Engineering

Refreshments at 3:30

Flow phenomena in porous media are relevant in many industrial applications including fabric filters, gas diffusion membranes, and biomedical implants. For instance, nonwoven membranes can be used as filtration media with tailored permeability range and controllable pore size distribution. However, predicting the structure-property relations that arise from specific porous microstructures remains a challenging task. Theoretical approaches have been limited to simple geometries and can often only predict the general trend of experimental data. Computer simulations are a cost-effective way of validating semi-empirical relations and predicting the precise relations between macroscopic transport properties and microscopic pore structure. To this end, mesoscopic simulation techniques have proven particularly successful in solving numerically the coupled partial differential equations for the complex boundary conditions in porous media. In this talk, I will review some lattice-based modeling approaches for mesoscopic modeling of flow in porous media. The techniques employed include the lattice Boltzmann method, link-flux method for electrokinetic transport, and moment propagation methods for advection-diffusion. I will then discuss how these techniques can be combined to simulate porous materials with realistic morphology based on experimental data. Results for permeability measurements will be presented to illustrate how the simulation data can provide feedback to experiment in order to optimize the materials properties of nonwoven fibrous membranes.

Transport properties of graphene and its bilayer in the presence of dilute alkali metal adatoms
Sung Oh Woo

Post Doctoral Researcher,
Department of Physics
North Dakota State University

Monday, October 29, 3:00-4:00pm,221 South Engineering

We present the electrical transport properties of graphene and its bilayer as a result of the decoration of dilute alkali metal adatoms at cryogenic temperatures. Upon Li and K deposition at T  20 K in ultra high vacuum (< 51010 Torr), electrons are donated to graphene, and the charge carrier mobility is decreased. With increasing temperature, the donated electrons to graphene are decreased, and charge carrier mobility is increased, signatures of the decrease of the Li or K adatoms. We show that the temperature-dependent transport properties of graphene are dominated by the reduction of adatoms, charged impurity scattering, which is induced by cluster formation of adatoms. To support our argument, we estimate the hopping time of the Li and K adatoms on graphene by Density Functional Theory, revealing that these adatoms are mobile even at cryogenic temperatures and become more mobile with increasing temperature, allowing for cluster formation. Utilizing the cluster formation of K adatoms, we explore the transport properties of bilayer graphene. In particular, conductivity changes from linear (σ  nα, α  1) to superlinear (α  1.5) with increasing temperature. To understand the temperature dependent conductivity of bilayer graphene, we fit the experimental data with Boltzmann transport theory, revealing that electrical transport of bilayer graphene in the presence of K adatoms originates from the Thomas-Fermi screening, which are induced by the increase of Coulomb potential inhomogeneity due to the cluster formation of K adatoms.

Multi-scale Modeling of Nanoconfinement and Interfaces in Polymer Materials
Wenjie Xia

Assistant Professor,
Department of Civil and Environmental Engineering
North Dakota State University

Monday, October 1, 3:00-4:00pm,221 South Engineering

Natural and engineered structural (load-bearing) composites often achieve remarkable mechanical performance by confining their microphases in smaller dimensions. However, understanding and predicting their thermomechanical properties are very challenging due to their complex hierarchical microstructures and interfaces within the systems. In this talk, I will present a multi-scale computational paradigm for understanding these complex phenomena occurring in nanocomposite materials. I will first present scale-bridging computational techniques, namely the coarse-grained modeling approach, for simulating polymer and 2D materials at extended time and length scales. Following this, I will discuss several cases where the coupling between nanoconfinement and interface leads to intriguing phenomena as observed in polymer nanostructured materials. I will describe how nanoconfinement could be utilized to achieve high strength and toughness in layered nanocomposite through bioinspiration. Drawing an analogy between thin films and nanocomposites, I will illustrate how understanding thin film can help us design better load-bearing composites using renewable materials, such as nanocellulose.

Resources from Students on the use of Eigentheory at the beginning of Quantum Mechanics
Warren Christensen

Associate Professor of Physics,
North Dakota State University

Monday, October 1, 3:00-4:00pm,221 South Engineering

In collaboration with Megan Wawro, a mathematician at Virginia Tech, an investigation into student thinking about quantum mechanics has uncovered a number of resources that students activate when discussing their ideas about eigentheory at the beginning of a Quantum Mechanics course. Resources are identified as fine-grained ideas from students' prior experience that are called upon when them seem useful based on how the student is framing a particular problem. We aim to better understand how students use mathematics with a goal of developing or aiding in the development of instructional materials. Using semi-structured interviews across two universities with students during the first week of quantum mechanics, we have elicited a variety of ideas and identified common resources used among students. Methodology and initial findings will be discussed.

mK to km: How Millikelvin Physics is Reused to Explore the Earth Kilometers Below the Surface
Dr. Robert L. Kleinberg

American Physical Society Distinguished Lecturer
on the Applications of Physics,
Senior Fellow,                                          
Institute for Sustainable Energy         
Boston University

Refreshments at 2:30

Monday September 17, 3:00-4:00pm,
221 South Engineering

investigations of the superfluid phases of liquid helium-3 would seem to have little application to the study of rock formations thousands of meters below the surface of the earth.  However, the physicist’s tool box is versatile, and techniques used in one field of study can be reused, with appropriate adaptation, in very different circumstances.

The temperature of liquid helium-3 in the millikelvin range can be measured using an unbalanced-secondary mutual inductance coil set designed to monitor the magnetic susceptibility of a paramagnetic salt.  The loss signal is discarded by phase sensitive detection.  Now consider the task of measuring the electrical conductivity, at centimeter scale, of the earth surrounding a borehole.  Turn the mutual inductance coil set inside out, with secondary coils arranged to be unbalanced with respect to the rock wall.  Instead of discarding the loss signal, use it to measure conductivity.  A sensor based on this principle has been implemented in a widely deployed borehole geophysical instrument, used to estimate the prevailing direction of the wind millions of years ago, or to decide where to drill the next well in an oilfield.

Nuclear magnetic resonance may seem a very improbable measurement of the rock surrounding a borehole.  Conventionally, we place the sample (which might be a human being) inside the NMR apparatus. In borehole deployment, the instrument is placed inside sample, the temperature is as high as 175C, pressure ranges to 140 MPa, and measurements must be made while moving at 10 cm/s.  Apparatus with these specifications have been deployed worldwide, and are used to measure a number of rock properties, including the distribution of the sizes of pores in sedimentary rock, and the viscosity of oil found therein.  They have also been used for geological and oceanographic studies in northern Alaska, and at the seafloor offshore Monterey, California.   

Helical Interactions of DNA
Dr. Aaron Wynveen

Teaching Associate Professor,
School of Physics and Astronomy
University of Minnesota

Refreshments at 2:30

Monday September 10, 3:00-4:00pm,
221 South Engineering

The interactions between DNA molecules are involved in a number of important physiological processes, such as packing of genetic material within cells and viral phage heads, the formation of mesoscopic DNA structures in vivo and in vitro, and the recognition of homologous genes required for DNA repair and homologous recombination.  Yet many of these processes, to this day, remain poorly understood, and theories that treat DNA as simple, unstructured polyelectrolytes are unable to fully explain many physical phenomena.  We have recently developed a theory that considers how the helical structure of DNA may govern its interactions. In this talk, I will review this theory, first describing classical experiments probing the double-helical structure of DNA -- and what was overlooked in these experiments -- and then explore the many physical manifestations of this theory, paying particular attention to recent theoretical and experimental work investigating molecular recognition of homologous (same sequence) DNA.

Numerical Simulations of Coupled Aero-Thermodynamic Systems Based on the Lattice-Boltzmann Method
Aleksandra Pachalieva

Lehrstuhl für Aerodynamik und Strömungsmechanik
Technische Universtiät München

Refreshments at 2:30

Monday August 27, 3:00-4:00pm,
221 South Engineering

We develop a framework based on the Lattice-Boltzmann Method (LBM) for simulating coupled aero-thermodynamic problems using High-Performance Computing systems (HPC) on Graphical Processing Units (GPUs). The framework can be applied in the automotive, IT and civil engineering industry for simulating packaging concepts for electric vehicles, cooling of microelements, or air conditioning and ventilation in buildings. The development of complex aero-thermodynamic systems requires reliable, high-fidelity simulations of high Reynolds number flows. We implemented a numerical solver based on the double-distribution LBM, that main advantage is its computational efficiency and intrinsic parallelism. The proposed model is used to simulate a heated generic object in a closed cavitation. The framework is extended to allow for a Direct Numerical Simulation (DNS) of a turbulent channel flow with passive-scalar heat transfer. The obtained results for both cases are in good agreement with prior data.

Curvature Elasticity of the Electric Double Layer
Sylvio May

Associate Professor and Chair,
Department of Physics and Astronomy,
North Dakota State University

Monday April 16, 3:00-4:00pm,
221 South Engineering

Mean-field electrostatics is used to calculate the bending moduli of an electric double layer for fixed surface charge density of a macroion in a symmetric 1:1 electrolyte. The resulting expressions for bending stiffness, Gaussian modulus, and spontaneous curvature refer to a general underlying equation of state of the electrolyte, subject to a local density approximation and the absence of dipole and higher-order fields. We present results for selected applications: the lattice-gas Poisson-Fermi model with and without asymmetric ion sizes, and the Poisson-Carnahan-Starling model.

<CANCELLED DUE TO SNOW>Helical Interactions of DNA
Aaron Wynveen

Post Doctoral Fellow,
Department of Physics and Astronomy,
University of Minnesota.

Refreshments at 2:30

Monday April 16, 3:00-4:00pm,
221 South Engineering

The interactions between DNA molecules are involved in a number of important physiological processes, such as packing of genetic material within cells and viral phage heads, the formation of mesoscopic DNA structures in vivo and in vitro, and the recognition of homologous genes required for DNA repair and homologous recombination.  Yet many of these processes, to this day, remain poorly understood, and theories that treat DNA as simple, unstructured polyelectrolytes are unable to fully explain many physical phenomena.  We have recently developed a theory that considers how the helical structure of DNA may govern its interactions.  In this talk,
I will review this theory, first describing classical experiments probing the double-helical structure of DNA -- and what was overlooked in these experiments -- and then explore the many physical manifestations of this theory, paying particular attention to recent theoretical and experimental work investigating molecular recognition of homologous (same sequence) DNA.

Charge Properties of TiO2 Nanotubes
Klemen Bohinc

Assistant Professor,
Faculty of Health Sciences and Faculty of Electrical Engineering,
University of Ljubljana.

Refreshments at 2:30

Monday April 30, 3:00-4:00pm,
221 South Engineering

Charging of material surfaces in aqueous electrolyte solutions is important to better understand interactions between biomaterials and surrounding tissue. In our work we studied the surface charge properties of titania nanotubes in NaNO3 solution using polyelectrolyte colloid titration measuring technique. High-resolution transmission electron microscopy imaging was used to determine the morphology of nanotubes. A theoretical model based on the classical density functional theory coupled with the charge regulation method in terms of mass action law was developed in order to understand the experiments.

Guiding Self-Assembly of Functionalized Nanoparticles by Computational Modeling of Effective Interactions
Vijay Shah

Candidate for M.Sc.,
Department of Physics,
NDSU

Monday April 9, 3:00 pm,

South Engineering 221

Nanoparticles, which have sizes between those of atoms and macroscopic objects, have attracted much attention because of their unusual physical properties, which are size-dependent at the nanometer scale. These properties allow for nanoparticles to be used in practical applications such as drug delivery and in photovoltaic cells, the latter of which exploits self-assembly into crystalline arrays (superlattices). The self-assembly of nanocrystals into superlattices can be facilitated by functionalizing the nanocrystals with ligand brushes, allowing bulk dispersions to be sterically stabilized against aggregation induced by van der Waals interactions. Others have used computational methods to study the self-assembly of gold nanoparticles coated with grafted dodecanethiol ligands at low concentrations, where cluster morphologies were observed. To more broadly characterize the self-assembly of gold nanoparticle dispersions, we performed Monte Carlo simulations and quantified the dependence of superlattice stability on nanocrystal concentration and ligand coverage. Experiments have shown that silver nanoparticles coated with adsorbed oleylamine ligands can self-assemble into equilibrium superlattices in the presence of free ligands. To better understand the role of adsorbed and free ligands in self-assembly, we extracted the effective force between two flat, ligated plates through molecular dynamics simulations. Our results are compared to the theoretical prediction and discrepancies are discussed.

Free Energy Minimization and Multicomponent, Multi-phase Lattice Boltzmann Simulations of van der Waals Fluid Mixtures
Kent Ridl

Candidate for M.Sc.
Department of Physics,
NDSU

Wednesday, April 5, 2:00 pm,

South Engineering 221

We develop a general framework for the lattice Boltzmann method to simulate multiphase systems with an arbitrary number of components. Theoretical expectations are easily visualized for binary mixtures, so we focus on characterizing the performance of the method by numerically minimizing the free energy of a binary van der Waals mixture to generate phase diagrams. Our phase diagrams contain very intriguing features that are not well-known in today’s physics community but were understood by van der Waals and his colleagues at the turn of the 20th century. Phase diagrams and lattice Boltzmann simulation results are presented in a density-density plane, which best matches with LB simulations performed at constant volume and temperature. We also demonstrate that the algorithm provides thermodynamically consistent results for mixtures larger numbers of components and high density ratios. All the theoretical phase diagrams are recovered well by our lattice Boltzmann method.

Speaking an Employer's Language
Olivia Scott

Career Educator,
NDSU

Monday February 26, 11:00am,

South Engineering 221

Learn how to translate your experience and transferable skills through interviews and on job applications to show employers your qualifications, as well as, how you will fit with their organization. Through this interactive seminar we will discuss ways to market yourself to industry employers, academic committees, and everywhere in between.

Methods for calculating effective interactions and pressure in charge-stabilized dispersions with application to filtration
Mariano Brito

Candidate for Ph.D.,
Institute of Complex Systems
Forschungszentrum Jülich GmbH

<Special Day>Tuesday February 13, 11:00am,

<Special Location>ABEN 201

Charge-stabilized suspensions have interesting static features, reflected in properties such as the suspension osmotic pressure and ionic microstructure. These properties are determined by electro-steric interactions between all ionic species. Due to the large size asymmetry between colloidal macroions and small microions, the degrees of freedom of the latter can be integrated out, resulting in an effective one-component interaction potential describing microion-dressed colloidal quasi-particles.

We present a comparison, and partial extension, of various methods of calculating effective colloidal interaction parameters including effective charges and screening constants as functions of concentration and ionic strength [1]. We discuss osmotic suspension pressure calculations for dispersions in Donnan equilibrium with a salt ion reservoir. Methods are discussed including cell-models, renormalized jellium models, and multi-colloid-centered mean-field models. The pros and cons of the various methods are assessed by comparison with primitive model based computer simulations.

As an application to a technologically relevant process, a parameter-free model for cross-flow ultrafiltration is presented [2]. In this process, a dilute charge-stabilized dispersion is concentrated and purefied by continouosly pumping it though an array of cylindrical membranes having nano-sized pores.

References
[1] M. Brito, J. Riest, A. Denton and G. Nägele, to be submitted (2017).
[2] M. Brito, J. Riest, O. Nir, M. Wessling and G. Nägele, work in progress.

Theory and simulation of dispersions with competing interactions applied to protein solutions
Gerhard Nägele

Professor,
Institute of Complex Systems (ICS-3),
Forschungszentrum Jülich GmbH

Monday February 12, 3:00-4:00pm,

<Special Location>Bacheller Technology Center 271

Dynamic clustering of globular particles in dispersions exhibiting competing short-range attraction and long-range repulsion (SALR) such as in low-salinity protein solutions has gained a lot of interest over the past years. We investigate the influence of clustering on the dynamics and structure of globular particle dispersions. For this purpose, we combine a semianalytic hybrid method where hydrodynamic interactions (HIs) are approximately included [1] with multi-particle collision (MPC) simulations accounting for the full many-particles HIs [2]. By this simulation-theory comparison, we establish the high accuracy of the hybrid method for calculating diffusion and viscosity properties of SALR systems in the dispersed fluid phase region. We show that a cluster peak is present also in the hydrodynamic function characterizing the short-time dynamics, in accord with neutron spin echo results on lysozyme solutions [3]. Enhanced short-range attraction leads to a smaller self-diffusion coefficient and a larger dispersion viscosity. The behavior of the (generalized) sedimentation coefficient is more intricate showing, e.g., non-monotonic interaction strength dependence. Inter- and intraclusters dynamics in the equilibrium cluster phase region is analyzed using MPC simulations [2]. Simulation results for the mean cluster lifetime, and the comparison with the dissociation time of an isolated particle pair reveal quantitative differences, pointing to the importance of many-particle HIs for the cluster dynamics. The cluster lifetime in the cluster-fluid phase increases far stronger with increasing attraction strength than in the dispersed-fluid phase.  Significant changes in cluster shapes are observed in the course of time.

[1] J. Riest and G. Nägele, Short-time dynamics in dispersions with competing short-range attraction and long-range repulsion, Soft Matter 11, 9273 (2015).
[2] S. Das, J. Riest, R.G. Winkler, G. Gompper, J.K.G. Dhont and G. Nägele, Clustering and dynamics of particles in dispersions with competing interaction: Theory and simulation, submitted (2017).
[3] J. Riest, G. Nägele, N.J. Wagner, Y. Liu (NIST), and D. Godfrin, Short-time dynamics of Lysozyme solutions with competing short-range attraction and long-range repulsion: Experiment and theory, submitted (2017).

Carrier Multiplication in Chiral Single-Walled Carbon Nanotubes and Silicon Nanowires: DFT-Based Study
Deyan Mihaylov

Ph.D. Candidate,
Department of Physics,
North Dakota State University

Monday December 4, 3:00-4:00pm,

221 South Engineering

The conclusion about multiple exciton generation (MEG) efficiency in a nanoparticle can only be made by comprehensive study of different relaxation channels, such as phonon-mediated carrier thermalization, carrier multiplication and recombination, etc. Here, we study time evolution of a photo-excited state using Boltzmann transport equation (BE) that includes phonon emission/absorption together with the exciton multiplication and recombination. BE rates are computed using non-equilibrium finite-temperature many-body perturbation theory (MBPT) based on DFT simulations. Exciton effects are included by solving Bethe-Salpeter equation with RPA-screened Coulomb interaction (with additional simplifying approximations). We compute rates for both all-singlet MEG and Singlet Fission channels, which are of order 1014 s-1. For all-singlet MEG we calculate internal quantum efficiency (QE), the number of excitons generated from a single absorbed photon. Efficient MEG in chiral single-wall carbon nanotubes (SWCNTs), such as (6,2), both pristine and Cl-doped, and (6,5) is present within the solar spectrum range. We predict QE = 1.3-1.6 at the excitation energy of 3 times the optical gap in (6,2) and (6,5). However, QE = 1 is found in CNT (10,5) which suggests strong chirality dependence of MEG. MEG efficiency in functionalized SWCNTs is enhanced compared to the pristine case. Also, we calculate corrections to the bi-exciton state energy due to exciton-exciton interactions.

Coarse-Grained Modeling of Microgels (Mostly)
Alan Denton

Professor,
Department of Physics
North Dakota State University

Monday January 29, 3:00-4:00pm,

221 South Engineering

Microgels are soft colloidal particles, composed of cross-linked polymer networks, that swell and can acquire charge when dispersed in a solvent.  The equilibrium size of a microgel depends on a delicate balance of elastic and electrostatic osmotic pressures, which can be tuned by varying single-particle properties and external environmental conditions, such as temperature, pH, ionic strength, and concentration.  Because of their tunable interparticle forces, variable size, and ability to encapsulate cargo (e.g., drugs or dyes), microgels have practical relevance for drug delivery, biosensing, carbon capture, filtration, and many consumer care products.  Combining Poisson-Boltzmann and Flory-Rehner theories with molecular dynamics and Monte Carlo simulations for coarse-grained models of charged, elastic particles, my group is exploring the influence of particle compressibility and polydispersity on structural and thermodynamic properties of bulk microgel suspensions.

1D Capillary Bending of a Thin, Floating Polymer Film
Timothy Twohig

Ph.D. Candidate,
Department of Physics,
North Dakota State University

Monday January 8, 3:00-4:00pm,

221 South Engineering

Interest in the application of capillary origami has been expanding to many different areas of science and technology. However, insight into the basic physics of the process behind how a thin film is pulled to cover a droplet of fluid is still lacking. For example, the role of the substrate is largely ignored, but can easily arrest the wrapping process. More interestingly, wrapping can be arrested due to friction and jamming in folds forming as the flat sheet is forced to wrap the spherical droplet. Our work seeks to further the basic understanding of this process by creating a one-dimensional experiment to avoid geometric frustration and easily control substrate interactions. Specifically, we create a long, approximately flat, triple line between a drop and a film. The film rests on a fluid bath which can be modelled as a Winkler foundation. The capillary force acts on the film in a one-dimensional manner, pulling perpendicular to the triple line, without creating wrinkles or folds in the floating film. The experiment allows the system to reach an equilibrium between the capillary forces, the bending of the film, and the displacement of the substrate. A model of the system allows the further characterization of the effects of each of these forces on the shape of the bend produced in the supported film.

The Role of Adhesion in Crumpling
Andrew B. Croll

Associate Professor,
Department of Physics,
Materials and Nanotechnology Program,
North Dakota State University

Monday November 27, 3:00-4:00pm,

221 South Engineering

Once passive barrier coatings, industry now expects much more of thin films, demanding they form thin electronics for devices or conformal antennae, or demanding they become the structural elements they formerly protected through origami and kirigami based design.  Despite the simplicity, there are still a significant number of unknowns in the basic physics of thin elastic sheets.  For example, it is a challenge to confine a thin sheet in a volume of three dimensional space smaller than the sheets original size.  Bending a film is the easiest way to imagine confinement taking place; a sheet of size L can be bent along its axis reducing the overall volume occupied.  However, if a second dimension is simultaneously reduced, the sheet cannot smoothly accommodate the boundaries.  It is forced to localize stress in a sharp point known as a developable cone (easily seen in bending a sheet along two orthogonal axis).  If compression continues, many localized structures form and the sheet is forced into a random structure known as a crumple ball.  In this talk we will use confocal microscopy to examine the crumpled ball in an attempt to understand the stiffness that arises from this unique structure.  Using polymer films created in house allows us to control many variables and create truly elastic sheets, plastic sheets and sheets in which adhesion can be manipulated.  Confocal microscopy allows us to image the structure during the compression.  Ultimately, we show a semi-empirical model is superior to current scaling theories and observe adhesion to play a significant role, increasing the ‘effective’ modulus of the structure by an order of magnitude.

Crowding in Polymer-Nanoparticle Mixtures
Wyatt Davis

Ph.D. Candidate,
Department of Physics,
North Dakota State University

Monday November 20, 3:00-4:00pm,

221 South Engineering

The structure and function of polymers in confined environments, e.g., biopolymers in the cytoplasm of a cell, are affected by macromolecular crowding. To explore the influence of solvent quality on conformations of crowded polymers, we model polymers as penetrable ellipsoids, whose shapes are governed by the statistics of self-avoiding walks. Within this coarse-grained model, we perform Monte Carlo simulations of mixtures of polymers and hard nanosphere crowders, including trial changes in polymer size and shape. Penetration of polymers by nanospheres is incorporated via a free energy cost predicted by polymer field theory. To analyze the impact of crowding on polymer conformations, we compute average polymer shape distributions, radius of gyration, and asphericity over ranges of crowder size and volume fraction. We compare simulation results with predictions of free-volume theory for polymers in good and theta solvents. Our results indicate that excluded-volume interactions significantly affect crowding, especially in the limit of small crowders. Our approach may help to motivate future experimental studies of polymers in crowded environments, with relevance for drug delivery and gene therapy.

Entangled Granular Materials
Scott Franklin

Professor,
School of Physics and Astronomy,
Rochester Institute of Technology.

<Special Day>Wednesday November 15, 9:00-10:00pm,

<Special Room>212 South Engineering

A jumble of clothes hangers is a nightmare to untangle, as the individual hangers hook onto and become entangled with one another. Entanglement-driven cohesion is a general phenomenon, occurring in many different systems involving irregularly shaped particles. I'll present a variety of studies on "geometrically cohesive” granular materials (GCGM), defined by the ability to cohere due to the particle shape. These include long, thin rods, which can be surprisingly rigid, and U-shaped staples that resist being pulled apart. The statistical theories that explain how these piles melt and disentangle are reassuringly simple and capture the fundamental mechanisms of entanglement and "weakest link” behaviors, which also share universal similarities with geologically relevant avalanche and earthquake dynamics. GCGMs have also been used to model the network behavior of fire ants, which form self-healing piles capable of supporting both compressional and extensional forces to migrate across rivers. The talk will conclude with recent work on complex granular mixtures, with widely varying particle sizes and shapes, which show anomalous weakening in certain circumstances.

The Full Spectrum Boost project in nanoparticle solar cells: Downconversion, Upconversion, Transport
Gergely Zimanyi

Professor,
Department of Physics,
University of California, Davis.

Refreshments at 3:30

Monday November 13, 3:00-4:00pm,
221 South Engineering

Recent progress in nanoparticle solar cells is reviewed. Colloidal nanoparticles provide a versatile platform to implement genuinely pathbreaking physical processes for solar energy conversion. Examples include the downconversion process of multiple exciton generation, where a single incoming photon generates more than one electrons. One of the most promising upconversion process is the intermediate band mechanism, where two low energy photons cooperate to excite one electron to the conduction band. The grand scheme of "Full spectrum boost" will be presented that uses this downconversion and upconversion processes to boost the energy conversion efficiency across the entire solar spectrum. In the second half, transport phenomena will be reviewed and different classes of the Metal-Insulator transition in nanoparticle solar cells will be presented.

Multi-functional liquid crystalline epoxy networks

Michael R. Kessler

Dean,
College of Engineering
North Dakota State University

Monday November 6, 3:00-4:00pm,

221 South Engineering

Liquid crystalline networks (LCNs) are versatile functional materials because of the unique properties of liquid crystalline molecules, e.g., self-organization, reversible phase transition, and macroscopic orientation under external fields. The coupling between LC molecules and polymer networks allows these remarkable properties to be transferred to the bulk material and has resulted in a number of functional LCNs that are thermally-responsive and can change their shape reversibly due to the reversible LC phase transition upon temperature cycling.  The incorporation of photo-responsive chromophores into LCNs allows the material to convert light energy into mechanical work because of the transformation between two geometrically different azobenzene isomers upon light irradiation.  Here we demonstrate a simple route to incorporate three functional building blocks, including azobenzene chromophores, liquid crystals, and dynamic covalent bonds, together into a liquid crystalline epoxy network. The three functionalities show good compatibility and the resulting material can exhibit various photomechanical responses, dual-stimuli induced shape memory and self-healing properties, and excellent processability and recyclability.

Superintegrability in classical and quantum mechanics

Bjorn Berntson

Visiting Faculty
Department of Mathematics
North Dakota State University

Monday October 30, 3:00-4:00pm,

221 South Engineering

Typically, Hamiltonian systems in either classical or quantum mechanics cannot be solved exactly and one must resort to approximation methods. There exist, however, very special, maximally symmetric systems whose behavior can be understood algebraically (i.e. without solving differential equations). Such systems, including a number of known, important models, are called superintegrable. I will discuss these known systems in the context of their superintegrability and introduce a number of new models related to quantum mechanics in the complex domain and the theory of nonlinear ordinary differential equations.

Electrostatic Interactions at Dielectric Interfaces: From Membranes to Colloids

Guilherme Bossa

Ph.D. Candidate
Department of Physics
North Dakota State University

Friday October 27, 3:00-4:00pm,

221 South Engineering

In this work we have investigated electrostatic interactions at dielectric interfaces using theoretical models based on the non-linear Poisson-Boltzmann theory and its extensions. We have focused on three major topics: 

(1) modeling the energetics and interactions of charged nanoparticles trapped at the air-water interface; 

(2) calculation of the line tension between domains in charged lipid membranes, lipid-lipid correlations, and how membrane curvature is influenced by charged peptides; and 

(3) extensions of the classical Poisson-Boltzmann theory by accounting for the influence of ion-specific solvent-mediated interactions. More precisely, ion-specificity has been accounted for using the Poisson-Helmholtz-Boltzmann formalism, which adds to the bare Coulombic interactions a Yukawa-like potential that accounts for the interacting hydration shells of ions. 

Motivated by recent experimental and computational results, all projects present here aim to provide a deeper understanding of fundamental physical properties of charged dielectric interfaces.

Rejection-free cluster Monte Carlo algorithms in the presence of symmetry-breaking fields

Yen Lee Loh* and Nathan Carlson**

*Associate Professor
Department of Physics
University of North Dakota
**Graduate Student
Department of Physics
University of North Dakota

Monday October 9, 3:00-4:00pm, Refreshments at 2:30.

221 South Engineering

Cluster Monte Carlo algorithms are extremely efficient methods for generating thermal equilibrium configurations of spin models, lattice gases, and other systems.  However, most cluster algorithms rely on self-inverse global symmetries.  For models with symmetry-breaking external fields, the approach in the literature is to augment the cluster algorithm with a Metropolis criterion, which produces large rejection ratios and kills efficiency.  We have recently developed two cluster algorithms that have a 100% acceptance ratio even in the presence of symmetry-breaking fields.   The auxiliary-spin cluster algorithm (ASCA) replaces the symmetry-breaking field by a symmetry-preserving coupling to an auxiliary spin.   Our tests indicate that ASCA is superior to existing algorithms for Ising ferromagnets in uniform field.   The replica-exchange cluster algorithm (RECA) evolves two copies of the system using “swaps” (cluster-exchange updates), interleaved with “rotations” (global symmetry operations that need not be self-inverse), and with other types of moves (such as Metropolis updates) to ensure ergodicity.  It is applicable to XY models in the presence of vector potentials.   A Monte Carlo algorithm combining ASCA, RECA, and Metropolis updates may be a promising way to simulate various systems including superconducting Josephson junction arrays in magnetic fields, frustrated magnets, magnetic skyrmions, and molecular fluids.  We are currently benchmarking the algorithms to compare their autocorrelation times with other Monte Carlo methods.

Lattice Gas with a Molecular Dynamics collision operator

Reza Parsa

Graduate Student,
Department of Physics
North Dakota State University

Monday October 2, 3:00-4:00pm, 

221 South Engineering

In this talk I will introduce lattice gas and lattice Boltzmann methods and a new approach of deriving them from an underlying Molecular Dynamics simulation. Previously lattice gas and lattice Boltzmann methods have been justified by showing that they are able to recover in the hydrodynamic limit the continuity and Navier-Stokes equations. Our approach aims at closing the link that directly connects a fluid with a corresponding lattice gas and lattice Boltzmann method. So far we have been able to establish this link for dilute gases and we are starting to see some interesting effects for denser gases that traditional lattice gas and lattice Boltzmann approaches miss.

This presentation is part of Reza's comprehensive examination.

Probing student reasoning approaches through the lens of dual-process theories

Cody Gette

Graduate Student,
Department of Physics,
North Dakota State University

Monday September 25, 3:00-4:00pm,

221 South Engineering

As part of an ongoing investigation into student reasoning approaches in the context of physics, we have been applying dual-process theories of reasoning to understand the factors and instructional circumstances which may affect reasoning approaches in the classroom. In particular, we are interested in contexts in which difficulties remain even after multiple refinements of research based instructional materials have been made. In this talk, we will discuss experimental findings in the context of buoyancy which illustrate the application of dual-process theories to interpret student responses and further develop instructional materials. Future experiments toward understanding student approaches in difficult contexts and possible instructional implications will also be discussed.

Differential Capacitance of Ionic Liquids with and without Added Solvent

Sylvio May

Associate Professor,
Department of Physics,
North Dakota State University

Monday September 18, 3:00-4:00pm,

221 South Engineering

Ionic liquids, which consist exclusively of ions that reside in the liquid state, have many applications, including the use as superpacacitors. The underlying process of double layer charging involves interesting physics, especially when a small amount of solvent is added. My talk discusses the structure of a double layer formed by an ionic liquid and the corresponding differential capacitance. Recent computer simulations suggest that adding solvent to an ionic liquid can lead to an "anomalous" non-monotonic behavior of the differential capacitance. I will argue that this behavior is actually captured by a 75-years-old simple lattice-based mean-field model and discuss extensions of the model to describe ionic liquids more realistically.

Molecular Dynamics Lattice Gas: a New Tool to Understand
Lattice Gases and Lattice Boltzmann

Alexander Wagner

Department of Physics
North Dakota State University

Monday September 11, 3:00-4:00pm,

221 South Engineering

 We introduce a new kind of lattice gas that consists of a coarse-graining procedure for an underlying Molecular Dynamics simulation (MDLG). We show that for appropriate choices of the MD system and the spatial and temporal coarse graining discretization we can match the collision operator of the MDLG system to a lattice Boltzmann collision operator, at least approximately. The fundamental connection between the MDLG algorithm to an underlying MD simulation ensures that the MDLG algorithm is fundamentally constrained to give the physically correct dynamics. This allows us to test properties of this ideal LG algorithm, which allows us to verify (or appropriately modify) current LG and lattice Boltzmann approaches for fluctuating lattice Boltzmann and, eventually, multi-phase and multi-component implementations.

Methods for Cyclic Coating Test Protocol Development

Aaron Feickert

Graduate Student
NDSU Dept. Physics

Monday August 28, 3:00-4:00pm.

221 South Engineering

Cyclic exposure testing is frequently used in the qualification and comparison of candidate coating systems. However, existing protocols often fail to capture the range of failure modes often seen in service, and are not particularly suited for multi-layer coating stackups. In this talk, we will discuss experimental and simulation methods used in the development of advanced test protocols that integrate mechanical flexing into the cycling process. Techniques like structural finite-element analysis and cycled moisture transport modeling are used to better understand the role of layering and periodic cycling of environmental factors.

This talk is intended to be at a general scientist/engineer level, so it will be broad and not particularly technical.

Mining Deep Underground for Neutrinos: DUNE

David DeMuth

Department of Physics,
Valley City State University

Monday May 1, 3:00-4:00pm, Refreshments at 2:30.

221 South Engineering

High Energy Particle Physics research has deepened our understanding of the universe through world class explorations of energy and matter at a fundamental scale, prompting amazing discoveries, innovation, and technologies. A new particle physics experiment at Chicago’s Fermi National Accelerator Laboratory will focus neutrinos to a deep underground laboratory 800 miles away in South Dakota, near Sturgis, at a depth of 4,850 feet. The design, construction, and operation of precision particle detectors is coordinated by a large international collaboration of scientists and engineers who hope to answer big questions related to the origin of matter, unification of forces, supernova mechanics, and black hole formation.

In this presentation, Valley City State University Professor David DeMuth, Jr. will set a context for high energy particle exploration, discuss motivations for building a Deep Underground Neutrino Experiment,  highlight engineering design and challenges, indicate timeline, and suggest implications for Dakota’s student’s participation in neutrino research.  For more on DUNE: www.dunescience.org

DNA Topology and Genomic Information Processing

Professor Laura Finzi

Department of Physics,
Emory University

Tuesday April 25, 3:00-4:00pm, Refreshments at 2:30.

271 Batcheller Technology Center <NOTE THE DIFFERENT ROOM>

DNA is a right-handed, double helical polymer that encodes genetic information. Its topology is dynamically changed by proteins and enzymes. In this talk, I will describe our use of single molecule techniques to understand critical topological changes that regulate genomic function.

A novel macroscopic Technique to measure the Nanomechanics of durable multifunctional Nanoshells

Abu Tafique

Graduate Student,
Department of Physics, NDSU

Monday April 24, 3:00-4:00pm,

221 South Engineering

In the recent advancement of nanotechnology, carbon nanotubes (CNTs) have shown promise and potential due to their excellent mechanical, electrical, and optical properties for a wide variety of applications. A network of single-wall carbon nanotubes (SWCNTs) is of particular interest due to its applications in flexible electronics, composites, constructional materials, and so on. Understanding the mechanical properties of these nanoscale thin films is critical in order to fully characterize their behavior, whereas only a few methods are in the literature to determine the deformation mechanics of these films. In order to put some insight into the large-deformation mechanics, and relative lack of proper measurement techniques of such films, we propose a novel, simple, direct method for evaluating the mechanical characteristics of the SWCNT thin films. We provide the theoretical background, experimental approach, and a MATLAB based analysis to extract the data. Finally, we compare our result to existing wrinkling-based measurements, which shows reasonable agreement between the two techniques.

Electronic and optical properties of monolayer and few-layer black phosphorus: A DFT Perspective

Deniz Cakir

Dept. Physics and Astrophysics
University of North Dakota

Monday April 10, 3:00-4:00pm, Refreshments at 2:30.

221 South Engineering

Phosphorene, or monolayer black phosphorus, is a new 2D layered material with high carrier mobility and direct semiconducting band gap. One of the most interesting characters of phosphorene is highly anisotropic electronic and optical properties due to its anisotropic puckered atomic structure, making it a very promising material for electronics and optoelectronics applications.

In this talk, I will discuss the effect of strain, staking type and twisting angle on the electronic and optical properties of monolayer and few-layer black phosphorus by the help of density functional theory calculations. We find that the optical properties (i.e. absorption spectrum and exciton binding energy) and the electronic properties (i.e. band structure and effective mass) are highly anisotropic and strongly depend on the amount of applied strain in monolayer black phosphorus and type of staking in few-layer black phosphorus. Due to reduced dimensionality and weak screening, the calculated exciton binding energies are quite large and found to be in the range of 0.3-0.8 eV depending on applied strain and staking type. In addition, strain and type of staking are able to tune the electronic band gap and optical gap of black phosphorus by 1.5 eV. Such a wide tuning ability of the electronic band gap and optical gap allows us to design novel optoelectronic devices that capture a broad range of solar spectrum.

Less is more: Extreme Optics with Zero Refractive Index

Eric Mazur,

Department of Physics,
Harvard University

Wednesday April 5, 11:15-12:15pm, <Note the Special Time>

221 South Engineering

Nanotechnology has enabled the development of nanostructured composite materials (metamaterials) with exotic optical properties not found in nature. In the most extreme case, we can create materials which support light waves that propagate with infinite phase velocity, corresponding to a refractive index of zero. This zero index can only be achieved by simultaneously controlling the electric and magnetic resonances of the nanostructure. We present an in-plane metamaterial design consisting of silicon pillar arrays, embedded within a polymer matrix and sandwiched between gold layers. Using an integrated nano-scale prism constructed of the proposed material, we demonstrate unambiguously a refractive index of zero in the optical regime. This design serves as a novel on-chip platform to explore the exotic physics of zero-index metamaterials, with applications to super-coupling, integrated quantum optics, and phase matching.

Exciton Dynamics and Optical Properties of Single Semiconductor
Carbon Nanotubes and Nanotube Bundles

Gianluigi Veglia

Department of Biochemistry, Molecular Biology and Biophysics,
Department of Chemistry, University of Minnesota

Tuesday April 4, 2:00-3:00pm, Refreshments at 1:30.

271 Batcheller Technology Center <NOTE THE DIFFERENT ROOM>

Eukaryotic protein kinases (EPKs) constitute a class of allosteric switches that mediate several signaling events. Protein kinase A is a prototypical kinase of paramount biological importance as it is involved in a myriad of cellular processes. In this talk, I will show how state-of-the-art nuclear magnetic resonance (NMR) techniques can trace the intramolecular allosteric network responsible for activation and deactivation of kinase A. In particular, I will emphasize to role of hydrophobic spines within the catalytic core that are essential for the catalytic turnover. In the apo form, the C-spine is disassembled, with the two lobes of the enzyme dynamically uncommitted. Nucleotide binding locks the architecture of the catalytic spine, synchronizing the motions (committed dynamics) in the hydrophobic core. While pseudo-substrate binding further rigidifies of the spines, the conformational dynamics of the core are retained with natural substrates. Since the C-subunit is highly conserved within the kinase family, the present study offers novel mechanistic insights into intramolecular signaling of protein kinases that can serve for the design of novel activators or inhibitors of kinases.

Carbon Nanomaterial Microelectrodes for Neurotransmitter Detection

B. Jill Venton

Department of Chemistry,
University of Virginia

Monday March 27, 3:00-4:00pm, Refreshments at 2:30.

271 Batcheller Technology Center <NOTE THE DIFFERENT ROOM>

Carbon nanotubes have interesting electrochemical properties including fast electron transfer kinetics and high surface areas. Common acid purification techniques also result in oxidative functional groups which can serve as adsorption sites for neurotransmitters. The use of CNTs in fabricating larger electrodes has been widely demonstrated but the reproducible production of small, sensitive CNT-based sensors is not as well studied. My lab has investigated several different techniques for making CNT-based microelectrodes and tested their response using fast-scan cyclic voltammetry. This talk will compare CNT-modified carbon-fiber microelectrodes, CVD grown aligned CNTs, CNT yarns, and CNT fibers for use as microelectrodes. In general, alignment of CNTs and chemical functional groups determine the sensitivity of the electrode for neurotransmitters. Also, CNT yarns and fibers have different adsorption properties that allow them to be used with higher temporal resolution without a decrease in sensitivity. We have also studied other types of carbon nanomaterials including carbon nanospikes and amorphous carbon on nanopipettes. Our work reveals that carbon nanomaterial-based microelectrodes are advantageous as electrochemical sensors for neurotransmitters.

Out-of-the-Box Solutions to Silicon Crystal Growth Problems

Harald Korb

Korb Consulting, LLC.

Thursday March 23, 3:30-4:30pm, Refreshments at 3:00.

271 Batcheller Technology Center

The Czochralski process for growing silicon crystals is capable of producing high quality crystals at high yield and reasonable cost. The expectation is that every wafer produced from those crystals will be free from measurable defects, and the manufacturing process will be error-free. If straightforward engineering changes can't provide solutions to quality problems, a more basic attack may help. I will describe three problems that benefitted from a different angle of attack: a) The creation of a method to pump liquid silicon using electromagnetic means; b) The identification of the mechanism by which small spherical bubbles can be grown into a silicon crystal, and c) (if time permits) The determination of the conditions under which electrical breakdown (arcing) in Ar should occur at high temperature and low pressure.

This is a special award double seminar.

Presentation of Awards by Dean Wood, followed by

The New Horizons Mission to Pluto

Darrell F. Strobel

Johns Hopkins University

Abstract: On 19 January 2006, NASA launched its first mission to Pluto the ninth planet. Called New HorizonsMission, its spacecraft is only the size of a grand piano and operated on just 200 Watts of power. Atlaunch it was one of the fastest spacecraft to leave the Earth and made the trip to Pluto in a mere 9 ½years. In this talk I will discuss the early history of the mission, the launch, the planning of missionoperations and the remarkable scientific return from this largest known Kuiper-belt object during the 14July 2015 flyby. At the end of the talk I will be open to discuss: the ninth planet at launch and a dwarfplanet at arrival and more than a frozen water ice ball. New Horizons will study Kuiper Belt object2014 MU69 during a flyby on 1 January 2019.

Followed by the second talk:

From Wheat Fields to Watt FieldsLinks in the Silicon Food Chain

Harold Korb

Korb Consulting, LLC

Abstract: The phenomenal advances in electronics in the past 70 years since the invention of the transistor havebeen driven by the exquisite understanding of the basic physics of semiconductors and semiconductor devices and by the creativity and investment in new technology in the semiconductor device industry.Less publicized is the role played by the availability of silicon wafers engineered and optimized toenable the economical fabrication of all silicon-based devices. For the first 25 years of this era, theemphasis was on making defect-free silicon with electronic (chemical) properties tailored to eachapplication. Since then, it has become essential to create families of defects in the silicon and toengineer the defect properties for each application. I will describe the physics and engineeringinvolved in creating and controlling these families of defects.

The Atmosphere of Pluto: Results from the New Horizons Mission

Darrell F. Strobel

Johns Hopkins University
+ ALICE (Leslie Young, Josh Kammer, Andrew Steffl et al.)
&REX (Dave Hinson et al.)Teams
and Xun Zhu, Johns Hopkins Applied Physics Laboratory

Tuesday March 21, 3:3-4:30pm, Refreshments at 3:00.

227 South Engineering

 
Abstract:
On 14 July 2015, NASA’s New Horizons spacecraft observed an ultraviolet solar occultation of Pluto's atmosphere with its ALICE ultraviolet spectrograph and performed a radio occultation that sounded Pluto's atmosphere down to the surface with radio signals transmitted simultaneously by four antennas of the NASA Deep Space Network, each radiating 20 kW at a wavelength of 4.2 cm. From the solar occultation data we derive line-of-sight (los) optical depths that yield los column densities for 5 molecular species, and extinction coefficients for haze. The radio occultation data yield N2 number density, pressure, and temperature profiles from the surface to about 110 km of altitude at two diametric points on the planet. We find a very stable, spherically symmetry, lower atmosphere, with well-mixed portion restricted to a planetary boundary layer (surface to 5 km), peak temperature of ~ 106 K at ~ 25km, cold isothermal temperature ~ 68 K in Pluto’s upper atmosphere, and inferred CH4 surface mixing ratio ~ 0.3±0.02%. The inferred enhanced Jeans escape rates are 5±2 x 1022 N2 s-1 and 6.4±1.6 x 1025 CH4 s-1 at the exobase (r ~ 2900 km, where the Kn = 0.7). 

Impact of Chain Conformation and Dynamics on Macroscopic Properties of Polymer Melts and Nanocomposites

Gerald Schneider,

Department of Chemistry,
Louisiana State University

Friday February 24, 3:00-4:00pm, Refreshments at 2:30.

221 South Engineering

Intense research has led to substantial progress towards understanding the fundamentals of polymer melts and polymer based nanocomposites. Characterizing the polymer dynamics at micro- and mesoscopic scales is often of particular interest. For example, it is important for modeling the macroscopic material response needed for the target-oriented engineering of new hybrid materials. It may lead to optimized materials ranging from the classical car tire to battery or fuel cell applications.

The presentation highlights the research on model nanocomposites well suited to act as interlink between a theoretical understanding and the technical application and sheds light on the influence of hard impenetrable surfaces on polymer melts. It presents the link from the morphology and the dynamics at microscopic and mesoscopic scales to the material properties, e.g. those measured by rheology experiments.

Curvature Matters: Modulation of Phase Morphology and Dynamics of Lipid Monolayers with Curvature of Air-Aqueous Interfaces

Amit Sachen

Department of Chemical Engineering and Materials Science,
University of Minnesota, MN

Monday February 6, 3:00-4:00pm, Refreshments at 2:30.

221 South Engineering

In this talk, we will present our tools and approaches to create monolayers on interfaces of different background curvatures (25-1000 µm radius), as well as to investigate time- and curvature-dependent changes in the morphology and dynamics. Using a biologically relevant multi-component surfactant, which forms coexisting ordered, ‘solid-like’ domains in the disordered, ‘liquid-like’ phase in a planar monolayer, we will show that as the curvature of the interface approaches the typical size of domains observed on the planar interface, the curved monolayer yields a unique phase-coexistence pattern where size, shape and connectivity of the domains dramatically change. A theoretical model will be presented, explaining how interplay between line tension and electrostatic (dipole:dipole) interaction energies is compromised in the curved monolayer, and gives rise to a modulated phase pattern. Surprisingly, the background curvature-mediated dramatic shift in phase-coexistence pattern impacts the dynamic properties of the monolayer as well, in particular the dilatational modulus, epsilon=A(d gamma/ dA).
These results have implications in rational engineering of smart and stable interfaces for different commercial applications, as well as in understanding the mechanically efficient breathing mechanism in air-breathing mammals. 

From Self-Assembly to Electrokinetics: Novel Predictive Capabilities for Dielectric Effects

Erik Luijten

Professor of Material Science and Engineering
Northwestern University

Friday February 3, 3:00-4:00pm, Refreshments at 2:30.

271 Batcheller Technology Center <NOTE THE DIFFERENT ROOM>

The ability of matter to self-organize in complex dynamic structures is increasingly used to generate new, active materials. Progress in this field critically depends on the predictive capabilities of reliable and efficient computer simulation strategies. Here, I will introduce new, computational methodologies for dielectric effects, and demonstrate that these methods make it possible to perform dynamic simulations that fully incorporate self-consistently calculated polarization charges. Notably, I will discuss how the impact of these developments ranges from the prediction and control of colloidal and nanoscale self-assembly and aggregation to the understanding of dynamical properties of self-propelled particles that form the basic building blocks of active matter. I will also show how these ideas have the potential to find application in the understanding of supercapacitors and other energy-related problems.

Diagnostic Imaging Physicists in the 21st Century: Ongoing Challenges and Future Developments

Dr. Ryan Bosca

Radiological Physicist
Sanford Health

Monday January 30, 3:00-4:00pm, Refreshments at 2:30.

221 South Engineering

Diagnostic imaging has a rich history rooted in Röntgen’s discovery of x-rays and the first radiographs taken over 120 years ago. Since that time, (medical) physicists have been heavily involved in the development of new imaging modalities and techniques, equipment performance evaluations, and operator education for the safe use of such equipment. Technological advancements, especially the development of computers, have resulted in the widespread use of imaging modalities such as computed tomography, positron emission tomography, digital radiography, and magnetic resonance imaging, to name a few. Continued developments from equipment manufacturers, professional organizations, and academic researchers require careful evaluation and implementation by clinical medical physicists. This presentation will briefly outline some of the physical principles used in safely acquiring medical images and summarize research opportunities in the Department of Imaging Physics at Sanford Health in Fargo.

Exciton Dynamics and Optical Properties of Single Semiconductor
Carbon Nanotubes and Nanotube Bundles

Andrei Piryatinski

Theoretical Division
Los Alamos National Laboratory

Monday January 23, 3:00-4:00pm, Refreshments at 2:30.

230 Minard Hall <NOTE THE DIFFERENT ROOM>

Semiconductor single-walled carbon nanotubes (CNTs) are near-perfect 1D materials with great potential for applications in opto-electronic and photonic devices. Their unique optical properties are determined by highly mobile interacting excitons. Motivated by experiment, we examine competition between exciton diffusion dynamics and their local interactions resulting in the exciton-exciton annihilation. [1] Our model explains experimentally observed dependence of the exciton emission profile on the intensity of the optical pump and further allows for the interpret of the photon counting statistics probed by measuring the 2 nd order photon number correlation function.

We also examine the effect of exciton states modulation by external periodic potential due to the acoustic wave propagating along CNT substrate, [2] and demonstrate that the potential induces Floquet sub-bands separated by dynamical gaps in the single particle spectrum. This leads to
redistribution of the exciton oscillator strength and subsequent fluorescence quenching. Finally, motivated by experimental studies, we examine spectral signatures of interacting intratube and intertube exciton states formed in bundles of CNTs. For this purpose, an exciton scattering model is developed. Considering optimized geometry hexagonal lattice CNT bundle, we identified the sites participating in the formation of the intertube excitons. These sites are treated as an interacting “impurity centers” giving rise to the delocalized intratube exciton scattering. Modeling of the Raman resonance excitation profiles in (6,5) CNTs demonstrates an appearance of a sharp feature at the red shoulder of the spectrum that has been observed experimentally. The model-based analysis confirms that the feature is due to the weakly coupled interband exciton states.

References:
[1] X. Ma, O.Roslyak, J. G. Duque, S. K. Doorn, A. Piryatinski, D. H. Dunlap, and H. Htoon. Influences of Exciton Diffusion and Exciton-Exciton Annihilation on Photon Emission Statistics of Carbon Nanotubes, Phys. Rev. Lett. 115 017401 (2015).
[2] O. Roslyak and A. Piryatinski. Effect of periodic potential on exciton states in semiconductor carbon nanotubes. Chem. Phys. 481 177 (2016).

Statics and Dynamics in Folded Thin Films

Andrew B. Croll

Associate Professor,
Department of Physics,
Materials and Nanotechnology Program,
North Dakota State University,

Monday December 5, 3:00-4:00pm.

221 South Engineering,

Folding sheets into complex geometries can create efficient light-weight structures with many fascinating dynamic properties not available to solid structures.  The adoption of origami based structures by industry, however, requires a careful characterization of real materials folded into creative structures.  Here we highlight our recent progress in understanding how thin polymer films respond to the extreme bending required by origami.  In this talk we update our progress in studies of the statics and dynamics of complex folded matter.  In particular, we use simple bending and three different polymers – one elastomer (poly dimethyl siloxane), and two glasses having different failure properties (polystyrene and polycarbonate) to create a set of model materials and geometries.   After characterizing the simplest origami component (a single bend), films are subjected to a second bend (orthogonal to the first) creating a d-cone, the most extreme focus of energy occurring in thin film systems.  Finally, the interaction of two d-cones is used to identify an important lengthscale in folded systems.  Adhesion is found to play a profound role in the creation and load baring ability these simplified origami components through creating additional energy storage and extreme changes in boundary conditions.  Ultimately, we introduce the conclusion that material is a critical feature determining the statics and dynamics of thin folded matter.

Mean-Field Prediction for Surface Tension of Yukawa Fluid

Sylvio May

Associate Professor and Chair,
Department of Physics,
North Dakota State University,

Monday November 28, 3:00-4:00pm.

221 South Engineering,

In this talk I will discuss a fluid with particles interacting through a repulsive or attractive (yet, not phase separating) Yukawa pair potential. A mean-field model will be presented and used to calculate the surface tension at the interface of a container. The goal of the talk is to derive an analytic expression of the surface tension in the limit of small particle density.

Density Functional Theory (DFT) + Many-Body Perturbation Theory (MBPT) Study of Multiple Exciton Generation in Semiconductor Nanostructures

Deyan Mihaylov

Graduate Student,
Department of Physics,
North Dakota State University,

Monday November 21, 3:00-4:00pm.

221 South Engineering,

Multiple exciton generation (MEG) in nm-sized H-passivated Si nanowires (NWs), and quasi 2D nanofilms depends strongly on the degree of the core structural disorder as shown by the perturbation theory calculations based on the DFT simulations. In perturbation theory, we work to the second order in the electron-photon coupling and in the (approximate) RPA-screened Coulomb interaction. We also include the effect of excitons for which we solve Bethe-Salpeter Equation. To describe MEG we calculate exciton-to-biexciton as well as biexciton-to-exciton rates.
We also study the time evolution of exciton occupation numbers using the Boltzmann transport equation (BE) which includes photon absorption and emission (exciton recombination) terms. We consider 3D arrays of Si29H36 quantum dots, NWs, and quasi 2D silicon nanofilms, all with both crystalline and amorphous core structures. We find that MEG in the amorphous nanoparticles is enhanced by the electron localization due to structural disorder. The exciton effects significantly red-shift QE vs. photon energy curves. Nm-sized a-Si NWs and films are predicted to have effective MEG within the solar spectrum range. Also,  we find efficient MEG in the pristine and Cl-doped chiral single-wall Carbon nanotubes.

Student thinking regarding coordinate systems in the upper division

Brian Farlow

Graduate Student,
Department of Physics,
North Dakota State University,

Monday November 14, 3:00-4:00pm.

221 South Engineering,

As part of a broader study on the content of and student thinking within mathematics in the undergraduate physics curriculum, we report on student thinking about coordinate systems in the upper division. Early evidence suggests that upper-division physics students struggle to solve problems and answer conceptual questions requiring the use of Cartesian and non-Cartesian coordinate systems. Specifically, students have difficulty identifying the direction of unit vectors and constructing symbolic expressions for position and velocity in the plane polar coordinates. We report findings from one-on-one interviews that used a think aloud protocol designed to shed light on student thinking within this domain.  We investigate the potential connection between student reasoning regarding Cartesian and non-Cartesian coordinates with emphasis on polar and spherical coordinate systems, and students’ ability to answer conceptual questions and solve problems requiring the use of those coordinate systems.

Defect physics as key to understanding functional materials

Khang Hoang

Research Scientist,
Center for Computationally Assisted Science and Technology,
North Dakota State University,

Monday November 7, 3:00-4:00pm.

221 South Engineering,

In this talk we use Monte Carlo simulations and various Mean-Field models to investigate the influence of soft, hydration-mediated ion-ion and ion-surface interactions on the differential capacitance of an electric double layer. We focus on a planar electrode surface at physiological concentration of monovalent ions in a uniform dielectric background. Hydration-mediated interactions are modeled on the basis of Yukawa potentials that add to the Coulomb and excluded volume interactions between ions. We present a mean-field model that includes hydration-mediated anion-anion, anion-cation, and cation-cation interactions of arbitrary strengths. In addition, finite ion sizes are accounted for through excluded volume interactions, described either on the basis of the Carnahan-Starling equation of state or using a lattice gas model. Both our Monte Carlo simulations and mean-field approaches predict a characteristic double-peak (the so-called camel shape) of the differential capacitance; its decrease reflects the packing of the counterions near the electrode surface. The presence of hydration-mediated ion-surface repulsion causes a thin charge-depleted region close to the surface, which is reminiscent of a Stern layer. We analyze the interplay between excluded volume and hydration-mediated interactions on the differential capacitance and demonstrate that for small surface charge density our mean-field model based on the Carnahan-Starling equation is able to capture the Monte Carlo simulation results. In contrast, for large surface charge density the mean-field approach based on the lattice gas model is preferable.

Nonadiabatic Dynamics in Semiconductor Nanomaterials

Dayton Jon Vogel

Graduate Student,
Department of Chemistry,
University of South Dakota,

Monday October 24, 3:00-4:00pm.

221 South Engineering,

Within photovoltaics and optoelectronic applications, pathways of energy dissipation of the generated charges following photoexcitation are of great importance. These processes can highlight charge carrier lifetimes, energy states facilitating the relaxation, radiative emission, and non-radiative relaxation. The presented work is a computational look into the application of nonadiabatic dynamics, using Redfield formalism, in MAPbI3,1 Si QD,2 and polyoxotitante clusters3 to study electronic population probability among electronic states to predict charge collection efficiency. Electronic density of states, absorption spectra, and partial change localization, among others, provide valuable insight to the specific atomic species and molecular motions that contribute the observed physical properties of a material. Synthesis of nanostructures allows fine-tuning of electronic and optical properties, resulting from quantum size effects. The increased control of the material size can lead to increased efficiency within photovoltaic and water-splitting devices. Many methods for increasing quantum efficiencies (QE) within photovoltaic and optoelectronic processes have been developed such as material interfacing, modified device architecture, and physical constraints to the photoactive material. This processes applied to small bandgap semiconductor quantum dots (QD) has achieved QE over 1. Understanding electronic relaxation mechanisms and their corresponding timescales allow for a clearer picture into which relaxation processes are of greatest importance and can be harnessed for maximum efficiency.

1.    Junkman, D.; Vogel, D. J.; Han, Y.; Kilin, D. S., Ab Initio Analysis of Charge Carrier Dynamics in Organic-Inorganic Lead Halide Perovskite Solar Cells. MRS Online Proceedings Library 2015, 1776, 19-29.
2.    Brown, S. L. V., D. J.; Miller, J. B. ; Inerbaev, T. M.; Anthony, R. J. ; Kortshagen, U. R.; Kilin, D. S.;  Hobbie, , Enhancing Silicon Nanocrystal Photoluminescence Through Temperature and Microstructure. J. Phys. Chem. C.  2016, 120 (33), 18909–18916.
3.    Vogel, D. J.; Kilin, D. S., First-Principles Treatment of Photoluminescence in Semiconductors. Journal of Physical Chemistry C 2015, 119 (50), 27954-27964.

Lattice Boltzmann for non-ideal fluids

Kent Ridl

Graduate Student,
Department of Physics,
North Dakota State University,

Monday October 17, 3:00-4:00pm.

221 South Engineering,

We recently heard a number of talks related to lattice Boltzmann. This talk will focus on the simple fundamentals of lattice Boltzmann, including high-level derivations with conceptual emphasis that tie the Boltzmann equation to hydrodynamics and discretize it for computational use. We show a simple one-dimensional implementation for a non-ideal gas and compare the results of this dynamical model in equilibrium with those of a free energy minimization. We close in discussing how this model can be extended to multiple components to simulate an experiment from the Hobbie group at NDSU, where the evaporation of solvent from a colloid-polymer mixture left complex structures in its wake.

Role of ion hydration for the differential capacitance of an electric double layer

Guilherme Bossa

Graduate Student,
Department of Physics,
North Dakota State University,

Monday October 10, 3:00-4:00pm.

221 South Engineering,

In this talk we use Monte Carlo simulations and various Mean-Field models to investigate the influence of soft, hydration-mediated ion-ion and ion-surface interactions on the differential capacitance of an electric double layer. We focus on a planar electrode surface at physiological concentration of monovalent ions in a uniform dielectric background. Hydration-mediated interactions are modeled on the basis of Yukawa potentials that add to the Coulomb and excluded volume interactions between ions. We present a mean-field model that includes hydration-mediated anion-anion, anion-cation, and cation-cation interactions of arbitrary strengths. In addition, finite ion sizes are accounted for through excluded volume interactions, described either on the basis of the Carnahan-Starling equation of state or using a lattice gas model. Both our Monte Carlo simulations and mean-field approaches predict a characteristic double-peak (the so-called camel shape) of the differential capacitance; its decrease reflects the packing of the counterions near the electrode surface. The presence of hydration-mediated ion-surface repulsion causes a thin charge-depleted region close to the surface, which is reminiscent of a Stern layer. We analyze the interplay between excluded volume and hydration-mediated interactions on the differential capacitance and demonstrate that for small surface charge density our mean-field model based on the Carnahan-Starling equation is able to capture the Monte Carlo simulation results. In contrast, for large surface charge density the mean-field approach based on the lattice gas model is preferable.

Thermal and Structural Properties of Microgel Suspensions

Alan Denton

Associate Professor,
Department of Physics,
North Dakota State University,

Monday October 3, 3:00-4:00pm.

221 South Engineering,

Microgels are microscopic gel particles that become swollen when dispersed in a solvent.  The equilibrium size of a microgel is governed by a balance of osmotic pressures, which can be tuned by varying single-particle properties and externally controlled conditions, such as temperature, pH, ionic strength, and concentration.  Because of their tunable size and ability to encapsulate dye or drug molecules, these soft colloidal particles have practical relevance for biosensing, drug delivery, carbon capture, and filtration.  Combining theory and simulation for a model of elastic, compressible particles, we demonstrate that, with increasing concentration, ionic microgels can deswell due to a redistribution of counterions, while nonionic microgels deswell due to steric interparticle interactions.  We further explore consequences of size polydispersity for the structure and thermodynamic phase behavior of microgel suspensions.

Lattice Boltzmann with collision operator derived from Molecular Dynamics

Reza Parsa

Graduate Student,
Department of Physics,
North Dakota State University,

Monday September 26, 3:00-4:00pm.

221 South Engineering,

We present here a mapping of a molecular dynamics simulation onto density transfers between cells, reminiscent of lattice gas and lattice Boltzmann methods. We show that while it is impossible for fundamental reasons to map this method on a standard lattice gas method, it may be possible to map an ensemble average of this method onto a lattice Boltzmann method.  We performed a sample Molecular Dynamics simulation for the underlying physical dynamics. The migration of particles was observed at each time-step and lattice cell with associate a lattice velocity vector. After another time-step particles were redistributed to new lattice sites and lattice velocities which can be understood as an effective Molecular Dynamics derived Lattice Boltzmann (MDLB) collision operator.  We show that the new MDLB collision operator shows promise for mapping it onto a standard collision operator of lattice Boltzmann.

Analytical and numerical approach to cyclic diffusion

Aaron Feickert

Graduate Student,
Department of Physics,
North Dakota State University,

Monday September 19, 3:00-4:00pm.

221 South Engineering,

Coating systems are used heavily to protect underlying substrates from corrosion, which causes aesthetic and structural degradation. Diffusion of water and other materials provides a method of ingress and egress into and out of coating systems, where the eventual buildup of fluid at the substrate can cause the onset and continuation of corrosion processes. Test methods used to examine and certify coatings for use in service often use cyclic weathering testing, in which a coating system is subject to wet and dry cycles along with other stressors designed to mimic environmental conditions in the field. While basic Fickian diffusion has been studied extensively in the literature, the precise nature of cyclic diffusion through polymer systems is not fully understood as it applies to typical test systems. We present very preliminary work into an analytical and numerical analysis of this process, showing the derivation of an analytic solution to a cyclic diffusion problem and a lattice Boltzmann verification and visualization of our solution. This is joint work with Alexander Wagner, Kyle Strand, and Kent Ridl

Developing a fluctuating lattice Boltzman method for the diffusion equation

Kyle Strand

Graduate Student,
Department of Physics,
North Dakota State University,

Monday August 29, 3:00-4:00pm.

221 South Engineering,

Lattice Boltzmann is a relatively new simulation method which is simple in its nature and has repeatedly provided good results when applied to hydrodynamics. Although it is powerful in its most basic form, it does raise issue when attempting to model soft-matter systems where discrete particle fluctuations are present. In this talk, we will briefly introduce lattice Boltzmann and fluctuating lattice Boltzmann methods, as well as highlight this new method for applying fluctuations to the lattice Boltzmann equation for use with the diffusion equation [1]. The simplicity that is seen in a diffusive system allows for a close look at the fundamentals of a considering fluctuations in lattice Boltzmann.

Mechanics of Ultrathin Polymer Films: Viscoelasticity, Dynamics andRubbery Response of Membranes

Gregory B. McKenna

Department of Chemical Engineering,
Texas Tech. University,

Tuesday April 26, 3:00-4:00pm, Refreshments at 2:30.

271 Bachellor Technology Center

Determination of the mechanical response of polymeric materials with dimensions less than 100 nm is a continuing challenge.  Here we describe a novel membrane (“nano-bubble”) inflation method we have developed for the purpose of making measurements of the creep response of ultrathin polymer films and show two major findings. The first is that the material dynamics as measured by the creep response of the membranes depends dramatically on film thickness.  For example, in polystyrene films, the dynamics is accelerated so much that the glass transition temperature Tg of a 11 nm thick film is reduced by approximately 50 K relative to the macroscopic Tg [1].  Furthermore, we have discovered that the nominal rubbery plateau in ultrathin films is stiffened by upwards of two orders of magnitude relative to the macroscopic state and the rate of stiffening (stiffening index S) correlates with the shape of the segmental relaxation in accordance with a recent model proposed by Ngai, Prevosto and Grassia [2].  We have elaborated this finding further and observe a strong correlation with the fragility index m that is related to glass formation according to the Angell categorization [3] of super cooled liquids.  These results will be discussed in terms of current understanding of the impact of nanoconfinement on the glass transition behavior of polymers. In addition to being able to characterize the creep response of the ultrathin polymer films, we have also succeeded in adapting the bubble inflation method to make measurements on a graphene/polymer nano-sandwich structure and show that the method can be used to not only extract the stiffness of the graphene inner layer of the composite but that the method can be used to extract the interfacial shear strength of the polymer-graphene couple [4].

 

 

[1]  P.A. O’Connell, S.A. Hutcheson and G.B. McKenna, Journal of Polymer Science: Part B: Polymer Physics, 46, 1952-1965 (2008).

[2] K.L. Ngai, D. Prevosto and L. Grassia, Journal of Polymer Science: Part B: Polymer Physics, 51, 214-224 (2013).

[3]  C.A. Angell, Journal of Non-Crystalline Solids, 73, 1-17 (1985).

[4]  X. Li, J. Warzywoda and G.B. McKenna, 55, 4976-4982 (2014).

Probing Single-Molecule Protein Conformational Dynamics in Enzymatic Reactions and Cell Signaling

H. Peter Lu

Ohio Eminent Scholar and Professor,
Department of Chemistry
Bowling Green State University,

Tuesday April 25, 3:00-4:00pm, Refreshments at 2:30.

271 Bachellor Technology Center

Enzymatic reactions are traditionally studied at the ensemble level, despite significant static and dynamic inhomogeneities.  Subtle conformational changes play a crucial role in protein functions, and these protein conformations are highly dynamic rather than being static. We applied single-molecule spectroscopy to study the mechanisms and dynamics of enzymatic reactions involved with kinase and lysozyme proteins.  Enzymatic reaction turnovers and the associated structure changes of individual protein molecules were observed simultaneously in real-time by single-molecule FRET detections.  We obtained the rates for single-molecule conformational active-site open-close fluctuation and correlated enzymatic reactions.  Our new approach is applicable to a wide range of single-molecule FRET measurements for protein conformational changes under enzymatic reactions and protein-protein interactions in cell signaling.  Using this approach, we analyzed enzyme-substrate complex formation dynamics to reveal (1) multiple intermediate conformational states, (2) conformational motions involving in active complex formation and product releasing from the enzymatic active site, and (3) conformational memory effects in the chemical reaction process. Furthermore, we have applied AFM-enhanced single-molecule spectroscopy to study the mechanisms and dynamics of enzymatic reactions.  We obtained the rates for single-molecule conformational active-site open-close fluctuation and correlated enzymatic reactions.  We have demonstrated a specific statistical analysis to reveal single-molecule FRET anti-correlated fluctuations from a high background of fluorescence correlated thermal fluctuations.  Our new approach is applicable to a wide range of single-molecule AFM-FRET measurements for protein conformational changes under enzymatic reactions, including AFM-FRET control of enzymatic reactivity by mechanical-force manipulating protein conformations.

Federal Research Grants and the New Public Access Mandates

Robert Correll

Library,
North Dakota State University,

Monday April 18, 3:00-4:00pm, Refreshments at 2:30.

221 South Engineering,

Over the past few years, federal funding agencies have begun implementing new requirements for researchers receiving grants to ensure public access to research data and publications. These requirements will have significant effects on both researchers and the broader scientific publishing ecosystem. This talk will situate the new mandates within the broader conversation about open access and scholarly communications, and discuss how researchers can comply with the new requirements in order to remain eligible for federal funding. Several common misconceptions about the mandates, public access, and copyright more generally will be addressed, and potential challenges for researchers, publishers and librarians will be discussed.

One Bend, Two Bend: Stepping Towards a Complex Folded Object

Andrew B. Croll

Department of Physics,
Materials and Nanotechnology Program,
North Dakota State University,

Tuesday April 11, 3:00-4:00pm,

221 South Engineering

Crumpled thin films form a very unique jammed state of matter.  They are both lightweight and ridged, suggesting broad industrial relevance.  While researchers have theorized over the origins of these properties, very little experimental work has been performed directly – collecting both structural and material properties in concert.  Without testing the strength and interplay of the basic structures making up the larger object (bends, folds, and d-cones) it is difficult if not impossible to completely trust the origin of various material properties and processes (modulus, aging behaviour).  Here we show our preliminary examination of these complex systems.  Specifically we will show that laser scanning confocal microscopy can be used to image geometry directly in concert with the recording of traditional macroscopic measurements (e.g. force vs displacement).  Our early results lead to two conflicting broad statements 1.) materials matter and 2.) there is still some hope for generality.

Soft Colloidal Particles in Crowded Environments

Alan Denton

Department of Physics,
North Dakota State University,

Tuesday April 4, 3:00-4:00pm,

221 South Engineering

Soft colloidal particles have inspired great interest recently for their rich and tunable properties, both individually and collectively.  Interdisciplinary research is driven by applications in the chemical, biomedical, food, consumer care, petroleum, and pharmaceutical industries.  Perhaps the simplest example of a soft colloid is a linear polymer coil, which can be modeled as a deformable particle, whose size and shape respond to confinement.  Another example, of particular interest for applications, is a microgel particle -- a microscopic porous network of cross-linked polymers that, when dispersed in water, swells in size and can acquire charge through dissociation of counterions.  The equilibrium size of an ionic microgel is governed by internal osmotic pressure, tunable by varying external stimuli, such as pH, salt concentration, and temperature.  By combining theory and simulation, we demonstrate that (1) crowding by nanoparticles affects the sizes and shapes of polymer coils, with implications for the structure and function of biopolymers, and (2) self-crowding induces microgels to deswell, enabling them to penetrate pores smaller than their dilute size, with potential importance for drug delivery, microfluidics, and filtration.

Single-Molecule Electronic Measurements of the Dynamic Flexibility of Histone Deacetylases

Jamie Froburg

Graduate Student,
Department of Physics
NDSU

Monday, February 22, 2015, 3:00-4:00pm

221 South Engineering

Due to their involvement in epigenetic regulation, histone deacetylases (HDACs) have gained considerable interest in designing drugs for treatment of a variety of human diseases including cancers. Recently, we applied a label-free, electronic single-molecule nano-circuit technique to gain insight into the contribution of the dynamic flexibility in HDACs structure during the course of substrates/ ligands binding and catalysis. We observed that HDAC8 has two major (dynamically interconvertible) conformational states, “ground (catalytically unfavorable)” and “transition (catalytically favorable)”. In addition, we found that its cognate substrates/ligands reciprocally catalyze the transition of the ground to the transition state conformation of HDAC8. Thus, we propose that both enzymes and their substrates/ligands serve as “catalysts” in facilitating the structural changes of each other and promoting the overall chemical transformation reaction. Such new information provides the potential for designing a new class of mechanism-based inhibitors and activators of HDAC8 for treating human diseases.

When experts struggle with physics, and how that can inform teaching

Alistair McInerny

Graduate Student,
Department of Physics
NDSU

Monday, February 22, 2015, 3:00-4:00pm

221 South Engineering

As experts, Physics instructors typically see the problems and concepts they assign differently than students.  The presented study attempted to place physics faculty (i.e., experts) in the shoes of their students (novices).  Specifically, 20 physics faculty were asked to solve a non-typical problem selected from the end of a chapter in a text book for an introductory physics course for science and engineering majors.  While the problem required the application of concepts and principles studied in an introductory course, problems of this type are not typically discussed in those courses.  As such, for most physics faculty, this problem presented a novel situation.   It turned out that most of the faculty were not able to solve that problem within 30 minutes, while they already knew the answer to an analogous question.  This phenomenon highlights some important aspects of teaching and testing that might be overlooked by instructors because as experts they are no longer able to relate to the difficulties students encounter as physics novices.

Basic Theory of Fluctuating Lattice Boltzmann

Alexander Wagner

Associate Professor,
Department of Physics
NDSU

Monday, February 1, 2015, 3:00-4:00pm

221 South Engineering

Lattice Boltzmann simulation is the name of a simulation method based on a discrete version of the Boltzmann method. These methods are most commonly used to simulate the Navier Stokes equations, but other applications include diffusion, Cahn Hilliard, shallow water, and even the Dirac equation.

Since the Boltzmann equation is an averaged equation, it the dynamics is deterministic and fluctuations are averaged out a priori. However, this precludes the methods from looking at a number of important phenomena including Brownian motion of a particle and nucleation phenomena. In this talk I will present a (more) consistent way of re-introducing fluctuations into the lattice Boltzmann method, using the simplest possible example of lattice Boltzmann for the diffusion equation.

Polymers, rivets, and airframe coatings 

Aaron Feickart

Graduate Student,
Department of Physics
NDSU

Monday, February 16, 2015, 3:00-4:00pm

221 South Engineering

For coatings that require long service lives, like those used in the automotive and aerospace industries, economic and environmental concerns necessitate a thorough understanding of expected coating failure modes. Such coatings are exposed to large temperature gradients, mechanical stresses, radiation, and aggressive species like salt and water, providing a host of variables to consider when testing and approving new coatings for widespread use. Little is understood about substantive complete links between microscopic degradation, mesoscopic ablation, and overall failure modes of coatings in service. Further, while test methods exist for general classes of automotive and aerospace coatings, such tests do not exist for specialty conductive coatings. We discuss first steps toward a test methodology for analyzing such coatings for suitability for field use and expected failure modes. In particular, we address characterization methods and design modeling for test coupons intended for environmental chamber inclusion.

Molecular dynamics for photocatalytic and gas storage applications: metal-organic super-containers and porous silica

Wendy Sapp

Department of Chemistry
University of South Dakota

Monday, January 25, 2015, 3:00-4:00pm, Refreshments at 2:30.

221 South Engineering

Molecular dynamics (MD) simulations are an invaluable tool in the computational chemistry field.1 Two centuries ago, the motions of gas molecules were theorized. Today, we can deterministically follow the law of motion for each molecule in an ensemble, representing interfaces of different phases: solid and gas or solid and liquid. Within each atomistic model, computer codes and software can describe interatomic forces with different approaches.

Two applications are presented that display different approaches to molecular dynamics simulations. First, force field MD are used to evaluate the utilization of metal-organic super containers as a particular case of metal-organic framework (MOF) gas storage devices. Data obtained from MD serves as a guide for the derivation of analytical equations that can be used to describe and explain the mechanism of gas desorption from within the cavity, which is activated by heat. Second, ab initio MD are used to investigate photocatalytic processes of a heterogeneous system which consists of a periodic and porous silica substrate with an imbedded titanium atom and water molecules filling the pore. A water-splitting mechanism can be identified using simulation of nonadiabatic electron dynamics, facilitated by MD and computed using the density matrix method.

Additionally, the modeling of this system has shown electron and hole trap states that could facilitate a chemical reaction.2
Ultimately, ground state thermal molecular dynamics enables electron dynamics through non-adiabatic coupling. Charge transfer dynamics, made possible via MD and triggered by a photoexcitation, leads to change of electronic configuration. A modification of electronic configuration leads to the change of inter atomic forces and thus changes the trajectory of MD leading to bond breaking and formation. The utilization of various forms of MD can provide useful data for a wide variety of properties and processes in various materials under different conditions for broad range of technology-related applications: from gas storage to solar energy harvesting.

1.    Marx, D. H., Jürg, Ab Initio Molecular Dynamics: Basic Theory and Advanced Methods. Cambridge University Press: London, 2012.
2.    Liu, I.-S.; Lo, H.-H.; Chien, C.-T.; Lin, Y.-Y.; Chen, C.-W.; Chen, Y.-F.; Su, W.-F.; Liou, S.-C., Journal of Materials Chemistry 2008, 18 (6), 675-682.

Destabilization of Lipid Membranes by Amphipathic Peptides

Carly Snell and Sylvio May

Department of Physics
North Dakota State University

Monday, November 30, 2015, 3:00-4:00pm, Refreshments at 2:30.

221 South Engineering

In this talk we investigate the influence of amphipathic alpha-helical peptides that reside at the hydrocarbon chain-headgroup interface of a lipid bilayer on the curvature elastic properties of the host membrane.  We specifically seek to describe a mechanism of how these peptides can shift the Gaussian modulus to positive values, possibly triggering an instability with respect to the formation of saddle curvatures. To this end, we employ a molecular lipid model that accounts for repulsive interactions between lipid headgroups, for the energy to stretch/compress the hydrocarbon chains, and for the interfacial tension acting at the hydrocarbon chain-headgroup interface. Electrostatic interactions are not yet included explicitly into the model. The peptides are modeled as uncharged cylinders that partition in a parallel orientation to the hydrocarbon chain-headgroup interface where they affect the space available to the lipid headgroups and chains. The penetration depth into the hydrocarbon chain region is determined by the angular size of the peptide's hydrophilic region. We demonstrate that peptides with a small angular size (but not those with a large angular size) have an intrinsic tendency to render the Gaussian modulus more positive. For realistic choices of the lipid's molecular interaction parameters we indeed find the possibility of a peptide-induced sign change of the Gaussian modulus. The corresponding mechanism the peptides employ reflects an effective decrease of the headgroup repulsion strength upon peptide insertion.

Electrostatic Interactions at Dielectric Interface: From Membranes to Colloids

Guilherme Volpe Bossa (Comprehensive Exam Talk)

Department of Physics,
NDSU

Electrostatic Interactions at Dielectric Interfaces: from Membranes to Colloids

Monday, November  23, 2015, 3:00-4:00pm

221 South Engineering

In this talk I will cover some of the projects that have been carried out during my PhD.  The first project refers to a spherical nanoparticle trapped at the dielectric interface between air and water.  In such a scenario, the mismatch of dielectric constants causes an asymmetric charge distribution over the particle surface
and gives rise to long range repulsive forces. Based on the Poisson-Boltzmann theory and solving a mixed boundary-value problem, we developed a model that determines the electrostatic potential everywhere in the system in terms of the distinct charge distributions, salt concentration, and the different dielectric constants in water, air and inside the particle.  In the second project we analyzed ion specific effects in a system constituted by a flat negatively charged solid surface exposed to a symmetric 1:1 electrolytic solution.  Upon the addition of a non-electrostatic cation-cation repulsion to the electrostatic Coulomb potential (an extension of the classical Poisson-Boltzmann theory known as the Poisson-Helmholtz-Boltzmann model), we could relate the formation of a Stern layer to the effective size of hydrated ions.  Besides these two major topics, I will also talk about the line tension between charged membrane domains and lipid correlations.

Infrared Astronomy from the Ground,  Air, and Space

Robert D. Gehrz

Minnesota Institute for Astrophysics,
University of Minnesota

Monday, November 9, 2015, 3:00-4:00pm, Refreshments at 2:30.

221 South Engineering

Infrared (IR) astronomical observations, beginning in the late 1960’s, have provided new understanding of the origin, content, and chemical evolution of the Universe.   Early observations in the field were made with ground-based observatories that set the stage for more advanced facilities recently developed by NASA.   A brief history of the field will be presented.  Observations with the Wyoming Infrared Observatory, and NASA’s Spitzer Space Telescope and Stratospheric Observatory for Infrared Astronomy (SOFIA) will be presented.  Anticipated observations with NASA’s James Webb Space Telescope will be described.

Investigating the Impact of Metacognitive Interventions on Student Reasoning

Nathaniel Grosz

Department of Physics,
NDSU

Monday, November 2, 2015, 3:00-4:00pm.

221 South Engineering

This study was motivated by research findings that suggest that, on certain topics, student conceptual and reasoning difficulties persist even after instruction expressly designed to address such difficulties. We have been developing a suite of different metacognitive interventions for use in introductory calculus-based physics courses. Interventions intended to promote (individual) student metacognition were administered in a web-based format outside of class. Interventions supporting socially mediated metacognition were implemented as part of laboratory instruction. The metacognitive interventions were multi-layered in order to address specific types of student responses. The impacts of these interventions were assessed on a course exam. In this presentation, preliminary results will be presented and implications for instruction will be discussed.

Electrical Double Layer Structure at the Water-Silica Interface: Role of Counterion Size, pH and Hydration Interactions

Matthew A. Brown

Laboratory for Surface Science and Technology,
Department of Materials,
ETH Zürich, Zurich Switzerland.

Monday, October 5, 2015, 3:00-4:00pm

Refreshments at 2:30.

221 South Engineering

The structure of the electrical double layer has been debated for well over a century, since it mediates colloidal interactions, regulates surface structure, controls reactivity, sets capacitance and represents the central element of electrochemical supercapacitors. The surface potential of such surfaces generally exceeds the electrokinetic potential, often substantially. Traditionally, a Stern layer of non-specifically adsorbed ions has been invoked to rationalize the difference between these two potentials; however, the inability to directly measure the surface potential of dispersed systems has rendered quantitative measurements of the Stern Layer potential, and other quantities associated with the Outer Helmholtz Plane, impossible. Here we describe our development of X-ray photoelectron spectroscopy (XPS) from a liquid microjet to measure the absolute surface potentials of silica nanoparticles dispersed in aqueous electrolytes. We quantitatively determine the impact of specific cations (Li+, Na+, K+, and Cs+) in chloride electrolytes on the surface potential, the location of the shear plane and the capacitance of the Stern layer. We find that the magnitude of the surface potential increases linearly with hydrated cation radius. Interpreting our data using the simplest assumptions and most straightforward understanding of Gouy-Chapman-Stern theory reveals a Stern layer (bounded by the Outer Helmholtz Plane) whose thickness corresponds to a single layer of water molecules hydrating the silica surface, plus the radius of the hydrated cation. We describe a modified Poisson-Boltzmann (PB) model that adds hydration repulsion between counterions to their Coulomb interaction. While retaining much of the simplicity of the classical PB model, this modified model predicts surface potentials and Stern layer thicknesses for the different counterions that are in excellent agreement with the experiments.

On How Being Soft and Squishy Affects Phase Behavior: The Case of Charged-Microgel Suspensions

Alberto Fernandez-Nieves

Associate Professor,
School of Physics,
Georgia Institute of Technology

Monday, October 5, 2015, 3:00-4:00pm

Refreshments at 2:30.

221 South Engineering

Microgels are an interesting class of mesoscopic soft particles that can deform and compress. When suspended in a solvent at large number density, they are able to crystallize and form glasses. However, for microgels, this depends on single-particle stiffness. We will discuss recent results with charged microgel particles with different stiffness. We will first show that the suspension osmotic pressure is controlled by those counterions in solution that are able to escape from the electrostatic attraction exerted by the microgel particles. We will then exploit this fact to obtain the particle volume fraction, f, and the microgel volume as a function of particle concentration, even in highly overpacked states, where the particles are forced to both change shape and compress. In terms of f, we find that the width of the fluid-crystal coexistence region decreases with decreasing microgel stiffness to eventually disappear for sufficiently soft microgels; in these cases, the suspensions remains fluid-like at all explored concentrations. By comparing our results with expectations from computer simulations, we propose possible interparticle-interactions that could potentially capture our experimental observations.

Connecting Physics Education Research across the Disciplines

Warren Christensen

Assistant Professor,
Department of Physics,
NDSU

Monday, September 28, 2015, 3:00-4:00pm

221 South Engineering

Discipline-based Education Research is the descriptor of research that focuses on the learning and teaching of science and mathematics at the Undergraduate level. Studying the connections that students make (or don't make) with physics from mathematics and biological sciences has driven my research since coming to NDSU. I will present on data from several current NSF funded studies. Investigating the flipped course curriculum of an algebra-based course designed around the physics of biomedical equipment has the challenges/benefits of a tremendous data set that we’re unpacking using a modification of Bloom’s Taxonomy, with undergraduate researchers Matt Urich and Levi Remily. An entirely different thread of research investigates content for a research-based Math Methods course. Brian Farlow and I are building on work of Marlene Vega, a 2015 REU student, investigating student’s use and understanding of coordinate systems through free-response questions and student interviews. I hope to touch on how these studies fit into my broader research agenda and what the outlook for these studies are going forward.

Pushing 2D Block Copolymers into the Third Dimension.

Andrew B. Croll

Assistant Professor,
Physics Department and Materials and Nanotechnology Program,
NDSU

Monday, September 21, 2015, 3:00-4:00pm

221 South Engineering

Block copolymers have received considerable attention due in large part to the fascinating nanoscopic patterns that form within.  Not surprisingly, this internal structure is heavily influenced by the system boundaries and many interesting differences arise when block copolymers are confined into thin film geometries.  While there is still much to be learned about the influence of thin film confinement itself, considerably less is known about how these quasi 2D objects are modified by their embedding in the 3rd dimension.  In this talk we examine some of the problems encountered as a thin diblock copolymer film is bent, when it it is quenched between different equilibrium states or when it is forced to exist on a closed spherical surface.

Pattern formation in block copolymer layers after a temperature quench

Alexander Wagner

Physics Department,
NDSU

Monday, February 16, 2015, 3:30-4:30pm

221 South Engineering

In recent experiments of Andrew Croll he found an interesting pattern formation phenomenon in thin block-copolymer films. These films will layer parallel to the substrate and subsequent changes in temperature can change the volume fraction of the top layer, leading to interesting patterns. Here we propose a simple numerical model of coupled two-dimensional phase-separation models that can reproduce many of the phenomena observed experimentally.

The Electrostatic Contribution to the Line Tension Between Lipid Membrane Domains

Guilherme Volpe Bossa

Graduate Student,
Department of Physics,
NDSU

Monday, Oct. 12, 2015, 3:00-4:00pm,

221 South Engineering

The line tension, which characterizes the excess free energy per unit length of the boundary between different lipid membrane domains, is one of the factors that determines domain size and dynamics. Consequently, experimental methods and corresponding modeling studies related to the line tension continue to attract significant interest. Considering a planar binary lipid layer with two domains consisting of neutral and anionic lipids, we calculate the electrostatic contribution to the line tension at the domain boundary using mean-field electrostatics. The influence of lipid mobility in each phase is studied through solutions of the Poisson-Boltzmann equation for different sets of boundary conditions that include fixing the local surface charge density or surface potential, or allowing the lipids to migrate subject to a demixing entropy penalty. We find the electrostatic contribution to the line tension to be negative with magnitudes on the order of piconewton close to physiological conditions.

A Robust Nonlinear Block Copolymer Nanoreactor-Based Strategy to Monodisperse Hairy Nanocrystals with Precisely Controlled Dimensions, Compositions and Architecture.

Dr. Zhiqun Lin

Professor, School of Materials Science and Engineering, Georgia Institute of Technology

<Special Time!!> Tuesday, May 5, 2015, 3:00-4:00pm (Refreshments will be served at 2:30).

<Special Location!!>271 Bachellor Technology Center

Nanocrystals exhibit a wide range of unique properties (e.g., electrical, optical, and optoelectronic) that depend sensitively on their size and shape, and are of both fundamental and practical interest. Breakthrough strategies that will facilitate the design and synthesis of a large diversity of nanocrystals with different properties and controllable size and shape in a simple and convenient manner are of key importance in revolutionarily advancing the use of nanocrystals for a myriad of applications in lightweight structural materials, optics, electronics, photonics, optoelctronics, magnetic technologies, sensory materials and devices, catalysis, drug delivery, biotechnology, and among other emerging fields. In this talk, I will elaborate a general and robust strategy for crafting a large variety of functional nanocrystals with precisely controlled dimensions (i.e., plain, core/shell, and hollow nanoparticles) by capitalizing on a new class of unimolecular star-like block copolymers as nanoreactors. This strategy is effective and able to produce organic solvent-soluble and water-soluble monodisperse nanoparticles, including metallic, ferroelectric, magnetic, luminescent, semiconductor, and their core/shell nanoparticles, which represent a few examples of the kind of nanoparticles that can be produced using this technique. The applications of these functional nanocrystals in energy-related applications (i.e., solar cells and photocatalysis) will also be discussed.

Combinatorial Properties of the Six-Vertex Model

Jessica Striker

Department of Mathematics
NDSU

Monday, May 4, 2015, 3:00-4:00pm.

221 South Engineering

Much work has been happening on the boundary of statistical mechanics and combinatorics in recent years, and, in particular, in the area of lattice models and exactly solvable models. In this talk, we focus on the six-vertex model on the square lattice with domain-wall boundary conditions. Configurations of this model on the n-by-n square lattice are known to be counted by a very nice formula; this long-standing combinatorial conjecture was proved using tools from physics. Moreover, these configurations are in bijection with alternating sign matrices, which are simply-described matrices with entries in {0,1,-1}. Alternating sign matrices exhibit many intriguing combinatorial properties and surprising relationships with other physics models, such as loop percolation. We give a survey of the various connections between alternating sign matrices and statistical physics, along with some very recent work and several open problems.  

Structure, Dynamics and Properties of Block Polymer Dispersions

Dr. Frank S. Bates

Regents Professor and Head, Department of Chemical Engineering and Materials Science, University of Minnesota

<Special Time!!>Wednesday, April 1, 2015, 3:00-4:00pm (refreshments served 2:30).

<Special Location!!>271 Batcheller Technology Center

Block copolymers belong to a broad class of amphiphilic compounds that includes soaps, lipids and nonionic surfactants. These macromolecules assemble into micelles with molecular dimensions on the order of 5 to 50 nm in size when mixed with excess solvent that preferentially solvates one block type. This presentation will explore two different aspects of block copolymer micelle formation.The fundamental thermodynamic and kinetic factors that control micelle shape and dynamics will be discussed based on small-angle x-ray and neutron scattering (SAXS and SANS) experiments and cryogenic transmission and scanning electron microscopy results. Although the structural features displayed by amphiphilic block copolymers resemble those associated with the self-assembly of lipids and simple surfactants (e.g., spherical and cylindrical micelles and vesicles) a macromolecular architecture leads to remarkably different dynamic properties, linked to a vanishingly small critical micelle concentration. As a consequence, molecular exchange is rapidly extinguished with increasing molecular weight resulting in non-ergotic behavior. These concepts have been exploited in developing a recently commercialized technology that provides immense improvements in the fracture toughness of thermosetting epoxy plastics, which also will be described.

Neutrinos: From Cosmic Rays and Accelerators to Old Iron Mines and the Fate of the Universe

Alec Habig

Department of Physics,
Minnesota State University, Duluth.
UCLA

Monday, March 23, 2015, 3:00-4:00pm, Refreshments at 2:30.

221 South Engineering

Neutrinos are fundamental particles which interact only weakly with other matter, and had been thought to be massless.  However, if they did have some non-zero mass, they would change flavors as they fly along.  This talk follows the search for these "oscillations" in neutrinos coming from cosmic-ray interactions with the Earth's upper atmosphere starting with the Super-Kamiokande experiment.  The MINOS experiment is making precise measurements of the phenomenon by creating an intense, well-understood neutrino beam at Fermilab (near Chicago) and observing it 735km away at the Soudan Mine in Northeast Minnesota.  The new NOvA experiment has started observations of the same beam in a different way, to help nail down more subtle neutrino changes.  Lastly, the implications of a whole lot of slightly-massive neutrinos sloshing around the universe are discussed.

A Bias-Free Algorithm for Diffusion-Limited Aggregation

Yen Lee Loh

Department of Physics,
University of North Dakota

Monday, March 9, 2015, 3:00-4:00pm, Refreshments at 2:30.

221 South Engineering

Diffusion-limited aggregation (DLA) is a process where a particle is launched randomly and wanders around via Brownian motion until it meets a cluster, at which point it sticks to the cluster.  Repeating this process many times causes the cluster to grow into a complex, fractal, tree-like pattern.  Just as the Ising model is a cornerstone of equilibrium statistical physics, DLA is a cornerstone of nonequilibrium statistical physics. 

Traditional simulations of DLA on a lattice suffer from bias when launching particles and when executing the random walks.  Does this affect the results?

In this talk I will give an interesting pedagogical introduction, aimed at undergrads and grad students, which connects continuum diffusion and lattice diffusion to electrostatics problems such as the capacitance of a cube and the resistance of a wire grid.  I will present results on lattice Green functions and use these to construct a DLA algorithm in which bias has been eliminated (i.e., reduced from about 1% to about 10^{-12}).  With this algorithm I grew clusters of 10^8 particles on 65536 x 65536 lattices.  The results are consistent with claims in the literature that lattice DLA clusters inevitably grow into anisotropic shapes, and that the fractal dimension evolves from the continuum DLA value (D = 1.71) for small disk-shaped clusters towards Kesten¹s bound (D=3/2) for highly anisotropic clusters with long protruding arms.  It appears that cluster geometry is influenced primarily by the anisotropy of the aggregation rule, rather than the anisotropy of the diffusion rule.  Thus, although simulations in the literature contained launching bias and diffusion bias, there is no significant error in the results.

Nanoparticles at an Air-Water Interface:a Mixed Boundary Value Problem

Guilherme Bossa

Graduate Student,
Department of Physics,
NDSU

Monday, February 23, 2015, 3:00-4:00pm.

221 South Engineering

Charged nanoparticles trapped at air-water interface have various applications in materials and biological sciences. Besides the difference of dielectric constants between the media, very often this system is characterized by an asymmetric distribution of charges  over the particle surface.  To account for these facts and describe the electrostatic potential in all regions , we model the particles as spheres with distinct charge densities at each half: one exposed to the air, where is valid the Laplace equation and the other one exposed to the water, where is valid the non-linear Poisson-Boltzmann equation. By solving a mixed (Robin) boundary value problem this model allows us to predict the effective charge at the air exposed surface and study how the interaction between the particles is affected by their dielectric constant and salt concentration.

Infectious diseases, auto-immune diseases, and opportunities for biophysics 

Gerard C. L. Wong

Bioengineering Dept., Chemistry & Biochemistry Dept.,
California NanoSystems Institute
UCLA

Monday, February 16, 2015, 3:00-4:00pm, Refreshments at 2:30.

221 South Engineering

We present examples from our group where biophysics impacts unsolved medical problems. We start with bacterial biofilms, which are structured multi-cellular communities that are fundamental to the biology and ecology of bacteria. The first step in biofilm formation, adaptation to life on a surface, requires the coordination of biochemical signaling, polysaccharide production, and molecular motility motors. These crucial early stages of biofilm formation are at present poorly understood. By adapting tracking algorithms from colloid physics, we dissect bacterial social behavior at the single cell level.  We will also discuss how we can learn from innate immunity peptides, and renovate antibiotic design via the biophysics of peptide-membrane interactions. Finally, we examine the pathological role of antimicrobial peptides in autoimmune diseases. 

How Fluid Flow Affects Phase-Separation Front Formed Morphologies

Eric Foard

Visiting Researcher,
Dept. Physics, NDSU

Monday, February 9, 2015, 3:00-4:00pm.

221 South Engineering

We previously studied the highly ordered morphologies generated in the wake of an idealized, abrupt, phase-separation front moving with an imposed velocity through a diffusive binary material.  We found such a system in two dimensions will form ordered lamelle oriented either parallel or orthogonal with respect to the front, and regular hexagonal droplet arrays, with the favored morphology dependent upon volume fraction and phase-separation front speed.  in three dimensions we find additional ordered cylindrical phases.

In this talk I explore what effect hydrodynamics has on the orthogonal lamella phase.  We find that, even at high viscosities, the effect of hydrodynamic flow is significant, and occur for reasons that may surprise you.

Void Formation in Cross-linked Polymer Networks

Aaron Feikhart

Dept. of Polymers and Coatings, and Dept. Physics,
NDSU

Monday, February 2, 2015, 3:00-4:00pm.

221 South Engineering

Crosslinked polymer networks play a pivotal role in materials research and the coatings industry, in part due to their resilience to the permeation of aggressive ionic species. In this talk, we present molecular dynamics simulations of such polymer networks. In particular, we investigate the presence and morphology of pore structures that result from cavitation during cooling, a phenomenon that takes place independently of the polymer's glass transition. We will also discuss applications of these results.

The Emerging Role of Network Analysis in Physics Education

Eric Brewe

Assistant Professor of Science Education,
Physics Education Research Group,
Florida Florida International University.

Monday, January 26, 2014, 3:00-4:00pm.  221 South Engineering

Join us for Refreshments at 2:30

Network Analysis is an approach to analyzing data which are relational in nature. With origins in quantitative sociology and more recent development in graph theory, Network Analysis is a rapidly growing interdisciplinary approach. The emergence of Network Analysis in education is the result of a recognition that student interactions naturally give rise to relational data, and that this has far-reaching consequences.  In this talk, I will provide several examples from physics education of how network analysis is being applied to the analysis of informal student communities, classroom communities, diagnostic tests such as the Force Concept Inventory, and even to investigate the retention and persistence of students in the physics major. 

GPS 101

Kent Ridl

Graduate Student
Department of Physics
North Dakota State University.

Monday, January 12, 2014, 3:00-4:00pm.

221 South Engineering

From its beginnings as a nearly-canceled experiment of the Department of Defense, the Global Positioning System has evolved to become the very definition of “ubiquitous” in today's technical society.  This seminar will provide a broad introduction to the history, components, and many uses of the technology.  The system's basic theory of operation will also be presented, touching on elements of receiver operation and satellite functionality.  Special emphasis will be placed on the many examples of applied physics throughout the system.

Elasticity-based mechanism for collective motion in natural and artificial swarms.

Cristián Huepe

Unaffiliated Research Scientist in Chicago supported by the National Science Foundation
and Visiting Scholar Applied Math Department
Northwestern University.

Monday, December 15, 2014, 3:00-4:00pm, Refreshments at 2:30.

221 South Engineering

Collective motion is one of the simplest forms of self-organization in systems of active components such as cell colonies, bird flocks, fish schools, or groups of autonomous robots.  Its emergence in fluid-like swarms with aligning interactions has been the focus of much research activity. In this talk, I will introduce a different model for collective motion, consisting of self-propelled particles connected by linear springs without explicit aligning dynamics. In this system, a simple elasticity-based mechanism drives the particles to self-organize by cascading self-propulsion energy towards lower-energy modes. Given its ubiquity, this mechanism could play a relevant role in various natural and artificial swarms.

2D block copolymer films embedded in a 3D world

Dr. Andrew B. Croll

Assistant Professor, Department of Physics and Materials and Nanotechnology, NDSU

Monday, December 8, 2014, 3:00-4:00pm.

221 South Engineering

Block copolymers are long chain molecules which are capable of self-assembling into many fascinating nanostructures.  Research has largely focused on adapting and understanding the material when it is in thin film form in order to meet industrial goals.  While our understanding of block copolymer thin films is now fair, very little research has investigated what new and unique functionality can be gained by forcing a film into a more complex three dimensional geometry.  Here we describe 3 excursions of a thin block copolymer film into the larger 3D world.  First we show how dynamic 'tricks' and thermodynamics can allow lamella forming block copolymer to develop much more complex out of plane morphologies.  We next discuss how adding curvature to a block copolymer film can lead, not just to a more interesting structure, but to unique insights into very basic material properties.  Finally, we discuss our recent investigations of popular "polymersomes" structure.  In particular, we show how the sessile drop geometry can form an ideal, minimal mechanical measurement scheme.

Microgels as Chemical Sensors and Drug Delivery Vehicles

Dr. Alan Denton

Associate Professor, Physics, NDSU

Monday, December 1, 2014, 3:00-4:00pm.

221 South Engineering

Microgels are microscopic gel particles that are swollen by a solvent.  Composed of porous, elastic networks of crosslinked polymers, microgels are soft colloids that can encapsulate dye molecules or drugs.  Their sensitive response to environmental conditions (e.g., temperature and pH) and influence on flow properties suit microgels to widespread applications in the chemical, pharmaceutical, food, and consumer care industries.  When dispersed in water, polyelectrolyte gels become charged through dissociation of counterions.  The permeability of ionic microgels, and the prevalence of tunable electrostatic forces, lead to unique materials properties.  Within a coarse-grained model of polyelectrolyte solutions, we compute the osmotic pressure of bulk dispersions via molecular dynamics simulations.  Within a cell model, we derive an exact formula for the electrostatic contribution to the osmotic pressure inside of a single gel particle, which we validate using Poisson-Boltzmann theory.  When combined with the elastic contribution to the osmotic pressure, our result can predict swelling and mechanical integrity of ionic microgels, with potential relevance for chemical sensing and drug delivery.

Nitrogen Hydrides Towards Massive Star Forming Regions

Cody Gette

Bonn University

Monday, November 24, 2014, 3:00-4:00pm.

221 South Engineering

Recent Herschel/HIFI observations toward a sample of 100 massive star forming clumps selected from the ATLASGAL submillimeter wavelength survey of the Galactic plane resulted in the detection of 64 sources in the NH 974 GHz 1_0-0_1 ground state rotational transition in absorption. In addition, 6 new sources were observed in NH_2 at 462 GHz with the Atacama Path Finder telescope (APEX). Previously obtained data increases that sample to 12 sources. Using this large sample, we aim to find new information regarding the location and properties of these two simplest nitrogen hydrides, for which relatively little data exists. We find that NH seems to be associated with H2O absorption and therefore is found in regions containing H_2. We also find NH can be used as a tracer of infall since 45% of sources show redshifted lines with a velocity shift greater than 1 km/s. Finally, the NH:NH2 column density ratio has been measured in 10 sources with an average ratio of 7.4:1, agreeing with other recent HIFI observations. This indicates that NH is on average more abundant than NH_2.

Effects of substrate on structural and electronic properties of laser-crystalized silicon films.

Matthew Semler

Physics, NDSU

Monday, November 17, 2014, 3:00-4:00pm.

221 South Engineering

A study on the impact of the substrate used for laser crystallization of silicon films will be presented.  The morphology, crystallinity, and electronic properties of the intrinsic and doped films on different substrates have been characterized using optical, atomic force, and scanning electron microscopy, Raman spectroscopy, X-ray diffraction, and resistivity measurements.  Preliminary results show a significant dependence of the properties of the film on the substrate.

Transdermal Therapeutic Systems: Structure, Function and Exelon(r) as a convincing example.

Dr. Alfred Fahr

Department of Pharmaceutics, Friedrich-Schiller-University Jena, Germany.

Monday, November 10, 2014, 3:30-4:30pm (Refreshments will be served at 3:00).

221 South Engineering

Transdermal Therapeutic Systems (TTS) have made their way over the last decades as an alternative to oral formulations not only for circumvention of the first pass effect as for example delivering nitroglycerine, but also for maintaining steady plasma level values and for reducing serious side effects during therapy. A very good example for the latter case is the Exelon TTS(r) with its active ingredient rivastigmine, which is used in therapies against Alzheimer's disease.

Encapsulation of solutes in lipid vesicles: Origins of life considerations.

Dr. Tereza de Souza

Research Associate, Earth and Planetary Sciences, Harvard University

<Special Time!!> Friday, November 7, 2014, 3:00-4:00pm (Refreshments will be served at 2:30).

221 South Engineering

In the past years we have investigated the entrapment of macromolecules inside vesicle by analyses of cryo-transmission electron microscopy of liposome populations created in the presence of ferritin, ribosomes and small macromolecular aggregates. And surprisingly, results reveal that the local (intra-liposomal) macromolecules concentration in these liposomes largely exceeds the bulk concentration. This seminar aims to summarize and discuss these results under the light of the origins of life scenario and synthetic biology, addressing question as: What is the minimal size of a minimal cell? Is the number of entrapped macromolecules inside vesicles homogeneous? Lipid compartments are frequently addressed as agents of confinement and protection, but could these compartments have a more active role in the pre-biotical scenario? In another words, does the formation of lipid compartments play a role in concentrating the "entrapped to be" macromolecules?

Promoting and assessing student metacognition in physics.

Alistair McInerny

Research Associate, Earth and Planetary Sciences, Harvard University

Monday, October 27, 2014, 3:00-4:00pm.

221 South Engineering

A scaffolded metacognition activity was incorporated into the laboratory component of the introductory physics course at Western Washington University (WWU) and Whatcom Community College (WCC). Each week, students wrote reflectively to contrast their initial and current understanding of a specific physics topic, and described the “trigger” events that led them to change their thinking. Goals were to enhance conceptual understanding as well as the depth and quality of student reflection. A coding scheme was developed to evaluate student reflections. We present the scaffolded activity and coding scheme, as well as preliminary findings about changes in student reflection over time and correlations between amount of reflection and conceptual learning.

How do Student Evaluations of Instruction Relate to Students’ Conceptual Learning Gains?

Dr. Warren Christinsen

Assistant Professor, Department of Physics, NDSU

Monday, November 3, 2014, 3:00-4:00pm.

221 South Engineering

Across the United States, Student Evaluations of Instruction (SEIs) are often the primary (if not only) metric used to evaluate the quality of an instructor. Although SEIs probably reflect student attitudes towards the class in some way, it is not clear to what extent SEI scores represent how much students learned in the class.  This study looks at data from faculty volunteers who were recruited from a pool of recent attendees of the APS’s New Faculty Workshop. The study solicited numerous forms of class artifacts from these faculty including student evaluations of instruction and multiple-choice conceptual survey data. The data indicate that there is no correlation between SEI ratings and normalized learning gains on the FCI, or other instruments. Thus, it appears that faculty receiving high (or low) evaluations from their students has no connection to how much conceptual understanding their students developed throughout the semester.

Ruga Mechanics of Folding Atomic-Layer Nanostructures.

Professor Kyung-Suk Kim,

Institute of Molecular and Nanoscale Innovation, Brown University.

Monday, October 20, 2014, 3:00-4:00pm (Refreshments will be served at 2:30).

<Special Location!!>  271 Batcheller Technology Center

When one folds a thin solid film that is only a few atoms thick, such as graphene, the film properties can be controlled for various functions. For example, a simple compression of the folded film can change optical, electrical, wetting and adhesion characteristics of the film, and can be used for making multifunctional materials such as transparent electric circuits, self-cleaning surfaces, oil-spill cleaning cloths and self-adjusting friction grips. Such atomic-layer nanostructures can be folded and self-organized by nonlinear large deformation of soft material substrates. In particular, nano science and technology has enabled us to explore new functional properties of hierarchically ruga-structured materials through folding or wrapping thin atomic-layer structures with nanometer scale features. The Latin word ruga means a state of a “large-amplitude” wrinkle, crease, fold or ridge to form various 1-D or 2-D patterns. As multi-scale surface morphologies of rugae determine effective properties such as wetting, adhesion, friction, flexoelectric and optoelectronic properties, ruga state control is considered as a viable method for real-time regulation of effective material properties. It is found that graded or layered elastic properties of the substrate can provide diverse bifurcation paths of the attached atomic-layer deformation under lateral compression, producing various atomic-layer ruga states. Nonlinear mechanics of soft-material substrate enables us to construct ruga-phase diagrams. As an example, a mathematical analysis of sequential bifurcation processes of hyper-elastic neo-Hookean substrates is used to construct generic ruga-phase diagrams. When an atomically layered structure such as multi-layer graphene is folded by ruga control, nano-scale crinkles are generated. In general, nano-scale crinkle ridges are invisible to conventional AFM due to its peculiar flexoelectric properties. Here, a new invention of “Dual-Tip AFM Interferometer” (DT-AFMI) will be introduced, which makes the invisible visible. The DT-AFMI image reveals that the crinkle ridge of a multi-layer graphene has its ridge width less than 1.8nm. The nano-crinkle ridges have strong flexoelectric characteristics, and the crinkle ridge networks of the top graphene layer exhibit high molecular adsorptivity. Potential applications of such high molecular adsorptivity localized along the nano-ridges will be discussed as well.

For more information, contact Erik K. Hobbie, 701-231-6103, erik.hobbie [at] ndsu [dot] edu

Developing Metacognitive Skills in Conjunction with Conceptual Understanding of Physics.

Nathaniel Grosz

Physics, NDSU

Monday, October 13, 2014, 3:00-4:00pm.

221 South Engineering

Effective learners possess a diverse repertoire of metacognitive skills that they consciously deploy to support and guide their thinking. Adopting new thinking approaches is complex and demanding for novice learners, but the process can be facilitated by instructors actively supporting the development of students' metacognitive skills. As part of an ongoing investigation of student reasoning approaches in physics courses, we wish to identify instructional strategies that are effective at promoting the development of metacognitive skills in conjunction with the development of conceptual understanding of physics. We have been probing the effectiveness of such strategies across multiple learning environments (e.g., interactive lectures, laboratory). We will present data from question sequences purposefully designed to evoke metacognitive behavior. Results from individual and group work will be presented and compared. Implications for instruction will be discussed.

Ab initio electron dynamics at metal–semiconductor nano-interfaces.

Dr. Dmitri Kilin

Assistant Professor of Chemistry, University of South Dakota

FRIDAY, September 26, 2014, 3:00-4:00pm (Refreshments will be served at 2:30).

221 South Engineering

Photo-induced charge transfer at the interface of two materials is a fundamental process in (i) photovoltaic and (ii) photocatalytic applications. The photo-induced time-dependent electron dynamics are computed for different interfaces by a combination of ab initio electronic structure and time-dependent density matrix methodology. A dissipative equation of motion for the reduced density matrix for electronic degrees of freedom is used to study the phonon-induced relaxation of hot electrons in the simulated systems. Non-adiabatic couplings between electronic orbitals are computed on-the-fly along nuclear trajectories. Equations are solved in a basis set of orbitals generated ab initio from a density functional.[1] For an application to photovoltaic effect, one explores light-induced electric current in a model of a simplified photovoltaic cell composed of a Si nano-crystal co-doped with p-and n- type doping, interfacing with Au electrodes. Charge carrier dynamics induced by selected photo-excitations show that hole relaxation in energy and in space is much faster than electron relaxation. Use of the continuity equation for electric current allows to identify substantial local currents at the Si/Au interfaces and small overall net charge transfer across the slab. [2] For an application to photocatalytic water splitting, charge transfer dynamics is explored at the interface of supported metal nanocluster and liquid water. The metal cluster introduces new states into the band gap of semiconductor TiO2 surface, narrows the band gap of TiO2, and enhances the absorption strength. The H2O adsorption significantly enhances the intensity of photon absorption, which is due to the formation of metal−oxygen (water) coordination bonds at the interfaces. The metal cluster promotes the dissociation of water, facilitates charge transfer, and increases the relaxation rates of holes and electrons. [3] Reported results help in understanding basic photophysical and protochemical processes contributing to harvesting solar energy by photovoltaics and photoelectrochemical water splitting.

1. Huang, S.; Kilin, D. S., Charge Transfer, Luminescence, and Phonon Bottleneck in TiO2 Nanowires Computed by Eigenvectors of Liouville Superoperator. J. Chem. Theor. Computation 2014, 10 (9), 3996-4005.

2. Han, Y.; Micha, D.; Kilin, D., Ab initio study of the photocurrent at the Au/Si metal semiconductor nano-interface. Mol. Phys. 2014, in print, DOI: 10.1080/00268976.2014.944598.

3. Huang, S.; Inerbaev, T. M.; Kilin, D. S., Excited state dynamics of Ru10 cluster interfacing anatase TiO2(101) surface and liquid water. J. Phys. Chem. Lett. 2014, 5, 2823–2829.

Bacterial Adhesion on Material Surfaces.

Dr. Klemen Bohinc

Faculty of Health Sciences, University of Ljublijana, Slovenia

Monday, May 19, 2014, 3:30-4:30pm (Refreshments will be served at 3:00).

221 South Engineering

Bacterial adhesion can be controlled by different material surface characteristics like surface roughness, on which we concentrate in our study. Different glass surfaces were prepared by polishing the glass plates with different gradations. The corresponding surface roughness was controlled by atomic force microscope and profilometer.  The rate of adhered bacteria on glass surfaces was determined with spectrophotometer and scanning electron microscopy. Our results showed that the rate of adhered bacteria increases with increasing surface roughness. The increased adhesion of bacteria on more rough surfaces is the interplay between the increasing effective surface and increasing number of defects on the surface.

Excitions in Nanoscale Semiconductor Structures from the Bethe-Salpeter Equation.

Deyan Mihaylov

Department of Physics, North Dakota State University

Monday, May 12, 2014, 3:00-4:00pm

221 South Engineering

Deyan is a graduate student working with Andrei Kryjevski

Excitons are bound states of electrons and holes held together by Coulomb attraction. In semiconductor nanoparticles of size comparable to or smaller than the Exciton-Bohr radius excitons have significant effect on the density of states of the system. The standard way of calculating energies and wave functions of excitons is to solve the Bethe Salpeter equation (BSE) in the basis of Kohn-Sham (KS) orbitals and energies, the output of Density Functional Theory. In this work, we derive BSE using basic tools of many body quantum mechanics. Namely, we compute the matrix of the full, interacting Hamiltonian in the basis of electron-hole KS states, and, in particular, derive direct and exchange Coulomb terms present in the standard BSE. The method is then applied to calculating energies and wave functions of low energy excitons in a nm-sized Hydrogenated crystalline Silicon quantum dot.

Spectroscopic Studies of the Jovian Magnetosphere and the Atmospheres of the Galilean Satellites with Voyager, Cassini and Hubble Space Telescope.

Dr. Darrell Strobel

Departments of Earth & Planetary Sciences and Physics & Astronomy, The Johns Hopkins University

Wednesday, April 30, 2014, 3:00-4:00pm (Refreshments will be served at 2:30).

Special Location - 271 Batcheller Technology Center

This talk will review remote sensing measurements of the Jovian system with ultraviolet spectrometers over the past 35 years which led to the discovery of the Io plasma torus in the inner Jovian magnetosphere, molecular oxygen atmospheres of Europa, Ganymede, and now Callisto, auroras on Io, Europa, and Ganymede, and water vapor plumes from Europa.  These discoveries were made as a Co-Investigator and Science Team Leader on the Voyager Ultraviolet Spectrometer Experiment, a key member of the Hopkins UV team using IUE and Hubble Space Telescope and as an interdisciplinary scientist on the Cassini Mission.

Answer First: Applying the heuristic-analytic theory of reasoning to examine student intuitive thinking in the context of physics.

Dr. Mila Kryjevskaia

Dept. Physics, North Dakota State University

Monday, April 28, 2014, 3:30-4:30pm

221 South Engineering

It is a common expectation that, after instruction, students will consciously and systematically construct chains of reasoning that start from established scientific principles and lead to well-justified predictions.  When student performance on course exams does not reveal such patterns, it is often assumed that students either do not possess a suitable understanding of the relevant physics or are unable to construct such inferential reasoning chains due to deficiencies in reasoning abilities.  Psychological research, however, suggests that in many cases thinking processes are strikingly different from those outlined above.  Dual-process theories suggest that there are two distinct processes involved in many cognitive tasks.  Process 1 supports reasoning that is quick, intuitive, and automatic, while Process 2 is slow, rule-based, analytical, and reflective.  In this project, we will apply the extended heuristic-analytic theory of reasoning proposed by Evans, which was specifically designed to explain a particularly puzzling phenomenon related to reasoning:  logical competence demonstrated on one task is often not exhibited in the performance of another related task.  Indeed, student often rely on a variety of intuitive (often erroneous) reasoning strategies even though they possess the knowledge and skills necessary to arrive at a correct answer.  In this study, we developed a methodology that allowed for the disentanglement of student conceptual understanding and reasoning approaches.  We then applied the heuristic-analytic theory of reasoning in order to account for, in a mechanistic fashion, the observed inconsistencies in student responses.  Data from introductory calculus-based physics courses will be presented and implications for instruction will be discussed.

CANCELLED!

Imaging of Complex Biological Systems at the Sub-Cellular, Cellular and Multicellular Levels

Dr. Alexander Khmaladze

University of Michigan

tuesday, April 15, 2014, 3:00-4:00pm (Refreshments will be served at 2:45).

Special Location - 271 Batcheller Technology Center

Non-invasive nature of optical microscopy enables researchers to study a great variety of materials under conditions approaching or similar to their "natural" environment. This is especially relevant to live biological specimens, which can be studied both in-vitro and in-vivo, providing a unique insight into the dynamic processes occurring in the live organisms. In recent years, the emphasis has been shifting towards the technologies that combine several different imaging techniques to study a particular system. Each technique then allows measuring a partially overlapping set of parameters, leading to deeper understanding of the processes occurring within that system. This talk presents several imaging and spectroscopic techniques, namely dual-wavelength digital holographic microscopy, hyperspectral coherent anti-Stokes Raman imaging and spontaneous Raman spectroscopy. I will show how these techniques proved to be useful to answer specific questions, and also how they can be applied to solve a wide range of problems in physics, chemistry, biology and medicine. Moreover, by combining them, a single objective, such as a comprehensive study nanoparticles entry into cells and tissues, can be achieved. Due to interdisciplinary nature of these research topics, they are particularly well suited for involving researchers with various scientific backgrounds and interests. This research also provides rich opportunities for students to explore and gain knowledge in optical design, software programming, mathematics and nano-biology.

Coherent Raman Standoff Detection with Shaped Femtosecond Pulses

Marshall Bremer

Applied Physicist, Appareo Systems.

Monday, April 14, 2014, 3:30-4:30pm, 221 South Engineering

Standoff detection of explosives in order to protect public spaces is extremely challenging. Explosives generally have very low vapor pressures, limiting the effectiveness of air sampling and promoting research into optical methods to detect thin residues or micro-particles on surfaces as indicators of concealed danger. With excellent chemical specificity, Raman spectroscopy is capable of detecting particular compounds within the chemically complex background manifest in everyday surfaces, but the weak signal of the spontaneous process prohibits quickly detecting such small quantities.

I will discuss my graduate work at Michigan State University in which stimulated Raman scattering was used to quickly detect and image trace quantities of explosives in a standoff configuration.¹ Stimulated Raman microscopy techniques generally employ two synchronized laser pulses and wavelength scanning to tune to the appropriate vibrational frequency.  Our approach uses a single femtosecond laser and pulse shaper to selectively excite a particular transition with the broad bandwidth. The transition is detected by simultaneously measuring stimulated Raman gain and loss using the diffusely reflected laser light from a single pulse. I will present images showing detection of single microcrystals of NH4NO3 on a variety of real world surfaces using a few laser shots and collecting the strong signal at ten meters.

1. Bremer, M. T. & Dantus, M. Standoff explosives trace detection and imaging by selective stimulated Raman scattering. Appl. Phys. Lett.103, 061119–061119–5 (2013).

Macromolecules on Lipid Membranes: Brownian Motion, Conformational Dynamics, and Local Perturbations

Dr. Eugene Petrov

Max Planck Institute of Biochemistry, Department of Cellular and Molecular Biophysics, Martinsried, Germany

Tuesday, April 8, 2014, 3:30-4:30pm (Refreshments will be served at 3:15).

Special Location - 271 Batcheller Technology Center

Interaction of polymer molecules and colloidal particles with lipid membranes is one of the important problems of the modern bio-inspired soft matter physics. Its understanding provides an insight into mechanisms of interaction of biological macromolecules with cell membranes. What is the effect of the lipid membrane on the conformational dynamics and Brownian motion of membrane-bound polymer molecules? How lipid diffusion and phase separation in the membrane are affected by local perturbations induced by macromolecules? In my talk I will address these and other related questions using my recent results for model membrane systems and macromolecular structures including DNA, DNA origami, fd-virus, and artificial FtsZ-based membrane cortex. Experimental results obtained using fluorescence video-microscopy, fluorescence correlation spectroscopy, and single-particle tracking will be compared with Monte Carlo simulations and theoretical models. I will discuss implications for understanding important biological issues such as anomalous diffusion in cell membranes, effect of the membrane cytoskeleton on cold-shock resistance of organisms, and spontaneous DNA uptake by living cells

Directed Assembly of Colloidal Particles at Liquid Crystal Interfaces

Dr. Mohamed Gharbi

University of Pennsylvania

Monday, March 31, 2014, 3:00-4:00pm (Refreshments will be served at 2:45).

Special Location - 271 Batcheller Technology Center

Colloidal particles organize spontaneously at fluid interfaces owing to a variety of interactions to form well-organized structures that can be exploited to synthesize advanced materials. While the physics of colloidal assembly at isotropic interfaces is well understood, the mechanisms that govern interactions between particles at complex fluid interfaces are not yet clearly established. In particular, nematic and smectic liquid crystal materials offer important degrees of freedom that can be used to direct particles into new structures. In this work, I report the behavior of solid micrometric beads with homeotropic anchoring confined at interfaces of liquid crystal films. First, I will detail the behavior of spherical solid particles at planar nematic liquid crystal (NLC) interfaces. Subsequently, I will report the behavior of particles at more complex NLC interfaces. I will review how the competition between anchoring conditions, liquid crystal elasticity, and topology of curved surfaces is responsible for the formation of new ordered structures in a self-assembly process. Second, I will report the behavior of silica beads confined at interfaces of thin smectic films. I study the interactions and self-assembly of these particles in both supported and free standing films. When particles are captured in thin membranes, they induce distortions of the smectic interface to satisfy wetting properties at particle boundaries, leading to capillary interactions. These forces compete with elastic ones induced by the distortion of the smectic layers. The resulting potential drives assembly of the spheres into new different structures. Recent progress in understanding the mechanism of particle self-organization is presented.

Single Molecule Bioelectronics

Dr. Yongki Choi

Department of Physics and Astronomy, University of California Irvine.

Thursday, March 27, 2014, 3:30-4:30pm (Refreshments will be served at 3:15).

Special Location - 271 Batcheller Technology Center

Nanoscale electronic devices like field-effect transistors have long promised to provide sensitive, label-free detection of biomolecules.  In particular, single-walled carbon nanotubes have the requisite sensitivity to detect single molecule events, and have sufficient bandwidth to directly monitor single molecule dynamics in real time.  Our recent work has demonstrated this premise by monitoring the dynamic, single-molecule processivity of three different enzymes: lysozyme, protein Kinase A, and DNA polymerase I.  With all three enzymes, single molecules were electronically monitored for 10 or more minutes, allowing us to directly observe rare transitions to chemically inactive and hyperactive protein conformations.  The high bandwidth of the nanotube transistors further allow every individual chemical event to be clearly resolved, providing excellent statistics from tens of thousands of turnovers by a single enzyme. Besides establishing values for processivity and turnover rates, the measurements reveal variability, dynamic disorder, and the existence of intermediate states.  Initial success with the three enzymes indicates the generality and attractiveness of the nanotube devices as a new tool to complement existing single molecule techniques.  Furthermore, our focused research on transduction mechanisms provides the design rules necessary to further generalize this architecture.

Determination of pK Values of Ionizable Residues in Pentapeptides and SNase Protein

Guilherme Volpe Bossa

Department of Physics, NDSU

Monday, March 24, 2014, 3:30-4:30pm.

South Engineering, Rm 221

 pK is a parameter related with the ionization process of ionizable groups, as amine and hydroxyl. The determination of amino acids pK values in an electrolytic solution is crucial to understand the dynamics of various biological processes as, for example, adsorption of peptides and their interactions with enzymes active site. In this work we developed a mean-field model to calculate the mean electrostatic potential on amino acids surface. Such potential, which takes into account the contributions exerted by neighboring groups and ions in solution, is responsible for determine the pK value of each residue. The proposed model is applied to amino acids Asp, Glu, Lys, His, Tyr and Cys. Since the results were consistent with those reported in experimental works, our model was extended and applied to the computation of pK in Gly and Ala pentapeptides and of ionizable residues of Staphylococcal Nuclease (SNase) protein. In this last case, we used an approach similar to the first-neighbors approximation and the results shown good agreement with other theoretical works. These facts and the tiny computational cost involved points to a promising applicability of the suggested approach to modeling force fields.

Revealing Protein Dynamics by Integrating Molecular Dynamics Simulations with Neutron Scattering Experiments

Dr. Liang Hong

Center for Molecular Biophysics, Oak Ridge National Laboratory, Oak Ridge, TN 37831

Wednesday, March 19, 2014, 3:00-4:00pm (Refreshments will be served at 2:45).

Special Location - 271 Batcheller Technology Center

Protein, the engine of life, carries out most functions in living things on the earth through characteristic modulation of its three-dimensional structure over time. Understanding the microscopic nature of the protein internal motion and its connection to the function and structure of the biomolecule is a central topic in biophysics, and of great practical importance for drug design, study of diseases, and the development of renewable energy, etc. Under physiological conditions, protein exhibits a complex dynamics landscape, i.e., a variety of diffusive and conformational motions occur on similar time and length scales. This variety renders difficult the derivation of a simplified description of protein internal motions in terms of a small number of distinct, additive components. This difficulty is overcome by our work using a combined approach of Molecular Dynamics (MD) simulations and the state-of-the-art Neutron Scattering experiments. Our approach enables quantitative characterization of distinct protein motions, furnishing an in-depth understanding of the connection between protein structure, dynamics, energy landscape and function.

Investigating student understanding of electric circuits:  New insights from introductory physics and upper-division analog electronics

MacKenzie R. Stetzer

Department of Physics and Astronomy,University of Maine

Monday, Mar. 10, 3:30-4:30pm
South Engineering 221

REFRESHMENTS AT 3:15!!

In recent years, large-scale undergraduate course transformation efforts have become an increasingly visible response to a well-documented need for improved STEM instruction at all levels.  The role that research-validated instructional materials play in such transformations, however, is sometimes overlooked.  As the focus of these efforts shifts from introductory to upper-division courses, there is an increasing need for the kind of in-depth studies of student understanding that may inform the development of effective instructional materials.  In this talk, I present examples from an ongoing, multi-year, multi-institutional investigation of student understanding of electric circuits and analog electronics.  The insights drawn from work conducted in both introductory and upper-division courses continue to guide efforts to minimize the disconnect between what we teach and what students learn in junior-level laboratory courses in analog electronics.

Multipole Excition Generation in Nanometer-Sized Silicon Nanoparticles 

Andrei Kryjevski

Department of Physics, NDSU

Monday, Feb. 10, 3:30-4:30pm
South Engineering 221

In a semiconductor absorption of an energetic photon with energy exceeding twice the bandgap results in a creation of an energetic electron-hole pair (or an exciton, to be more precise). Typically, this excess energy is lost to heat due to phonon emission. An alternative path for the time evolution of an energetic exciton is the carrier multiplication (CM), or multiple exciton generation (MEG) mechanism, where two (or more) electron-hole pairs (excitons) are created from one absorbed photon. This mechanism is expected to be appreciable in nano-structures since electrostatic electron interactions here are enhanced by the spatial confinement. So, in nanoparticles it may be possible to channel the excess photon energy into creation of an additional electron-hole pair instead of being lost to heat. This is one reason why nano-material based photovoltaic devices are hoped to exceed the Shockley–Queisser efficiency limit of about 30%. In this talk, we will discuss MEG calculations in several semiconductor nanosystems. The calculations are based on Density Functional Theory (DFT) combined with the many body perturbation theory. Hydrogen-passivated Si29H36 quantum dots (QDs) with crystalline and amorphous core structures, the quasi one dimensional (1-D) arrays constructed from these QDs, as well as crystalline and amorphous Si nanowires have been studied. Amorphous Si nanostructures are predicted to have more effective carrier multiplication. Also, we will discuss our ongoing work on the carrier energy loss due to phonon emission. The ultimate goal of this project is to make predictions of efficiency of both MEG and electron energy loss to heat in the same nanoparticle. This should help in selecting nanomaterials suitable for photovotaic applications.

Molecular Simulation of Physical Aging in Ultrathin Polymer Film

Qiyun Tang

Post. Doc., Department of Physics, NDSU

Monday, Feb. 03, 3:30-4:30pm
South Engineering 221

Physical aging in glassy polymer films has attracted much attention in the past decades due to their strong correlation with the lifetime of polymer-based nano-devices. In the past years, many simulations were performed to understand the physical aging of bulk polymers while little attention was focused on the aging of ultrathin polymer films. Here we performed a Monte Carlo simulation by introducing the vacancy diffusion and vanishing mechanisms to investigate the physical aging of ultrathin polymer films. The obtained results are consistent with that observed in experiments, such as the linear increase of the local average density of segments with the logarithm of time, and the responses of aging rates to temperatures. Especially, at the extremely small thickness, the accelerated physical aging commonly observed experimentally in thin polymer films slows down, and the phenomenon is suppressed in the case of low molecular-weight ultrathin films. This anomalous behavior can be attributed to an inversed vacancy diffusion process caused by the sliding motion of chain molecules between the two free surface layers in free-standing polymer films. More interestingly, we verify that the average length of short chain-fragments defines a critical film thickness and demonstrate the existence of a new confinement effect at the nanoscale. The outlined approach could indeed provide new insights from the molecular levels to understand the physical aging of ultrathin films, and can be easily extended to unveil the correlations between the structure and dynamics in ultrathin polymer films with complex architectures.

A Tale of Two Colloids: Order and Disorder on a Soft Substrate

Andrew B. Croll

Department of Physics, NDSU

Monday, Jan 27, 3:30-4:30pm
South Engineering 314

The buckling of thin films has recently received considerable attention in both the materials and the continuum elasticity communities.  To the former, elastic instabilities form a platform for the mechanical measurement of material properties under increasing degrees of confinement.  To the latter, instabilities represent a testing ground for advanced elastic theory.  Buckling is also of considerable importance in the evolution of granular systems, which often show deformations that resemble those of continua.  Previously, we documented several differences between continuum theory and discrete elasticity in a discrete model of a thin film experimentally constructed from a well ordered (hexagonally packed) layer of colloid scale particles.  Here we consider how the structure of the 2D layer influences the buckling process.  In particular, we examine the details of how a complex, disordered (glassy) 2D layer resting on soft foundations responds to in-plane compressive stress.  We show how the fundamental buckling lengthscale remains identical to that of ordered layers, despite considerable heterogeneity in the motion of the particles.

Hydration and Screening in Ionic Mixtures: Multiscale Modeling

Alan Denton

Department of Physics, NDSU

Monday, November 25, 3:30-4:30pm
South Engineering 314

Ionic mixtures have long been a focus of soft matter research, owing to both their rich variety of interparticle forces and their diverse applications, from batteries to biology. On length scales > 1 nm, electrostatic forces dominate and can stabilize colloidal suspensions and polyelectrolyte solutions, with practical importance for foods and pharmaceuticals. On shorter length scales, non-electrostatic forces (e.g., due to hydration of ions) become relevant and entail ion-specific effects, such as the classic Hofmeister series, which classifies ions according to their ability to salt out or salt in proteins. The physical mechanism by which a solvent (water) mediates effective ion-ion interactions, however, is still poorly understood. In modeling such complex mixtures, multiscale approaches often prove essential to surmount computational challenges posed by broad length and time scales. I will outline our recent efforts to derive from perturbation theory a coarse-grained model of ions interacting via both long-range Coulomb and short-range solvent-induced forces. By inputting effective interactions into molecular simulations, we explore ion-specific properties of electrolyte solutions

Quantitative kinetic theory of self-propelled particles

Thomas Ihle

Department of Physics, NDSU

Monday, November 18, 3:30-4:30pm
South Engineering 318 (note the room change)



The Vicsek-model [1] for self-propelled agents such as birds, fish or bacteria, where agents try to align with their neighbors, is considered. Starting from a Markov chain in phase space and assuming Molecular Chaos I derive a kinetic equation for the evolution of the one-particle density [2]. This equation is solved numerically by a novel Lattice-Boltzmann-like algorithm which relies on about 1000 microscopic velocities [3]. Steep soliton-like invasion waves are observed. It is shown that these waves change the order of the phase transition to collective motion from continuous to discontinuous. For large particle speeds, the shapes of the waves agree quantitatively with agent-based simulations. Attempts to go beyond the mean-field assumption of Molecular Chaos and to include correlation effects are also discussed.

[1] T. Vicsek et al, Phys. Rev. Lett. 75 (1995) 1226.
[2] T. Ihle, Phys. Rev. E 83 (2011) 030901.
[3] T. Ihle, Phys. Rev. E 88 (2013) 040303.

Including Water-Mediated Effective Interactions into Electrolyte Mean-Field Models

Sylvio May

Department of Physics, NDSU

Monday, November 4, 3:30-4:30pm
South Engineering 118 (note the room change)

This talk gives an introduction to ion specific effects in electrolyte solutions, including the Hofmeister effect, and then discusses a systematic method to include non-electrostatic interactions into the (mean-field) Poisson-Boltzmann model. These non-electrostatic interactions account for the influence of the aqueous solvent (beyond merely providing a background dielectric constant). We will employ a separate field to account for the presence of Yukawa-like interactions in addition to the Coulomb potential. Extensions to more realistic oscillating Yukawa-like interactions are briefly sketched.

Nanoscale Carbon and Silicon at the Hard-Soft Interface

Erik Hobbie

Department of Physics, NDSU

Monday (Oct 28) 3:30-4:30pm
South Engineering 118 (note the room change)

Leveraging ‘soft matter’ at the nanoscale to simplify materials processing and improve material performance is becoming a reality, with potentially profound implications for a number of emerging technologies. A critical element of this is the manipulation and assembly of colloidal nanoparticles of exceptional purity through fluid-phase processing, and many of the current approaches for this are being drawn from polymer science. Aspects of our experimental research on single-wall carbon nanotubes and nanocrystalline silicon will be presented in this context, with an emphasis on applications related to flexible electronics and photoluminescent spectroscopic imaging.

Student difficulties measuring distances in terms of wavelength: Lack of basic skills or failure to transfer?

Mila Kryjevskaia

Department of Physics, NDSU

Monday, October 21, 3:30-4:30pm
South Engineering 221

We will discuss  student reasoning on problems involving two-source and thin-film interference. In both cases, interference arises from differences in the path lengths traveled by two waves. We found that some students (up to 40% on certain questions) had difficulty with a task that is fundamental to understanding these phenomena: expressing a physical distance, such as the separation between two sources, in terms of the wavelength of a periodic wave. We administered a series of questions to try to identify factors that influence student performance. We concluded that most incorrect responses stemmed from erroneous judgment about the type of reasoning required, not an inability to do said reasoning. A number of students do not seem to treat the spacing of moving wave fronts as analogous to immutable measurement tools (e.g., rulers).

Production of a thin solar cell using Si6H12 and type-sorted carbon nanotubes

Matthew Semler

Department of Physics, NDSU

Monday, October 14, 3:30-4:30pm
South Engineering 221

This talk is part of Matt's comprehensive examination.

Thin film silicon has primarily been made by depositing amorphous silicon (a-Si) via plasma enhanced chemical vapor deposition (PECVD), which is energy intensive.  A decade ago, NDSU formulated a novel technique to produce a silane solution, cyclohexasilane (CHS), that can be spin-cast onto a substrate, producing an a-Si film.  Using a pulsed laser, we have crystallized these a-Si films, producing polycrystalline silicon films.  In this presentation, I will discuss laser crystallization of a-Si and its potential use in solar cells, as well as other research I have thus far completed in fulfillment of my graduate comprehensive presentation.

Using a lens of Resources and Framing to make sense of students' ideas about matrix multiplication

Warren Christensen

Department of Physics, NDSU

Monday October 7, 3:30-4:30pm

South Engineering 221

In principle, a student who has completed both Linear Algebra and Quantum Mechanics should have a wealth of conceptual and procedural knowledge that has been attained from both mathematics and physics classes.  However in practice, it seems that many students come into our physics courses with an apparent lack of skills that we know were taught in math courses. This investigation casts light on students' thinking about matrix multiplication and how their thinking appears to be influenced by their framing of the problem as either a mathematics or physics question. We use the framework of Framing and Resources to describe a single student's thinking during an interview. Using an interview protocol written by mathematicians from a study in Mathematics Education, we explicitly probed mathematical thinking, and investigated if (and when) students attempted to relate mathematical problems to physics. Using lexicon analysis, we find students seem to shift from a "mathematical frame" to a "physics frame" and back again, but struggle to successfully transfer concepts between those frames. I will highlight the markers for these frame shifts and explore the potential instructional consequences of this work.

Drying of colloidal suspensions

Alexander Wagner

Department of Physics, NDSU

Monday, September 30,  3:30-4:30pm

South Engineering 221

This will be an interactive research talk where I report on recent simulations of evaporation or colloid laden drops. The research is driven by interesting experimental results by Erik Hobbie et al. in our department. I will show some results of my numerical investigations which show the effect of hydrodynamics and of Marangoni flows.

Local fluctuations in the lattice Boltzmann method

Goetz Kaehler

Department of Physics, NDSU

Monday, September 23, 3:30-4:30pm

South Engineering 221

The implementation of thermal fluctuations in lattice Boltzmann methods has been an active research topic in the last ten years. While implementations at high number density have been used successfully, even in some cases of non-ideal systems, examples at low number density still exhibit significant problems. It has been shown shown earlier that it is possible to largely avoid Galilean invariance violations by implementing a locally velocity dependent set of transforms to moment space. Recent calculations indicate that at low number densities the locality of the BGK collision operator leads to results different in nature than those obtained from Langevin theory. In this presentation I will discuss, in detail, our most recent results in obtaining a closed fluctuation dissipation theorem.

Skyrmion wormholes and Skyrmion black holes

Terry Pilling

Department of Physics, NDSU

Monday, Sept 16 3:30-4:30pm

South Engineering 221, NDSU

Skyrmions are soliton solutions to the non-linear sigma model lagrangian with a `Skyrme term'. The Skyrme model originated in the early 1960's as an attempt to model nucleons as solitons in a meson background field with the conserved topological charge identified as the baryon number. 

In 1983 Witten showed that the Skyrme model constitutes a low energy effective field theory of QCD and it is expected that the Skyrme model will give access to non-perturbative phenomena to which perturbative QCD is not accesible. There are a number of groups, most notably that of Nick Manton at DAMTP in Cambridge who are still working on the Skyrme model of nuclear physics and low energy QCD. 

In 2008, Manton, Diakonov, and I began studying the coupling Skyrmions to gravity to explore some previously discovered properties such as violations of the `no-hair' conjecture. Dmitiri Diakonov, until he passed away last year, was the Deputy Director, and the head of the Theory group, at the Petersburg Nuclear Physics Institute in Russia. 

After Diakonov passed away, I sent all of my calculations and computer code on skyrmion black holes to his students at PNPI and began instead to try and construct a Morris-Thorne wormhole with a skyrmion matter source in the hope that the conserved topological SU(2) charge would prevent the usual collapse and allow one to form a stable, traversable, wormhole needing only a vanishing amount of negative energy.

Membrane-Macromolecule Interactions: A Physical View on Complex Systems

Dr. Miha Fošnarič

Laboratory of Biophysics, Faculty of Electrical Engineering, University of Ljubljana

Monday, August 26th at 3:30 p.m.
South Engineering 221, NDSU
Refreshments at 3:00 p.m. in SE 216

After an introduction in the physical perspective of biological systems, we will present three examples of modelling membrane-macromolecule interactions: the influence of rigid membrane inclusion on membrane elasticity [1]; vesicle wrapping of a charged colloid [2]; and vesicle accommodation of a linear semi-flexible polymer [3]. Phenomenological modelling and Monte-Carlo simulations will be discussed. Most of the talk will be relatively general, suitable for graduate and undergraduate students interested in biophysics and complex systems. 

[1] M. Fošnarič, A. Iglič, S. May: Influence of rigid inclusions on the bending elasticity of a lipid membrane, Phys. Rev. E, 2006. 

[2] M. Fošnarič, A. Iglič, D.M. Kroll, S. May: Monte Carlo simulations of complex formation between a mixed fluid vesicle and a charged colloid, J. Chem. Phys., 2009. 

[3] M. Fošnarič, A. Iglič, D. M. Kroll, S. May: Monte Carlo simulations of a polymer confined within a fluid vesicle, Soft Matter, 2013.

Phospholipids are where and do what?

Dr. Edgar E. Kooijman

Department of Biological Sciences, Kent State University

Tuesday, July 09 at 2:00 p.m.
South Engineering 221, NDSU
Refreshments at 1:45 p.m. in SE 216

Phospholipids are everywhere; from the biological membranes surrounding cells and intracellular organelles to the phospholipid monolayer in your lungs and the monolayer surrounding VLDL/LDL and intracellular lipid droplets. In this talk I will introduce two distinct lines of research carried out in my lab. The first of these lines is concerned with how
specific phospholipid species (might) function in biological membranes.  The goal of this work is to characterize the physicochemical properties of these lipids such as their effective molecular shape and ionization  properties. As an example I will discuss some recent results on a lipid primarily found in plant membranes that is involved in stress (e.g. salinity and drought) signaling.

The second line of research concerns the structure of intracellular lipid droplets. Specifically, we are interested in how a subset of LD binding proteins interacts with the phospholipid monolayer covering the neutral lipid core. In the talk I will introduce the structure of these intracellular "organelles" and then discuss some monolayer data on a model protein that actually functions in protein transport in insects.  For both lines I will discuss why these projects are interesting and why you should care about them.

Liposomes loaded with a lipophilic drug: Is there a transfer of the drug to blood components before the liposome is reaching its target and is this behaviour good or bad?

Dr. Alfred Fahr

Department of Pharmaseutical Technology, Friedrich-Schiller University Jena, Jena, Germany

Wednesday, March 13th at 4:00 p.m.
South Engineering 221, NDSU
Refreshments at 3:30 p.m. in SE 216

We have incorporated the very lipophilic photosensitizer temoporfin into liposomes of varying membrane composition, cholesterol content, vesicle size and life-prolonging liposome modifications.  We measured the resulting pharmacokinetic profile of the liposomal carrier and the incorporated temoporfin in a rat model. A novel pharmacokinetic model allowed distinguishing between temoporfin eliminated together with the liposomal carrier and temoporfin that is first transferred to other blood components (e. g. plasma proteins) before being eliminated from the blood. Our analysis using this model demonstrates that a fraction of temoporfin is released from the liposomes prior to being eliminated from the blood. In case of unmodified liposomes this temoporfin release was observed to increase with decreasing bilayer fluidity, indicating an accelerated temoporfin transfer from gel-phase liposomes to e.g. plasma proteins. This was predicted by earlier in vitro experiments using a liposome-liposome transfer system. The advantages of such a “leaky” carrier system for therapy will be discussed.

Monte Carlo simulations of a polymer confined within a fluid vesicle

Dr. Sylvio May

Department of Physics, North Dakota State University, Fargo, North Dakota, United States

Monday, January 28th at 3:30 p.m.
South Engineering 221, NDSU

Monte Carlo simulations are employed to study a fluid vesicle that contains a single worm-like polymer chain. We vary the degree of polymer confinement in our simulations by increasing the persistence length of the polymer. The vesicle is represented by a randomly triangulated self-avoiding network that can undergo bending deformations. Upon increasing the persistence length of the polymer beyond the size of the vesicle, we observe a transition of the polymer from an isotropic disordered random conformation to an ordered toroidal coil. Concomitantly, the vesicle adopts an oblate shape to allow for some expansion of the polymer coil inside the vesicle. It is convenient to characterize both polymer and vesicle in terms of the asphericity, a quantity derived from the gyration tensor. At the onset of the polymer's ordering transition, the asphericity passes through a minimum for both polymer and vesicle. The increase in vesicle asphericity for a semi-flexible polymer can be understood in terms of ground state energy calculations, either for a simplified representation of the vesicle shape (we specifically discuss a disk shape with a semi-toroidal rim) or involving a full vesicle shape optimization. The asphericity of the polymer's coil results from conformational fluctuations and can be rationalized using Odijk's deflection length of strongly curved semi-flexible polymers.

Dynamics of Microstructural Transitions in Block Copolymer Melts

Dr. Robert Wickham

Department of Physics, University of Guelph, Guelph, Ontario, Canada

Monday, January 21st at 3:30 p.m.
South Engineering 221, NDSU
Refreshments at 3:00 p.m. in SE 216

Soft materials are dynamical by nature and the study of the dynamics of soft materials is an exciting, rich area of current interest. During macromolecular self-assembly, as occurs in block copolymers, long structural relaxation timescales due to collective molecular motion are often seen. How microstructure influences the dynamics, the existence and lifetime of metastable states, and the dynamics of long-lived non-equilibrium structures are all poorly-understood issues. Our large-scale dynamical simulations address these questions in the context of the nucleation of one microstructured phase out of another in a block copolymer melt. Specifically, we simulate the model-B dynamics of the conserved monomer density driven by a Landau-Brazovskii free-energy, which is appropriate for diblock copolymer melts. I will discuss our simulations of nucleation at the cylinder-to-lamellar transition, and our recent study of the kinetics of crystallization of the body-centered cubic phase of spherical micelles from disorder.

Dr. Heiko Heerklotz

Faculty of Pharmacy, University of Toronto, Toronto, Ontario, Canada

Wednesday, January 16th at 3:30 p.m.
South Engineering 221, NDSU
Refreshments at 3:00 p.m. in SE 216

All are welcome!

Dr. Adam Raegen

Department of Physics, University of Guelph, Guelph, Ontario, Canada

Monday, January 14th at 3:30 p.m.
South Engineering 221, NDSU
Refreshments at 3:00 p.m. in SE 216

All are welcome!

Matthew Semler

Department of Physics, North Dakota State University, Fargo, North Dakota, United States

Monday, December 3rd at 3:30 p.m.
South Engineering 221, NDSU

All are welcome!

John Harris

Department of Physics, North Dakota State University, Fargo, North Dakota, United States

Monday, November 26th at 3:30 p.m.
South Engineering 221, NDSU

All are welcome!

Dr. Andrew B. Croll

Department of Physics, North Dakota State University, Fargo, North Dakota, United States

Monday, November 19th at 3:30 p.m.
South Engineering 221, NDSU

All are welcome!

Dr. Jun Kyung Chung

Department of Physics, North Dakota State University, Fargo, North Dakota, United States

Monday, November 5th at 3:30 p.m.
South Engineering 221, NDSU

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Dr. Artem Novozhilov

Department of Mathematics, North Dakota State University, Fargo, North Dakota, United States

Monday, October 29th at 4:00 p.m.
South Engineering 221, NDSU

All are welcome!

Dr. Alan Denton

North Dakota State University, Fargo, North Dakota, United States

Monday, October 22nd at 3:30 p.m.
South Engineering 221, NDSU

All are welcome!

Dr. Tung, Li-Chun Richard

University of North Dakota, Grand Forks, North Dakota; National High Magnetic Field Laboratory-FSU, Tallahassee, Florida, USA

Monday, October 15th at 3:30 p.m.
South Engineering 221, NDSU

All are welcome!

Kimberly Kreutzer

Graduate Student, North Dakota State University, Fargo, North Dakota, United States

Monday, October 8th at 3:30 p.m.
South Engineering 221, NDSU

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Dr. Alexander Wagner

North Dakota State University, Fargo, North Dakota, United States

Monday, September 17th at 3:30 p.m.
South Engineering 221, NDSU

All are welcome!

Dr. Miha Fošnarič

Faculty of Electrical Engineering, University of Ljubljana, Ljubljana, Slovenia

Monday, September 10th at 3:30 p.m.
South Engineering 221, NDSU

All are welcome!

Dr. Monica Olvera de la Cruz

Lawyer Taylor Professor, Materials Science & Engineering, Chemical & Biological Engineering and Chemistry, Northwestern University, Evanston, Illinois, United States

Wednesday, May 2nd at 3:30 p.m.
FLC 124, NDSU
Refreshments at 3:00 p.m. in SE 216

All are welcome!

Dr. Yuri Dahnovski

Department of Physics and Astronomy, University of Wyoming, Laramie, Wyoming, United States

Wednesday, April 25th at 4:00 p.m.
South Engineering 221, NDSU
Refreshments at 3:30 p.m. in SE 216

All are welcome!

Dr. Bret Ludwig

Communications Manager, 3M Innovation Center, St. Paul, Minnesota, United States

Thursday, April 12th at 3:30 p.m.
FLC 124, NDSU
Refreshments at 3:00 p.m. in SE 216

All are welcome!

Dr. Yen Lee Loh

Physics Department, University of North Dakota, Grand Forks, North Dakota, United States

Wednesday, April 4th at 4:00 p.m.
South Engineering 221, NDSU
Refreshments at 3:30 p.m. in SE 216

All are welcome!

Dr. JiaJia Dong

Physics Department, Hamline University, St. Paul, Minnesota, United States

Thursday, March 29th at 3:30 p.m.
South Engineering 221, NDSU

All are welcome!

Dr. An-Chang Shi

Physics Department, McMaster University, Hamilton, Ontario, Canada

Monday, March 12th at 3:00 p.m.
(Refreshments at 2:45 p.m. in SE 216.)
South Engineering 221, NDSU

Cosponsored by the Department of Coatings and Polymeric Materials, with support from the NDSU Cooperative Sponsorship Committee.

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Dr. Mila Kryjevskaia

Physics Department, North Dakota State University, Fargo, North Dakota, United States

Wednesday, February 22nd at 4:00 p.m.

All are welcome!

Dr. Yen-Liang Chou

Physics Department, North Dakota State University, Fargo, North Dakota, United States

Wednesday, February 15th at 4:00 p.m.

All are welcome!

Dr. Andrew Croll

Physics Department, North Dakota State University, Fargo, North Dakota, United States

Wednesday, February 1st at 4:00 p.m.

All are welcome!

Dr. Sylvio May

Physics Department, North Dakota State University, Fargo, North Dakota, United States

Wednesday, January 25th at 4:00 p.m.

All are welcome!

Dr. Ananda Shastri

Physics Department, Minnesota State University Moorhead, Moorhead, Minnesota, United States

Thursday, October 27th at 2:00 p.m.

All are welcome!

Dr. Matthew L. Beckman

Biology Department, Augsburg College, Minneapolis, Minnesota, United States

Thursday, October 27th at 2:35 p.m.

All are welcome!

Kathrin Kaeß

Department of Pharmaceutical Technology, Friedrich-Schiller-Universität Jena, Germany

Thursday, October 27th at 3:10 p.m.

All are welcome!

E. F. “Joe” Redish

Department of Physics, University of Maryland, College Park, Maryland, United States

Thursday, October 6th at 4:00 p.m.
(Refreshments at 5:00 p.m.)
South Engineering 116, NDSU

All are welcome!

Dr. Andrew Baruth

Department of Chemical Engineering and Materials Science, University of Minnesota - Minneapolis, Minneapolis, Minnesota, United States

Wednesday, March 30th at 4:00 p.m.
(Refreshments at 3:45 p.m. in SE216.)
South Engineering 221, NDSU

All are welcome!

Dr. Bruno Tomberli

Department of Physics and Astronomy, Brandon University, Brandon, Manitoba, Canada

Wednesday, March 9th at 4:00 p.m.
(Refreshments at 3:45 p.m. in SE216.)
South Engineering 221, NDSU

All are welcome!

Dr. Nuri Oncel

Department of Physics and Astrophysics, University of North Dakota, Grand Forks, North Dakota, USA

Thursday, March 3rd at 4:00 p.m.
(Refreshments at 3:45 p.m. in SE216.)
South Engineering 221, NDSU

All are welcome!

Dr. Jens Glaser

Department of Physics, Leipzig University and Department of Chemical Engineering, University of Minnesota, Minneapolis, Minnesota, USA

Wednesday, February 9th at 4:00 p.m.
(Refreshments at 3:45 p.m. in SE216.)
South Engineering 221, NDSU

 ^{Sponsored by NDSU Geosciences and Physics Department, and by the Cooperative Sponsorship program}

All are welcome!

Dr. Svetlana V. Kilina

Department of Chemistry, North Dakota State University, Fargo, North Dakota, USA

Monday, December 6th at 4:00 p.m.
South Engineering 221, NDSU

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Dr. Dmitri S. Kilin

Department of Chemistry, University of South Dakota, Vermillion, South Dakota, USA

All are welcome!

Dr. Luis Manzoni

Department of Physics, Concordia College, Moorhead, Minnesota USA

All are welcome!

Dr. Andrew B. Croll

Department of Physics, North Dakota State University, Fargo, North Dakota, USA

Thursday, September 16th at 4:15 p.m.
South Engineering 221, NDSU

All are welcome!

Dr. Kevin J. Landmark

Department of Physics, Augsburg College, Minneapolis, Minnesota, USA

Thursday, September 16th at 3:15 p.m.
South Engineering 221, NDSU

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Dr. Ahmed A. Heikal

Department of Chemistry and Biochemistry, and Department of Pharmaceutical Sciences, University of Minnesota, Duluth, USA

Thursday, February 4th at 4:00 p.m.
(Refreshments at 3:45 p.m. in SE216.)
South Engineering 221, NDSU

All are welcome!

Dr. Mark Stevens

Center for Integrated Nano-Technologies, Sandia National Laboratories, Albuquerque, New Mexico, USA

Wednesday, January 27th at 4:00 p.m.
(Refreshments at 3:45 p.m. in SE216.)
South Engineering 120, NDSU

All are welcome!

Dr. Reinhard Lipowsky

Director of the Department of Theory & Bio-Systems, Max-Planck Institute of Colloids and Interfaces, Potsdam, Germany

Wednesday, December 9th at 4:00 p.m.
(Refreshments at 3:45 p.m. in SE216.)
South Engineering 221, NDSU

All are welcome!

Dr. Wayne Barkhouse

Department of Physics and Astrophysics, University of North Dakota, Grand Forks, North Dakota, USA

Wednesday, December 2nd at 4:00 p.m.
(Refreshments at 3:45 p.m. in SE216.)
South Engineering 221, NDSU

All are welcome!

Dr. Jerome Delhommelle

Department of Chemistry, University of North Dakota, Grand Forks, North Dakota, USA

Wednesday, November 18th at 4:00 p.m.
(Refreshments at 3:45 p.m. in SE216.)
South Engineering 221, NDSU

All are welcome!

Dr. Marian Bocea

Department of Mathematics, North Dakota State University, Fargo, North Dakota, USA

Wednesday, November 4th at 4:00 p.m.
South Engineering 221, NDSU

All are welcome!

Dr. Erik K. Hobbie

Department of Physics, Department of Coatings and Polymeric Materials, North Dakota State University, Fargo, North Dakota, USA

Thursday, October 8th at 4:15 p.m.
South Engineering 221, NDSU

All are welcome!

Dr. Pavel Belik

Department of Mathematics, Augsburg College, Minneapolis, Minnesota, USA

Thursday, October 8th at 3:15 p.m.
South Engineering 221, NDSU

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Dr. Miha Fosnaric

Faculty of Electrical Engineering, University of Ljubljana, Slovenia

Wednesday, August 26th at 4:00 p.m.
(Refreshments at 3:45 p.m. in SE216.)
South Engineering 221, NDSU

All are welcome!

Dr. Andrew B. Croll

Department of Polymer Science and Engineering, University of Massachusetts Amherst, Massachusetts, United States

Wednesday, July 15th at 11:00 a.m.
(Refreshments at 10:45 a.m. in SE216.)
South Engineering 221, NDSU

All are welcome!

Dr. Guiseppe Gonnella

Dipartimento di Fisica, Università degli Studi diBari, Italy

Wednesday, May 27th at 4:00 p.m.
(Refreshments at 3:45 p.m. in SE218.)
South Engineering 221, NDSU

All are welcome!

Dr. James Forrest*

Department of Physics and Astronomy, Waterloo Institute for Nanotechology, University of Waterloo, Waterloo, Iowa, United States

Friday, April 17th at 1:30 p.m.
EML 370, NDSU

All are welcome!

Prof. Mustafa Yavuz*

Waterloo Institute of Techology, University of Waterloo, Waterloo, Iowa, United States

Monday, April 13th at 1:00 p.m.
Reimers Conference Room, Alumni Center, NDSU

[1] W. Wu, A. Hu, X. Li, J.Q. Wei, K.L. Wang, M. Yavuz  and N. Zhou, “Vacuum Brazing of Carbon Nano Tube Bundles”, Materials Letter (Elsevier), vol. 62, pp. 4486-4488, 2008. [2]S. Sahin, M. Yavuz and N. Zhou, Handbook ofMicrojoining and Nanojoining, “Chapter 18: Introduction to Nanojoining”, 70 pages, editor: N. Zhou, Woodhead Publishing Ltd., 2007.

All are welcome!

Dr. Ravindra Pandey*

Department of Physics, Michigan Technology University, Houghton, Michigan, United States

Wednesday, April 8th at 1:30 p.m.
Reimers Conference Room, Alumni Center, NDSU

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Dr. Erik K. Hobbie*

National Institute of Standards and Technology, Gaithersburg, Maryland, United States

Wednesday, March 25th at 10:00 a.m.
Reimers Conference Room, Alumni Center, NDSU

All are welcome!

Dr. Benjamin Stottrup

Department of Physics, Augsburg College, Minneapolis, Minnesota, United States

Thursday, March 12th at 2:00 p.m.
(Refreshments at 1:45 p.m. in SE 216)
South Engineering 221, NDSU

All are welcome!

Dr. Warren Christensen*

Department of Physics and Astronomy, University of Maine, United States

Wednesday, February 11th at 4:00 p.m.
(Refreshments at 3:45 p.m. in SE 216)
South Engineering 221, NDSU

All are welcome!

Dr. Alex Travesset

Iowa State University and Ames Lab, Ames, Iowa, United States

Wednesday, January 21st at 4:00 p.m.
(Refreshments at 3:45 p.m. in SE 216)
South Engineering 221, NDSU

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Dr. Stuart J. Haring

North Dakota State University, Fargo, North Dakota, United States

Wednesday, November 19th at 4:00 p.m.
South Engineering 221, NDSU

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Dr. Victoria Gelling

North Dakota State University, Fargo, North Dakota, United States

Monday, November 3rd at 2:00 p.m.
South Engineering 221, NDSU

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Dr. William Schwalm

University of North Dakota, Grand Forks, North Dakota, United States

Wednesday, October 22nd at 4:00 p.m.
South Engineering 221, NDSU

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Dr. Alexander Wagner

North Dakota State University, Fargo, North Dakota, United States

Wednesday, October 8th at 4:00 p.m.
South Engineering 221, NDSU

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Dr. Shuang Yang

North Dakota State University, Fargo, North Dakota, United States

Wednesday, September 24th at 4:00 p.m.
South Engineering 221, NDSU

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Dr. Terry Pilling

North Dakota State University, Fargo, North Dakota, United States

Wednesday, September 10th at 4:00 p.m.
South Engineering 221, NDSU

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Waipot Ngamsaad

Mahidol University, Bangkok, Thailand

Wednesday, August 27th at 4:00 p.m.
(Refreshments at 3:45 p.m. in SE 216)
South Engineering 221, NDSU

Many lipid bilayers will undergo phase-separation. It is believed that this
phase-separation may play significant roles in cell membranes. For sup-
ported lipid bilayers, the effect of substrate-induced immobilization of the
lower monolayer is important. It has been observed that lipid domains
in two leaves can be in or out of registration depending on the friction
between the lower monolayer and the substrate. We model the supported
lipid bilayers as two two-dimensional binary fluids that are coupled through
a simple interaction term as well as friction terms between each other and
the substrate. We developed a lattice-Boltzmann method (LBM) to numer-
ically investigate this model. In our simulations we found several dynamic
regimes for domain coarsening, including diffusive coarsening in the lower
monolayer, hydrodynamic coarsening in the upper monolayer and arrested
growth in the upper monolayer due to the coupling interaction. By sim-
plifying the membrane morphology patterns, we are able to find analytical
solutions for the arrested length-scale of domains in the upper monolayer.
Our simulation results support extending to more complex situations.

(<link fileadmin physics.ndsu.edu pdf_files seminar_waipot.pdf>Seminar Announcement)

All are welcome!