Fall 2018
These meetings occur on Fridays from 4:00pm to 5:00pm in Robeson 304 (unless otherwise indicated)
Friday 4:00pm
304 Robeson Hall
(poster)
Faculty Meeting (No CSB Discussion Meeting)
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Deepali Shirsekar (Mechanical Engineering)
“Bidirectional Reflectance Measurement of Black Coating Z302 for use in Optical Instrument Design ”
The bidirectional reflectance distribution function (BRDF) plays a fundamental role in the optical characterization of a surface. The BRDF is a measure of the amount of light incident from one direction that is scattered by a surface in another direction. This talk introduces the concept of BRDF and presents the thesis research of graduate student, Deepali Shirsekar, to investigate the BRDF of black coating, Aeroglaze Z302. Work includes design and fabrication of a highaccuracy bidirectional reflectometer and its use to measure the bidirectional reflectance of a black absorber Aeroglaze Z302®. A BRDF model consisting of diffuse, glossy, and specular components is fitted to the experimental results. Finally, the Monte Carlo raytrace (MCRT) method is used to simulate the performance of any optical instrument which has Z302 material coated on its active surfaces.
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Faculty Meeting (No CSB Discussion Meeting)
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Nazia Munir (Mechanical Engineering)
"Investigation of the GoldBlack Absorption Mechanism"
The material called goldblack is an absorptive material frequently used in various thermodetector. The advantages of goldblack that make it preferable over the other absorptive material is that it has the absorptivity almost one (α=1) in the visible and infrared range which means that it absorbs all the radiation incident on it and appears black in an observer’s eye. For this unique property goldblack has been used in thermal detector such as in microbolometer. The microbolometer converts the incident radiation to an electrical signal. Goldblack is used as a coating on the microbolometer to ensure a 100% absorption of the radiation. Microbolometer with goldblack coating has several applications specially in various program of Earth Radiation Budget where the global warming is closely monitored with satellite having microbolometer attached on it. The purpose of this effort is to establish a model of goldblack so that it can be used more efficiently in various detector. We seek a firstprinciple model for predicting the spectral absorptivity of goldblack. Goldblack has been widely used in various thermal and optical applications for more than a century. In most relevant contributions to the literature, goldblack is treated as a homogeneous layer whose behavior is governed by its bulk optical properties. However, on the microscopic level goldblack more closely resembles a fuzzy layer of moss or a miniature forest. This suggests that the optical behavior of goldblack can be better characterized by taking into account its actual morphology. We propose to model a layer of vacuumdeposited goldblack as a “fractal forest” where each branch of each tree is isolated and considered as an individual building block. In this treatment each individual branch acts as a dipole antenna with the forest as a whole behaving as a randomfractal antenna array. The approach of the current effort is to develop a model for the conversion of incident electromagnetic (EM) radiation to sensible heat by an individual branch behaving as a lossy antenna. The output of such a model would be the energy conversion efficiency (absorptivity), corresponding to a given wavelength, of a single branch having a specified length, diameter, and orientation with respect to incident EM radiation. The overall absorptivity of the forest at that wavelength would then be based on the statistical description of the spatial and angular distributions of branches of various length and diameter. The required statistical rules would be derived from microscopic study of actual goldblack layers.
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Faculty Meeting (No CSB Discussion Meeting)
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
"Fall Break" (No CSB Discussion Meeting)
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Harrison Wood (Biomedical Engineering and Mechanics)
“A study on the effects of inplane swelling gradients on orthotropic plates”
In this study, we examine the effects of inplane swelling gradients on resulting shapes of thin, orthotropic plates. Emphasis is placed on understanding how different swelling gradients and orthotropic material properties result in different shapes. This talk focuses on introducing the topic of incompatible elasticity applied to programming swelling functions and shapes in plates, and summarizes the current research of graduate student Harrison Wood on swelling and warping of engineered wood products. Several surface parameterizations are explored to explain warped shapes of orthotropic plates. An energy expression based on midplane strains and curvatures is minimized with respect to surface parameters, and competition between stretching and bending energy terms is studied to determine equilibrium shapes. Using some simple toy models of plate warp as inspiration, some scaling arguments are being developed to validate certain behaviors and shapes, such as the case where a specific inplane swelling gradient results in a cylindricallike shape at equilibrium for an orthotropic plate.
Organizer:Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Prof. Michael Flatte (University of Iowa)
Meeting with Students
Organizer: Giti Khodaparast
Friday 4:00pm
304 Robeson Hall
(poster)
Prof. William Ducker (Chemical Engineering)
"Absorption at Confined Interfaces"
Thin liquid films have different properties than bulk solutions because of the effects of the fields extending from the boundaries. These altered properties are important in determining the stability of colloid and nanoparticle suspensions, wetting films, adsorption in confined spaces, and in the fabrication and application of nanoscale devices. Our interest is in adsorption, which affects many of these applications: there is a multitude of applications where surfactants, polymers, ions, etc. are adsorbed to effect changes in thin films, for example, to alter the stability of colloidal particles. We describe measurements of adsorption between two flat plates when the plates are separated by 0 – 65 nm and several results for several examples: depletion of a simple ion in dilute solution and adsorption in very concentrated salt solutions. These measurements have been made possible by our development of new technique. Measurement of all separations is achieved simultaneously by measuring visiblelight interference in a wedgeshaped crack created between an oxidizedsilicon wafer and a glass wafer. The adsorbed amount is measured from the fluorescence emission of a dye, after accounting for the optical interference. The specific measurement is of the depletion of a divalent anion, fluorescein, in aqueous solution between two anionic solids. For dilute solutions at large separations between the flat plates, the dye is depleted relative to the bulk concentration. At smaller separations, the depletion of the dye decreases. The range of the depletion and the magnitude of depletion decrease with shorter Debyelength. Both of these effects are consistent with a simple calculation using the PoissonBoltzmann equation. For concentrated solutions, results do not agree with PoissonBoltzmann theory. That theory predicts that the surface potential decays exponentially with a decay length (Debyelength) that decreases with increasing concentration. Results are consistent with an increase in decay length with increasing concentration. We make comparisons to results in ionic liquids and drawn conclusions for crystal growth through particle attachment. We
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Faculty Meeting (No CSB Discussion Meeting)
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Thanksgiving Break (No CSB Discussion Meeting)
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Michael Kane (Mechanical Engineering)
"Topography and Mechanical Properties of Nanostructured PNIPAM Films"
PNIPAM is a thermoresponsive polymer that has wide applications in biological applications, including its use as a cell growth scaffold. In this talk, we will discuss some of the recent measurements that we have done on PNIPAM films on nanostructured substrates. Using Atomic Force Microscopy, we investigate the surface topography of the films at different temperatures as well as their mechanical properties in different parts of the sample.
Organizer: Rana Ashkar
Friday 4:00pm
304 Robeson Hall
(poster)
First Day of Exams (No CSB Discussion Meetings)
Organizer: Vinh Nguyen
Summer 2018
These meetings occur on Fridays from 1:30pm to 2:30pm in Robeson 304 (unless otherwise indicated)
Friday 1:30pm
304 Robeson Hall
(poster)
Organizer: Vinh Nguyen
Friday 1:30pm
304 Robeson Hall
(poster)
Organizer: Vinh Nguyen
Friday 1:30pm
304 Robeson Hall
(poster)
Organizer: Vinh Nguyen
Friday 1:30pm
304 Robeson Hall
(poster)
Organizer: Vinh Nguyen
Friday 1:30pm
304 Robeson Hall
(poster)
Ruslan Mukhamadiarov (Physics, Virginia Tech)
"Transverse Temperature Interface in KatzLebowitzSpohn Model"
Driven lattice gas with attractive nearest neighbor interactions and periodic boundaries demonstrate intriguing dynamics, when parts of lattice held at different temperatures. In two dimensions, this complex system experiences a jamming transition in the high temperature zone, and forms stripes in the low temperature regions. Density profiles are strikingly similar with those for Asymmetric Exclusion Process (ASEP) with open boundary conditions when injection and ejection rates are equal. In this talk, I will discuss the dynamics of twotemperature driven lattice gas system and characterize its density profile using analytical results and Monte Carlo simulations.
Organizer: Vinh Nguyen
Friday 1:30pm
304 Robeson Hall
(poster)
Prof. Uwe Tauber (Virginia Tech, Physics)
"Interactive Discussion: Manuscript writing"
Organizer: Vinh Nguyen
Friday 1:30pm
304 Robeson Hall
(poster)
Shadi Esmaeili
"An Exploration of Characteristics of System of Kuramoto Oscillators"
Coupled oscillators and emergent synchronized patterns can be found in many phenomena in nature. Kuramoto model is the simplest model of coupled oscillators with an exact solution that can explain many such phenomena. By choosing a proper coupling constant and topology the system shows multistability. Also, by choosing nonhomogeneous frequencies long period orbits emerge in the system. We study the effects of the change in different parameters of the system (e.g. coupling constant and width of frequency distribution) as well as the response of the system to external noise.
Organizer: Vinh Nguyen
Friday 1:30pm
304 Robeson Hall
(poster)
Ahmadreza Azizi (Physics, Virginia Tech)
"Microscopic description of Generalized Voter Model"
The Langevin equation of critical phenomena in the presence of two symmetric absorbing states is considered as a novel macroscopic description of generalized Voter model (GVM). Numerical integration of GVM in two dimensions shows that the direct transition from a disorder phase to either of the absorbing states is described by voter critical point. Also, indirect transitions to the ordered state can happen where the Voter critical point is split into Ising and Directed percolation (DP) critical points. Although the Langevin description of GVM is successful, there is no known microscopic version of GVM in two dimensions which clearly presents all three critical points together. We will study one of the possible ways to achieve a microscopic version of GVM with Voter, Ising and DP critical points.
Organizer: Vinh Nguyen
Friday 1:30pm
304 Robeson Hall
(poster)
Prof. John B. Phillips (Biological Sciences, Virginia Tech)
"Quantum Biology meets Behavioral Biology (and a Behavioral Biologist): a new sensory system and a new class of sensory receptors in the mammalian retina"
The ability of animals to detect the Earth’s magnetic field remains the least understood of the major senses. Many vertebrates have two functionally distinct magnetoreception mechanisms: a lightdependent, photoreceptorbased mechanism that provides directional (‘compass’) information and a nonlightdependent, magnetitebased mechanism that provides positional (‘map’) information. The lightdependent magnetic compass (LDMC) is mediated by a manifestly quantum process thought to involve a lightdependent radical pair reaction that forms longlived, spincoordinated radical pair intermediates (“radical pair mechanism” or RPM). The most compelling evidence for the RPM is the finding that magnetic compass orientation in a variety of animals can be altered or abolished by exposure to lowlevel radio frequency (RF) fields (> 1nT) that can alter the electronspin dynamics of the radial pair. Interest in the RPM spans a wide range of disciplines, and has been a primary impetus for the emerging field of Quantum Biology. Studies of murine rodents (mice, rats, etc.) have played a central role in both basic and applied (i.e., biomedical) research on mammalian spatial behavior and cognition. A number of wellcharacterized spatial cells (e.g., head direction cells, place cells, grid cells, boundary vector cells, and velocity cells; see 2014 Nobel Prize in Medicine) underlie a path integration system that encodes the animal’s spatial position as it moves through the environment. However, the spatial circuitry characterized to date only provides accurate navigational information over distances of a few 10s of meters, falling well short of the 100s of meters routinely moved by even small rodents like deer mice (20g) under natural conditions. A magnetic compass sense can dramatically increase both the range and accuracy of a path integration system, as well as play important roles in many other aspects of spatial behavior and cognition. Nevertheless, the consensus of the literature is that murine rodents do not rely on magnetic cues, despite evidence that a magnetic compass is virtually ubiquitous in other animals, including some mammals (bats, mole rats, dolphins). Contrary to the prevailing view in the literature, we have found that mice and rats have a welldeveloped magnetic compass. However, consistent behavioral and neurophysiological responses to magnetic cues can only be elicited reliably when the testing apparatus is shielded to screen out lowlevel RF noise. We have also identified photoreceptors in animals as different as flies, frogs, and mice that appear specialized for detection of the geomagnetic field. In this talk, I’ll briefly discuss evidence: (1) that there are a specialized photoreceptors in which the response to light is dependent on the alignment of an earthstrength magnetic field, (2) that in animals where specialized photomagnetoreceptors are located in the compound eye (flies) or retina (birds, mice), the magnetic field may be perceived as a 3dimensional pattern of light intensity and/or color superimposed on the animal’s surroundings, (3) that both behavioral and neurophysiological responses to magnetic cues can be altered or abolished by lowlevel radio frequency noise at intensities commonly found in laboratory environments, and (4) that the magnetic field plays multiple, previously unrecognized, roles in the spatial behavioral and cognition of murine rodents over a variety of spatial scales.
Organizer: Vinh Nguyen
Friday 1:30pm
304 Robeson Hall
(poster)
Professor Michel Pleimling (Physics, Virginia Tech)
"Aging processes in systems far from equilibrium I: An overview of the phenomenology of physical aging"
Physical aging scaling is encountered in numerous systems with slow dynamics. In this talk I introduce the phenomenology of physical aging and show that many of the characteristic features of physical aging can be understood through the investigation of simple coarsening systems. Dynamical scaling of twotime quantities like the autoresponse and autocorrelation functions is discussed for systems with a single timedependent length scale.
Organizer: Vinh Nguyen
Friday 1:30pm
304 Robeson Hall
(poster)
Professor Uwe Tauber (Physics, Virginia Tech)
“Interactive Discussion on Applications”
Organizer: Vinh Nguyen
Friday 1:30pm
304 Robeson Hall
(poster)
Professor Michel Pleimling (Physics, Virginia Tech)
"Aging processes in systems far from equilibrium II: Systems with complex ordering processes"
In this talk I first discuss aging scaling properties of a manyspecies system undergoing coarsening with nontrivial indomain dynamics. The second part of the talk is devoted to physical aging in interacting skyrmion matter. Twotime correlation functions are analyzed to study the nonlinear stochastic relaxation dynamics in the aging regime.
Organizer: Vinh Nguyen
Spring 2018
These meetings occur on Fridays from 4:00pm to 5:00pm in Robeson 304 (unless otherwise indicated)
Friday 4:00pm
304 Robeson Hall
(poster)
"No CSB Discussion Meeting Scheduled"
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
"No CSB Discussion Meeting Scheduled"
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
"No CSB Discussion Meeting Scheduled"
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
"No CSB Discussion Meeting Scheduled"
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
"No CSB Discussion Meeting Scheduled"
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Jacob Carroll (Department of Physics, Virginia Tech)
"Sparsely Encoding Convolutional Neural Networks II"
Neural networks are a family of models that range from the biologically inspired recurrent networks that serve as models of the brain, to the feedforward, deeplearning networks that have been at the forefront of machine learning in recent years. This talk will continue to introduce a specific type of neural network that while biologically inspired, has been developed for the purpose of machine learning and computer science: the sparsely encoding convolutional neural network. This talk will explain how these systems are used for imaged denoising, and how finite size scaling was observed in these networks as they denoised images across many different values of sparsity. This finite size scaling implies that these systems undergo a continuous as sparsity is varied.
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Priyanka (Department of Physics, Virginia Tech)
"Study of anomalous behavior in onedimensional harmonic system"
I will start with some theoretical models which have been developed to understand the violation of Fourier's Law in the lower dimension. Anomalous transport, nonlinear temperature profile etc, are the key feature of these model. In detail, I will talk about one of these models (harmonic chain with volume exchange) and present some its analytical and numerical results. I will present exact expression of twopoint function in a stationary state and also shows that the dynamics are governed by fractional Laplacian.
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Weigang Liu (Department of Physics, Virginia Tech)
"A numerical study of the twodimensional complex GinzburgLandau equation"
The complex GinzburgLandau equation with additive noise is a stochastic partial differential equation that describes a remarkably wide range of physical systems: coupled nonlinear oscillators subject to external noise near a Hopf bifurcation instability; spontaneous structure formation in nonequilibrium systems, e.g., in cyclically competing populations; and drivendissipative BoseEinstein condensation, realized in open systems on the interface of quantum optics and manybody physics. We employ a finitedifference method to numerically solve the noisy complex GinzburgLandau equation on a twodimensional domain with the goal to investigate the coarsening dynamics following a quench from a strongly fluctuating defect turbulence phase to a longrange ordered phase. We start from a simplified amplitude equation, solve it numerically, and then study the spatiotemporal behavior characterized by the spontaneous creation and annihilation of topological defects (spiral waves). We check our simulation results against the known dynamical phase diagram in this nonequilibrium system, tentatively analyze the coarsening kinetics following sudden quenches between different phases, and have begun to characterize the ensuing aging scaling behavior.
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Physics Faculty Meeting (No CSMB Discussion Meeting)
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
"Spring Break" (No CSB Discussion Meeting Scheduled)
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
CSB Faculty Meeting (No CSMB Discussion Meeting)
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Ali Charkhesht (Department of Physics, Virginia Tech)
"Probing Collective Motions of Proteins and Hydration Dynamics by a Wide Range Dielectric Spectroscopy"
Studying dynamics of proteins in their biological milieu such as water is interesting because of their strong absorption in the terahertz range that contain information on their global and subglobal collective vibrational modes (conformational dynamics) and global dynamical correlations among solvent water molecules and proteins. In addition, water molecules dynamics within protein solvation layers play a major role in enzyme activity. However, due to the strong absorption of water in the gigahertztoterahertz frequencies, it is challenging to study properties of the solvent dynamics as well as conformational changes protein in water. In response, we have developed a highly sensitive megahertztoterahertz dielectric spectroscopy system to probe the hydration shells as well as largescale dynamics of these biomolecules. . Thereby, we have deduced the conformation flexibility of proteins and compare the hydration dynamics around proteins to understand the effects of surfacemediated solvent dynamics, relationships among different measures of interfacial solvent dynamics, and proteinmediated solvent dynamics based on the complex dielectric response from 50 MHz up to 2 THz by using the system we developed. Comparing these assets of various proteins in different classes helps us shed light on the macromolecular dynamics in a biologically relevant water environment.
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Physics Faculty Meeting (No CSMB Discussion Meeting)
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Physics Faculty Meeting (No CSMB Discussion Meeting)
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Riya Nandi (Department of Physics, Virginia Tech)
"ShortTime Dynamics of ThreeDimensional Magnetic Systems with Heisenberg Interaction"
This project aims to explore the initial relaxation dynamics of Heisenberg ferro and antiferromagnets. It involves a new simulation technique of combining reversible mode coupling dynamics with the simple diffusive relaxation dynamics in order to obtain the correct dynamic exponent and identify the correct universality class. The system undergoes critical aging and relevant exponents identified. Finally, for a system with nonconserved orderparameter, i.e., the antiferromagnet, theory predicts nonuniversal initial slip exponent. This work aims to study its dependence on the width of the initial distribution of the conserved quantities. This is a work in progress, at best just beginning to show promising results.
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Physics Faculty Meeting (No CSMB Discussion Meeting)
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Physics Faculty Meeting (No CSMB Discussion Meeting)
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Exam Week (No CSMB Discussion Meeting)
Organizer: Vinh Nguyen






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Fall 2018
These meetings occur on Mondays from 4:00pm to 5:00pm in Robeson 304.
Refreshments are served before the semnars (unless otherwise indicated)
Monday 4:00pm
304 Robeson Hall
Joint CM Seminar
(poster)
Prof. Surita Bhatia (Stony Brook University, NY)
"Stratification in Colloidal Films"
Multicomponent films based on colloidal dispersions have a wide range of applications, including antimicrobial coatings for medical instruments, conductive textiles for flexible electronics, antireflective coatings for optical devices, paints for humid environments that are resistant to mold growth, and drugloaded coatings for medical implants. Often, there is a need to spatially control location of certain components in the film. For example, silver nanoparticles can be used to impart antimicrobial activity to paints, but this component is expensive and may only be needed in the top few layers of the coating, not throughout the entire film. In principle, evaporative drying of multicomponent dispersions can be used to create films with a prescribed vertical concentration profile in a onestep process. In this talk, we present our recent results from atomic force microscopy (AFM) and smallangle Xray scattering (SAXS) on films prepared from binary colloidal dispersions containing large and small particles of varying size and initial volume fraction. Our results show evidence of different types of stratification behavior, including largeontop (e.g., large particles migrating to the top surface of the film), smallontop, and “sandwich”like layering. We discuss these results in terms of recent theories for stratification during evaporative drying..
Host: Shengfeng Cheng
Monday 4:00pm
304 Robeson Hall
Joint CM Seminar
(poster)
Dr. Kunal Mondal (North Carolina State University)
"SoftNanomaterials, Interfaces, and MicroNanofabrication to Build Tools and Functional Devices"
New competitive technologies should be developed to deal with the world’s emerging problems in healthcare, environmental, agriculture, energy and security sectors to benefit a broad spectrum of society while using minimal resources. Multifunctional interfaces of nanomaterials can be used to tackle such glitches by developing sensors and detection devices such as biosensors, explosives trace detectors, mechanicalstress sensors, wastewater management systems and energy storage devices owing to their nanoscopic surface properties. Considering this, several catalytic and photocatalytic metal/metaloxide semiconductor nanostructures have been synthesized and used for environmental remediation, pointofcare diagnostics and energy storage applications. Several fabrication techniques including electrospinning, microfabrication, 3D printing etc. have been used to made functional nano/micro devices. Various physicochemical characterization techniques are used to study their properties in nanoscale. Furthermore, effort has been made on surface patterning and fabricating stretchable electronics by integration of conducting liquid metal in soft elastomers to explore ways to utilize these ‘softer than skin’ materials for bioelectronic applications. Finally, this concludes with an outlook and future challenges of these materials within this context.
Host: Rana Ashkar
Monday 4:00pm
304 Robeson Hall
(poster)
Labor Day "No CSB Seminar Scheduled"
Host:
Monday 4:00pm
304 Robeson Hall
(poster)
Host:
Monday 4:00pm
304 Robeson Hall
(poster)
Chengyuan Wen (Virginia Tech, Physics)
Host: Vinh Nguyen
Friday, 2:30pm
210 Robeson Hall
Special Seminar
(poster)
Prof. Gary Grest (Sandia National Laboratories, Albuquerque, NM)
"Going up in time and length scales in modeling polymers"
Polymer properties depend on a wide range of coupled length and time scales, with unique macroscopic viscoelastic behavior stemming from interactions at the atomistic level. The need to probe polymers across time and length scales and particularly computational modeling is inherently challenging. Here new paths to probing long time and length scales including introducing interactions into the traditional beadspring model that has been widely used for the past thirty years and coarse graining of atomistic simulations will be compared. Using linear polyethylene as a model system, the degree of coarse graining with two to six methylene groups per coarsegrained bead derived from a fully atomistic melt simulation were probed. Using these models we were successful in probing highly entangled melts and were able reach the longtime diffusive regime which is computationally inaccessible using atomistic simulations. We simulated the relaxation modulus and shear viscosity of wellentangled polyethylene melts for scaled times of a microsecond. The long time and length scale is coupled to the macroscopic viscoelasticity where the degree of coarse graining sets the minimum length scale instrumental in defining polymer properties and dynamics. Results will be compared to those obtained from the beadspring model to demonstrate the additional insight that can be gained from atomistically inspired coarse grained models.
Host: Shengfeng Cheng
Monday 4:00pm
304 Robeson Hall
(poster)
Host:
Friday, 2:30pm
210 Robeson Hall
Special Seminar
(poster)
Prof. Daniel I Goldman (Georgia Tech )
"Robophysics: Physics Meets Robotics"
Robots will soon move from the factory floor and into our lives (e.g. autonomous cars, package delivery drones, and searchandrescue devices). However, compared to living systems, locomotion by such devices is still relatively limited, in part because principles of interaction with complex environments are largely unknown. In this talk I will discuss efforts to develop a physics of moving systems  a locomotion ``Robophysics''  which we define as the pursuit of the discovery of principles of selfgenerated motion [Aguilar et al, Rep. Prog. Physics, 2016]. We use the methods of physics to examine successful and failed locomotion in simplified laboratory devices using parameter space exploration, systematic control, and techniques from dynamical systems. Drawing from examples from my group and our collaborators, I will discuss how robophysical studies in terrestrial environments have inspired new physics questions in low dimensional dynamical systems (including creation of analog quantum mechanics and gravity systems) and soft matter physics, have been useful to develop models for biological locomotion in complex terrain, and have begun to aid engineers in the creation of devices that begin to achieve lifelike locomotor abilities on and within complex environments. The rapidly decreasing cost of constructing sophisticated robot models with easy access to significant computational power bodes well for scientists and engineers to engage in a discipline which can readily integrate experiment, theory and computation.
Host: Uwe Tauber
Monday 4:00pm
304 Robeson Hall
Condensed Matter Seminar
(poster)
Dr. Jennifer Cano (Princeton University)
"TBD"
Host: Kyungwha Park
Monday 4:00pm
304 Robeson Hall
(poster)
Prof. David M. Leitner
(University of Nevada, Reno )
“Watching energy transport in proteins: Identifying dynamics networks and thermodynamic properties”
Energy transport in a protein mediates protein function and represents the early events following a reaction or photoexcitation. New timeresolved measurements, and a variety of computational and theoretical methods allow us to map out and describe energy transport in great detail. I will describe some of our theoretical and computational work on the nature of energy transport in proteins, with focus on what we can learn about protein dynamics and thermodynamics by watching energy flow in proteins. By coarse graining energy transport dynamics from the allatom to residue level, we have identified a relation between conformational dynamics at equilibrium and rates of energy transfer across nonbonded contacts. Measurements of rates of energy transfer thus provide a window into equilibrium dynamics of proteins and entropy associated with the dynamics of the contact.
Host:Vinh Nguyen
Monday 4:00pm
304 Robeson Hall
(poster)
Jacob Carroll (Virginia Tech, Physics)
“The effects of inhibitory neuron fraction on the dynamics of an avalanching neural network”
The statistical analysis of the collective neural activity known as avalanches provides insight into the proper behavior of brains across many species. In this paper we present a neural network model based on the work of Lombardi, Herrmann, de Arcangelis et al. that captures the relevant dynamics of neural avalanches, and we show how tuning the fraction of inhibitory neurons in this model removes exponential cutoffs present in the distributions of avalanche strength and duration, and transitions the power spectral density of the network into an epileptic regime, as well as effecting the evolution of the network structure over time. We propose that the brain operates away from this regime of low inhibitory fraction to protect itself from the dominating avalanches present in these extended distributions.
Host: Uwe Tauber
Friday, 2:30pm
304 Robeson Hall
No Colloquium
(poster)
Fall Break (No Colloquium)
Host:
Monday 4:00pm
304 Robeson Hall
Condensed Matter Seminar
(poster)
Alex Grutter
"TBD"
Host: Satoru Emori
Monday 4:00pm
304 Robeson Hall
(poster)
Shadi Esmaeili (Virginia Tech, Physics)
"From disorder to selforganization: A cyclic predatorprey system and a system of frustrated coupled oscillators"
Selforganization is the emergence of spontaneous order as a result of local interactions among the elements of a system. Systems far from equilibrium that are evolving toward their selforganized state show very interesting dynamic behaviors. We study the dynamic behavior of two systems: a cyclic predatorprey system with a complex spatiotemporal pattern, as well as a system of coupled oscillators with antagonistic coupling. In the predatorprey model, the response of the system to external perturbation is used as an approach to gain insights about its dynamic behavior. On the other hand, the breaking of time translation invariance was observed during the spontaneous relaxation of a system of coupled oscillators after a parameter quench in the absence of any stochastic fluctuation.
Host: Michel Pleimling
Friday, 2:30pm
304 Robeson Hall
Colloquium
(poster)
Prof. Michael Flatte' (University of Iowa)
"Quantum Coherent Electronic Technologies"
Electrons in most materials experience dramatic and frequent scattering from other electrons, phonons, and a variety of other excitations. Such scattering events often rapidly dissipate any memory the electron had of its quantum state, so the electrons can be described as an ensemble that is near local thermal equilibrium. If the electrons can retain a good memory of their quantum state, however, then they are quantum coherent and can be used for very unusual and exciting tasks such as quantum computing. Realizing these quantum technologies has traditionally been expected to require very special elements such as superconducting devices or very high mobility transistors, as well as very low temperatures, in order to avoid rapid loss of quantum coherence (decoherence). Over the past fifteen years we and others have identified remarkable examples of roomtemperature quantum coherent behavior in condensed matter electronic systems, usually involving spin coherence. Predicting the behavior of these spin coherent systems requires integrating theoretical techniques to cope with energy scales ranging from far smaller than the thermal energy to far larger. I will describe some examples of quantum coherent technologies and identify some of the features they share.
Host: Giti Khodaparast
Monday 4:00pm
304 Robeson Hall
CM Seminar
(poster)
Brian Skinner (Massachusetts Institute of Technology)
“Percolative Phase Transition in the Dynamics of Quantum Entanglement”
When left unobserved, manybody quantum systems tend to evolve toward states of higher entanglement. Making a measurement, on the other hand, tends to reduce the amount of entanglement in a manybody system by collapsing one of its degrees of freedom. In this talk I discuss what happens when a manybody quantum system undergoes unitary evolution that is punctuated by a finite rate of projective measurements. Using numerical simulations and theoretical scaling arguments, we show that for a 1D spin chain there is a critical measurement rate separating two dynamical phases. At low measurement rate, the entanglement grows linearly with time, producing a volumelaw entangled state at long times. When the measurement rate is higher than the critical value, however, the entanglement saturates to a constant as a function of time, leading to arealaw entanglement. We map the dynamical behavior of the entanglement onto a problem of classical percolation, which allows us to obtain the critical scaling behavior near the transition. I briefly discuss generalizations of our result to higher dimensions, and its implications for the difficulty of simulating quantum systems on classical computers.
Host: Uwe Tauber
Monday 4:00pm
304 Robeson Hall
Condensed Matter Seminar
(poster)
Dr. David Pappas (NIST)
"TBD"
Host: Sophia Economou
Monday 4:00pm
304 Robeson Hall
(poster)
Thanksgiving Holiday No Seminars scheduled
Host:
Monday 4:00pm
304 Robeson Hall
Condensed Matter Seminar
(poster)
Shannon Serrao (Physics, Virginia Tech)
"Fluctuation effects on a cyclic predatorprey system(MayLeonard model)"
Owing to close proximity with observed cyclic predatorprey dynamics in nature, we study the cyclic predatorprey model of MayLeonard with three species. The MayLeonard model is characterized by strong fluctuation induced effects to its nonequillibruim stationary state, notably the noise induced spatiotemporal spiral patterns on the two dimensional lattice; and the extinction of the longlived coexistence state on account of large but rare fluctuations. We study both these stochastic effects by firstly, characterizing the size of the aforementioned spiral patterns to the lowest order using the DoiPeliti coherent state path integral formalism and encoding the pattern quantitatively in the coefficients of the noisy complex GinsburgLandau equation. Secondly, on the wellmixed version of the model, we obtain the extinction times of all but one species driven by large fluctuations from a stable coexistence state and compare our results to Gillespie simulations across the transcritical bifurcation in the system.
Host: Uwe C. Tauber
Monday 4:00pm
304 Robeson Hall
(poster)
Prof. Sarah Perry (University of Massachusetts, Amherst)
"Molecular Engineering of Polyelectrolyte Complex Materials"
Electrostatic interactions and polyelectrolyte complexation can be used in the selfassembly of a wide range of responsive, bioinspired soft materials ranging from dehydrated thin films and bulk solids to dense, polymerrich liquid complex coacervates, as well as more complex hierarchical structures such as micelles and hydrogels. This responsiveness can include swelling and dissolution or solidification, which can be harnessed to facilitate encapsulation and the subsequent fabrication of functional materials. In particular, we draw inspiration from biomolecular condensates, or membraneless organelles, which utilize liquidliquid phase separation to create transient compartments in cells. These condensates are commonly formed due to weak, multivalent interactions involving intrinsically disordered proteins. Furthermore, these materials have been shown to enable the selective uptake of specific enzymes. We utilize polypeptides as model sequencecontrolled polymers to study how the patterning or presentation of charges and other chemical functionalities can modulate the potential for liquidliquid phase separation via complex coacervation. We further examine how the distribution of charge on globular proteins can be used to facilitate selective uptake into coacervate phases, and how such materials can be used to stabilize proteins against denaturation. This molecularlevel understanding of polyelectrolyte complexation is further enhanced by detailed rheological and thermodynamic examinations of the molecular nature of the various material transitions present in these systems. Our experimental efforts are supported by the parallel development of computational approaches for modeling and predicting the phase behavior of patterned polymeric materials. Our goal is to establish molecularlevel design rules to facilitate the tailored creation of materials based on polyelectrolyte complexation that can both illuminate selfassembly phenomena found in nature, and find utility across a wide range of realworld applications.
Host: Vinh Nguyen
Spring 2018
These meetings occur on Mondays from 4:00pm to 5:00pm in Robeson 304.
Refreshments are served before the semnars (unless otherwise indicated)
Monday 4:00pm
304 Robeson Hall
(poster)
Martin Luther King Holiday. No Seminar Scheduled.
Monday 4:00pm
304 Robeson Hall
(poster)
Physics Faculty Search. No CSB Seminar Scheduled
Monday 4:00pm
304 Robeson Hall
(poster)
Physics Faculty Search. No CSB Seminar Scheduled.
Monday 4:00pm
304 Robeson Hall
(poster)
Physics Faculty Search. No CSB Seminar Scheduled
Monday 4:00pm
304 Robeson Hall
(poster)
Physics Faculty Search. No CSB Seminar Scheduled.<./b>
Monday 4:00pm
304 Robeson Hall
Joint CM Seminar
(poster)
Dr. Michael Cooney ( NASA Langley Research Center )
MEDLI2, ARCSTONE and Broadband Photodetectors for measuring radiative flux
Dr. Michael Cooney engineer in the Electronic Systems Branch, will discuss three projects he currently supports. The first is a space flight project, the Mars Entry Descent and Landing Instrumentation 2 (MEDLI2). MEDLI2 will extend and enhance the dataset from the MEDLI mission, which flew on the Mars Science Laboratory (MSL) in 2012 and was the first instrument to characterize Mars’ aerothermal environment. MEDLI2 is scheduled to fly on the MARS 2020 mission. The second project is ARCSTONE, a lunar spectral reflectance instrument in response to the 2007 and 2017 Earth Science Decadal Surveys. Instrument intercalibration is a vital tool to maintain consistent datasets across various instruments and ensure historical continuity. The Moon is considered to be an excellent exoatmospheric calibration source, however the accuracy of the Moon as an absolute reference is limited to 510%. An orbiting spectrometer flying on a small satellite in low Earth orbit will provide lunar spectral reflectance with accuracy sufficient to establish an SItraceable absolute lunar calibration standard for past, current, and future Earth weather and climate sensors. The final project is a research activity in partnership with Virginia Tech to develop broadband photodetectors for measuring radiative flux in response to the Earth Science Decadal survey. Existing Earth science radiation budget instruments rely on radiometers with relatively difficult custom manufacturing processes and slow readout speed. To provide a lower cost future mission options, photon based photosensors have the possibility to lower mission cost while enabling new mission architectures.
Host: Vinh Nguyen
Monday 5:00pm
304 Robeson Hall
Special Seminar
(poster)
Dr. Michael Cooney ( NASA Langley Research Center )
Job/Internship Possibilities at NASA Langley
Host: Vinh Nguyen
Monday 4:00pm
304 Robeson Hall
CM Seminar Only
(poster)
Prof. Sumanta Tewari (Clemson University)
"TBD"
Host: Ed Barnes
Monday 4:00pm
304 Robeson Hall
(poster)
Spring Break Week and APS March Meeting. No Seminar Scheduled.
Monday 4:00pm
"Canceled and Rescheduled"
Joint CM Seminar
(poster)
Weigang Liu (Department of Physics, Virginia Tech)
"A study of the complex GinzburgLandau equation: analytical and numerical results"
Rescheduled for March 14, 2018
Host: Uwe Tauber
Wednesday 4:00pm
400 Hahn Hall, North
Joint CM Seminar
(poster)
Weigang Liu (Department of Physics, Virginia Tech)
"A study of the complex GinzburgLandau equation: analytical and numerical results"
The complex GinzburgLandau equation (CGLe) is a stochastic partial differential equation that describes a remarkably wide range of physical systems: coupled nonlinear oscillators subject to external noise near a Hopf bifurcation instability; spontaneous structure formation in nonequilibrium systems, e.g., in cyclically competing populations; and drivendissipative BoseEinstein condensation, realized in open systems on the interface of quantum optics and manybody physics. We employ the perturbative fieldtheoretic renormalization group method to analytically investigate the universal critical behavior near the continuous nonequilibrium phase transition in the complex Ginzburg–Landau equation with additive white noise. We show that to first order in the dimensional expansion about the upper critical dimension, the initialslip exponent in the complex Ginzburg–Landau equation is identical to its equilibrium model A counterpart. In our second project, we have employed a finitedifference method to numerically solve the noisy complex GinzburgLandau equation on a twodimensional domain with the goal to investigate the coarsening dynamics following a quench from a strongly fluctuating defect turbulence regime to a longrange ordered phase. We study the spatiotemporal behavior characterized by the spontaneous creation and annihilation of topological defects (spiral waves). We check our simulation results against the known dynamical phase diagram in this nonequilibrium system, tentatively analyze the coarsening kinetics following sudden quenches between different phases, and have begun to characterize the ensuing aging scaling behavior. Moreover, we are currently extracting the activation energy barrier for the nucleation process of the stable spiral wave structures.
Host: Uwe Tauber
Monday 4:00pm
304 Robeson Hall
Joint CM Seminar
(poster)
Chengyuan Wen (Department of Physics, Virginia Tech)
"Evaporation of Liquids and Solutions"
Evaporation of a liquid is a ubiquitous phenomenon. It drives the water cycle and can be used for cooling. It is also a useful tool for materials fabrication such as evaporationinduced selfassembly of colloidal particles and thinfilm deposition via spin coating. In the first part of this talk, we will present millionatom scale molecular dynamics (MD) simulations of the evaporation process of water. An enhancement of water density near the liquidvapor interface is found during fast evaporation. The temperature profiles based on both translational and rotational degrees of freedom are calculated at different stages of evaporation and evaporative cooling of the liquidvapor interface is observed, which accounts for the higher water density at the interface. The velocity distribution of water molecules in the vapor phase during evaporation is also computed at various distances relative to the interface and fit to the MaxwellBoltzmann distribution. Results indicate that local thermal equilibrium holds in the liquid phase, though the whole system is driven out of equilibrium. In the second part of this talk, we will focus on evaporating behavior of polymer solutions. In particular, polyelectrolyte solutions show rich physical behavior because of electrostatic interactions. We use MD simulations to study the evaporation of a solution of polyanionic chains (sodium polystyrene sulfonate). The polymers are represented by MARTINItype beadspring chains. Water is included as an explicit solvent and described with a model taking into account polarization effects. Counterions and salts are also explicitly included as mobile single beads. Our results show that the polyelectrolyte chains form layered structures with alternating polymerrich and counterionrich layers, indicating that onepot evaporation technique may be developed to fabricate multilayer polyelectrolyte films that are currently mainly produced via a layerbylayer deposition process. We will discuss the effects of polymer concentration, salt concentration, and evaporation rate on the structure of the resulting film. Finally, we will also briefly discuss our recent study of the evaporation of polymer solutions containing both polyanionic and polycationic chains.
Host: Shengfeng Cheng
Monday 4:00pm
304 Robeson Hall
Joint CM Seminar
(poster)
Xiangwen Wang (Department of Physics, Virginia Tech)
"Datadriven modeling of heavytailed distributions and scaling laws in human dynamics"
Studying human behavior is of fundamental importance in many social applications. Yet, it remains a challenging problem due to the high complexity of human activities. In recent years, advances in information technology have resulted in the collection of vast amounts of human activity logs, thus enabling the quantitative modeling of human behavior. Using a variety of metrics like probability distributions, we prove the wide existence of heavytailed distributions and scaling laws in human behavior. In human online searches we describe the search behavior as a foraging process that takes place on the semiinfinite line. A pairwise powerlaw distribution respectively exponential distribution is reported for steplengths in longrange respectively shortrange displacements, indicating that the search process is a combination of Brownianmotion local phases and truncatedLevyflight relocation phases. These results are confirmed through the analysis of mean squared displacements. In human online gambling, we view the net change of income of each player as a random walk and find that the win/loss distributions follow power laws with exponential cutoffs. The mean squared displacement of these net income random walks exhibits a transition between a superdiffusive and a diffusive regime. We present a model that allows to reproduce this behavior and identify the key features needed for observing this transition. For human movements in both real and virtual spaces, heavytailed steplengths are also reported.
Host: Michel Pleimling
Monday 4:00pm
304 Robeson Hall
Joint CM Seminar
(poster)
Ali Charkhesht (Department of Physcis, Virginia Tech)
"Probing collective motions and hydration dynamics of biomolecules"
Studying dynamics of proteins in their biological milieu such as water is interesting because of their strong absorption in the terahertz range that contain information on their global and subglobal collective vibrational modes (conformational dynamics) and global dynamical correlations among solvent water molecules and proteins. In addition, water molecules dynamics within protein solvation layers play a major role in enzyme activity. However, due to the strong absorption of water in the gigahertztoterahertz frequencies, it is challenging to study properties of the solvent dynamics as well as conformational changes protein in water. In response, we have developed a highly sensitive megahertztoterahertz dielectric spectroscopy system to probe the hydration shells as well as largescale dynamics of these biomolecules. . Thereby, we have deduced the conformation flexibility of proteins and compare the hydration dynamics around proteins to understand the effects of surfacemediated solvent dynamics, relationships among different measures of interfacial solvent dynamics, and proteinmediated solvent dynamics based on the complex dielectric response from 50 MHz up to 2 THz by using the system we developed. Comparing these assets of various proteins in different classes helps us shed light on the macromolecular dynamics in a biologically relevant water environment.
Host: Vinh Nguyen
"Special Seminar"
Friday, 2:30pm
210 Robeson Hall
Colloquium
(poster)
Prof. Jeff Chen (Department of Physics & Astronomy, University of Waterloo)
"The Onsager model for liquid crystals "
The Onsager model in liquid crystal theory holds the status of the Ising model for phase transitions. They both take a different view from the phenomenological Landaude Gennes model [liquid crystals] and Landau model [phase transitions] by relating the microscopic properties to the physical world. Identifiable molecular parameters are used in the Onsager model, allowing direct interpretation of experiments and computer simulation results. While the original model was proposed 70 years ago to deal with the bulk isotropicnematic transition, adding geometric frustrations gives the model a new life. In this talk, the solutions of the model for a number of confined systems of current interest, which display topological defects due to the frustrations between geometry and the nematic ordering field, are presented.
Host: Shengfeng Cheng
Monday 4:00pm
304 Robeson Hall
(poster)
Mengsu Chen
Exploring quantum manybody systems via lattice model and exact diagonalization
The quantum manybody problem of solving Schrodinger equation of a large number of interacting microscopic particles is generally considered impossible to tackle analytically. Numerical simulations becomes the essential tools to study these systems, especially at the strongly correlated regime. We used exact diagonalization (ED), the only unbiased numeric method, to study the newly proposed interactioninduced states such as fractional Chern insulators (FCI), topological Mott insulators (TMI), emergent kinetics on various quantum lattice model describing many physic systems including 2D materials and optical lattices. We study quantum phase transitions between different parameter regimes of Hamiltonian, and exotic properties such as fractional charges, spontaneous timereversal symmetry breaking in these phases.
Host: Vito Scarola
Monday 4:00pm
304 Robeson Hall
Joint CM Seminar
(poster)
Professor Mark Dykman ( Physics, Michigan State University)
"Timetranslation symmetry breaking in vibrational Floquet systems"
A periodically driven system has discrete timetranslation symmetry with the period of the driving. Its quantum dynamics is described in terms of the Floquet states. Generally, if the system is in a Floquet state, its dynamical variables oscillate with the period of the driving. Recently much interest have attracted systems where the time symmetry is broken, the “time crystal” effect. Nonlinear oscillators, including Nano mechanical systems and modes in electromagnetic cavities, provide an ideal platform for studying this effect. We will show how the symmetry breaking occurs in an individual oscillator in the quantum coherent regime. We will then discuss the classical and quantum phase transitions to the brokensymmetry state in systems of coupled oscillators.
Host: Uwe Tauber
Monday 4:00pm
304 Robeson Hall
CM Seminar Only
(poster)
Sriram Ganeshan (Stony Brook, New York)
"Odd Surface waves in twodimensional incompressible fluids"
In everyday fluids, the viscosity is the measure of resistance to the fluid flow and has a dissipative character. Avron, Seiler, and Zograf showed that viscosity of a quantum Hall (QH) fluid at zero temperature is nondissipative. This nondissipative viscosity (also known as ‘odd’ or ‘Hall’ viscosity) is the antisymmetric component of the total viscosity tensor and can be nonzero for parity violating fluids. I will discuss free surface dynamics of a twodimensional incompressible fluid with the odd viscosity (not quite quantum Hall hydro). For the case of incompressible fluids, the odd viscosity manifests itself through the free surface (no stress) boundary conditions. We first find the free surface wave solutions of hydrodynamics in the linear approximation and study the dispersion of such waves. As expected, the surface waves are chiral. In the limit of vanishing shear viscosity and gravity, we derive effective nonlinear Hamiltonian equations for the surface dynamics, generalizing the linear solutions to the weakly nonlinear case. In a small surface angle approximation, the equation of motion results in a new class of nonlinear chiral dynamics which we dub as chiral Burgers equation. I will briefly discuss how this program can be extended to the free surface of quantum Hall hydrodynamics.
Host: Ed Barnes
"Special Seminar"
Friday, 2:30pm
210 Robeson Hall
Colloquium
(poster)
Prof. Timothy HalpinHealy (Department of Physics, Columbia University)
"Within & Beyond the Realm of KPZ "
We discuss significant events in the recent Renaissance triggered by the enigmatic and elusive, but reach stochastic nonlinear PDE of Kardar, Parisi & Zhang, * a celebrated equation whose reach far exceeds its grasp, touching such diverse phenomena as nonequilibrium stochastic growth, optimal paths in illcondensed matter, the dynamics of driven lattice gases, as well as the extremal statistics of random matrix eigenvalues. *J. Stat. Phys. 160, 794 (2015).
Host: Uwe Tauber
Monday 4:00pm
304 Robeson Hall
Joint CM Seminar
(poster)
Prof. Juan Vanegas (University of Vermont)
"Mechanics at the nanoscale: Local stress calculations in Biomolecular systems"
The microscopic or local stress field provides a unique connection between molecular simulations and mechanics of materials at the nanoscale. Lateral stress profiles are routinely used to understand the mechanical behavior of liquid interfaces such as lipid membranes from molecular dynamics (MD) simulations. However, the 1dimensional stress profiles are not adequate to understand the multidimensional mechanical state in complex asymmetrical systems such as membrane proteins or other macromolecular structures. Furthermore, the fact that the microscopic stress from MD simulations is not uniquely defined is a theoretical consideration that is most often ignored, which has acute practical consequences when atomistic models are considered. I will present our recent work on the development of objective 3D local stress calculations by way of expressions that satisfy balance of linear and angular momentum for forcefields with arbitrarily high manybody interactions. I will show how some definitions of the microscopic stress violate mechanical equilibrium through various examples including defective graphene, lipid membranes, and fibrous proteins. I will also demonstrate the use of the traction vector, computed from the microscopic stress, as a powerful tool to visualize the local balance of forces at an interface. Focusing on the bacterial mechanosensitive channel MscL, I will show how the traction vector allows identification of a unique association pattern of lipids at specific sites on the MscL surface that may mediate gating of the bacterial channel by membrane tension or other stimuli.
Host: Shengfeng Cheng
Monday 4:00pm
304 Robeson Hall
(poster)
Final Exam Week. No Seminar Scheduled.







