Summer 2019
These meetings occur on Fridays from 1:30pm to 2:30pm in Robeson 304 (unless otherwise indicated)
Friday 1:30pm
304 Robeson Hall
(poster)
"CSB Symposium Week"
"No Discussion Meeting"
Special Time: Friday 4:00pm
304 Robeson Hall
(poster)
Ruslan Mukhamadiarov and Shengfeng Deng (Physics, Virginia Tech)
"Central Concepts of Nonlinear Dynamics and Chaos"
In a series of Center summer meetings, Shengfeng Deng and I will explore the central concepts of nonlinear dynamics and chaos. We will demonstrate how these abstract ideas can be applied in physics and biology. The list of topics that we plan to cover includes one and twodimensional flows, bifurcations, Lorenz equations, onedimensional maps, and fractals. This talk series is intended mainly for graduate students and will be held in the form of informal discussion meetings.
Host: Uwe Tauber
Friday 1:30pm
304 Robeson Hall
(poster)
TuoXian Tang (Biological Sciences, Virginia Tech)
"The Functional Basis of Phafin2 in Autophagy"
Autophagy is a highly conserved cellular pathway in the eukaryotic cells. A portion of the cytosol, which contains invading pathogens and longlived proteins, is taken up by an autophagosome. This doublemembrane organelle fuses with lysosomes, where the contents were digested by the lysosomal enzymes. Previous data showed that Phafin2 was involved in autophagy. After the induction of autophagy, Phafin2 and Akt accumulate on the lysosomal membranes through the interactions between Phafin2 and phosphatidylinositol 3phosphate (PtdIns(3)P). Phafin2 has two domains, one Nterminal PH (Pleckstrin Homology) domain and one Cterminal FYVE (Fab 1, YOTB, Vac 1, and EEA 1) domain. In this study, the binding affinity between PtdIns(3)P and Phafin2 was studied by surface plasmon resonance. Results showed that Phafin2 binds PtdIns(3)P with high affinity, triggering minor conformational changes in the protein. We also demonstrated that Phafin2 FYVE domain is responsible for the binding of PtdIns(3)P. Another interesting finding is that Phafin2 can cause membrane curvature, which may be required for tethering of lysosomes to autophagosomes, and consequently initiating autophagy.
Host: Vinh Nguyen
Friday 1:30pm
304 Robeson Hall
(poster)
Shengfeng Deng (Physics, Virginia Tech)
"Phase Portraits of TwoDimensional Flows"
Higherdimensional phase spaces can display more intricate and interesting dynamics. Two dimensional phase space already shows fixed points with different kinds of stabilities, and winding trajectories etc. We first discuss linear systems in two dimensions and the classification of fixed points, which will play an important role in the classification for fixed points of nonlinear systems. The phase portraits then allows us to predict the longterm behaviors of many physical systems.
Host: Uwe Tauber
Friday 1:30pm
304 Robeson Hall
(poster)
James Stidham (Physics, Virginia Tech)
"Ordering in Magnetic Skyrmion Lattices"
Ordering in magnetic skyrmion lattices is an active area of research for skyrmion systems. In this talk, I will present recent results obtained using Langevin molecular dynamic simulations, based on a previously derived particle model of skyrmion. Using a Vornoi cell algorithm, we examined the effect of the Magnus force present in skyrmion systems and how it affects ordering when noise is both present and absent in the system. We observed power law behavior during late time ordering in these skyrmion systems. We also found power law behavior when looking at the difference in time of consecutive events as the system orders.
Host: Michel Pleimling
Friday 1:30pm
304 Robeson Hall
(poster)
Ruslan Mukhamadiarov (Physics, Virginia Tech)
"Nonlinear Dynamics and Chaos: Limit Cycles and 2D Bifurcations"
Ubiquitous in nature, limit cycles are inherently nonlinear phenomena that can model systems with selfsustained oscillations. I am going to outline the existence conditions for the limit cycles, and I will show how the concept of limit cycles can be applied to study nonlinear oscillation problems. In the second part of the talk we will revisit the bifurcations and extend the concepts that we covered in the first meeting to the phase plane. I am also going to consider the other possible scenarios that arise in two dimensions, namely Hopf bifurcations and global bifurcations of cycles.
Host: Uwe Tauber
Friday 1:30pm
304 Robeson Hall
(poster)
"Last day of Classes of Summer Session I"
No Discussion Meeting
Friday 1:30pm
210 Robeson Hall
(poster)
Prof. David Minh (Chemistry, Illinois Institute of Technology)
"New Computational Tools for Designing Compounds Active Against Biological Macromolecules"
Most pharmaceuticals are small organic molecules that work via noncovalent interactions with biological macromolecules. Although drugs have saved or improved countless lives, drug discovery remains an inexact science that involves much trial and error. The main focus of my research group is the development of computer modeling tools to quickly characterize noncovalent proteinligand interactions. Most of our tools are based on implicit ligand theory, a theoretical framework that I derived to predict how tightly molecules bind and how they influence the population of conformations accessed by their targets. At this point, we have established that our methods are able to reproduce results of more computationally expensive approaches. We are working on making them more efficient and feasible to use with large libraries of chemical compounds. We have also advanced the theory of endpoint binding free energy methods, in which binding affinity is predicted based on molecular simulations of the bound complex.
Host: Igor Tolokh
Friday 1:30pm
304 Robeson Hall
(poster)
Austin Warren
(Physics, Virginia Tech)
"Nonlinear Dynamics and Chaos: Lorenz System and 1D Maps"
Despite being entirely predictable in principle, many deterministic systems display extremely complicated, apparently random long term behavior. This chaotic behavior appears in many practical everyday problems, from understanding how populations grow to predicting tomorrow's weather. In this talk, I'll be discussing what we mean when we talk about chaos and what it looks like in practice. In particular, we'll be looking at how chaos emerges from order in two wellknown systems: the continuous Lorenz system and the discrete logistic map.
Host: Uwe Tauber
Friday 1:30pm
304 Robeson Hall
(poster)
Connor Mackert
(Physics, Virginia Tech)
"Gray Scott Model Parameter Adjustment Effects"
The Gray Scott Model has been subject of numerous investigations. Due to the nonlinear nature of the reactiondiffusion system many studies have used overly broad strokes. Through systematic parameter adjustment we are able to find novel system pattern formations that were previously overlooked.
Host: Michel Pleimling
Friday 1:30pm
304 Robeson Hall
(poster)
Vinh Ho
(Physics, Virginia Tech)
“Broadband and High Responsivity Graphenebased Photodetectors at Roomtemperature”
Ability to covert light of graphene occurs in an ultrabroadband spectral range from violet to midinfrared region, making graphene as desirable photodetectors for various technology applications in imaging, sensing, spectroscopy and telecommunication. However, the low responsivity of graphene photodetectors about 10 mA/W, due to the ultrafast recombination of photocarriers, limits their potential applications. Here, we have engineered the interface between graphene and dielectric films to introduce trapping centers. The interface layer efficiently convert the photon energy into a large electrical signal. Thus, our graphenebased photodetectors have showed a high sensitivity up to 2 × 10^5 A/W together with a fast response time in a broadband spectral at room temperature.
Host: Vinh Nguyen
Friday 1:30pm
304 Robeson Hall
(poster)
"TBD"
Host:
Friday 1:30pm
304 Robeson Hall
(poster)
"Exams Begins for Summer Session II
Friday 1:30pm
304 Robeson Hall
(poster)
Anri Karanovich (Physics, Virginia Tech)
“Nonlinear Dynamics and Chaos: Fractals and Strange Attractors”
The emergence of deterministic chaos in many nonlinear systems, including the Lorentz map and the logistic map, is closely related to the existence of strange attractors  nontrivial closed subsets of the phase space, fractal in nature, to which nearby trajectories are converging. In order to gain a deeper insight into the patterns of the system evolution and its chaotic behavior, we need to study the main properties and geometric characteristics of the strange attractors. In the first part of the talk, I will define and provide basic examples of fractals, and then discuss the notion of fractional dimensionality and the various ways to measure it. In the second part, we will consider the simple examples of strange attractors, their characteristic features, methods of analysis, and relation to the chaotic properties of the system.
Host: Vinh Nguyen
Spring 2019
These meetings occur on Fridays from 4:00pm to 5:00pm in Robeson 304 (unless otherwise indicated)
Friday 4:00pm
304 Robeson Hall
"No CSB Discussion Meeting Scheduled"
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
"No CSB Discussion Meeting Scheduled"
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
"No CSB Discussion Meeting Scheduled"
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(Flyer)
Center for Soft Matter and Biological Physics Workshop (Location Blacksburg Community Center)
"No Discussion Meeting"
Organizers: William Ducker & Katrina Loan
"Special Date and Time" Monday 5:30pm
304 Robeson Hall
Student discussion with Dr. Rui Zhang (University of Chicago)
Host: Shengfeng Cheng
Friday 4:00pm
304 Robeson Hall
Student discussion with Dr. Ting Ge (Duke University)
Organizer: Shunsaku Horiuchi
Friday 4:00pm
304 Robeson Hall
Student discussion with Dr. Daniel Sussman (Syracuse University)
Organizer: Michel Pleimling
Friday 4:00pm
304 Robeson Hall
Student discussion with Dr. Antonia Statt (Princeton University)
Organizer: Uwe Tauber
"Special Date and Time" Monday 5:30pm
304 Robeson Hall
Student discussion with Dr. Cihan Nadir Kaplan (Harvard University)
Host: Rana Ashkar
Friday 4:00pm
304 Robeson Hall
"APS March Meeting Practice"
Organizer: Vinh Nguyen
"Special Date and Time" Monday 5:30pm
304 Robeson Hall
Student discussion with Dr. Trung Dac Nguyen (Northwestern University)
Host: Daniel Capelluto
Friday 4:00pm
304 Robeson Hall
"Faculty Meeting"
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
"APS March Meeting" (Boston)
"No discussion meeting scheduled"
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
"Spring Break continuing ends March 17"
"No CSB Discussion Meeting Scheduled"
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
"Faculty Meeting"
"No Discussion Meeting"
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
"Faculty Meeting"
:No Discussion Meeting"
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Nicole Abaid (Biomedical Engineering and Mechanics, Virginia Tech)
"Passive and active sensing in the Vicsek model
While many social animals move in groups using passive senses like vision, bats form very large colonies while using on sonar for navigation. Sonar is a socalled active sense, which relies on a selfgenerated signal (sound) rather than one already present in the environment (e.g., light for vision). From an engineering perspective, using active sensing in large groups poses many challenges centered around interference between signals produced by peers. However, experimental work with bats suggests that these animals may be capable of using their peers' signals for passive sonar, which may ameliorate some of these complications. Taking this system as inspiration, we explore the possibility of combining passive and active sensing in a wellstudied model which shows collective behavior, the Vicsek model. The Vicsek model enforces a local alignment rule in groups of selfpropelled particles perturbed by noise. Phase transition is observed in both the presence and absence of passive sensing, yet the range of parameters for which ordered and disordered group states exist dramatically changes when passive sensing is used. Notably, we find numerous cases of the model for which the implementation of passive sensing increases the robustness of the collective behavior to noise.
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
"Faculty Meeting"
"No Discussion Meeting"
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Dr. Saptarshi Chakraborty (Physics, Virginia Tech)
"PolymerStabilized Colloidal Catalysts: Role of Polymers and Strategies for Recovery and Reuse"
Gold nanoparticles (AuNPs) have attracted enormous attention due to their unique catalytic activities. Colloidal AuNPs provides the benefit of selectivity, greater surface area per mass of catalyst compared to supported catalysts, catalyzes reactions under mild conditions and are very effective for chiral catalysis. On a per metal basis colloidal AuNPs demonstrate higher catalytic activity than their supported counterparts. Colloidal AuNPs however requires surface functionalization with ligands to prevent aggregation which causes surface passivation and significant reduction in catalytic activity. Colloidal AuNP catalytic activity is strongly dependent on ligand packing and conformation on the AuNP surface. Large polymeric ligands demonstrate increase in available surface area leading to increased catalytic activity, while small molecule ligands lead to complete AuNP surface passivation. A major drawback of colloidal AuNPs as catalysts is the catalyst recovery from the product stream. We have employed small pH sensitive ligands as AuNP stabilizers to show that AuNPs can be recovered from the product stream by altering the pH to selectively precipitate or phase transfer catalyst into organic solvents. However, due to the high small molecule packing density on AuNP surface, complete surface passivation was observed. Catalytic activity could be recovered by partially functionalizing the AuNP surface, however at the cost of recoverability. To maintain catalytic activity in recoverable catalysts, we have developed pH sensitive polymer ligands by synthesizing thiolated polymer ligands. AuNPs functionalized with thiolated polymer, demonstrate similar recoverability while retaining high catalytic activity. Application of colloidal AuNP as catalysts, thus entails fine tuning ligand MW, structure, catalytic activity and recoverability.
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Prof. Jonathan Boreyko (Mechanical Engineering, Virginia Tech)
"Fun with Water: Catching Fog, Building Trees, and Freezing Bubbles"
Nature displays incredible feats of fluid mechanics that have much to teach us. Here, we study and exploit four different kinds of natureinspired fluid phenomena: two involving liquidphase behavior and two involving freezing water. First, we’ll explain how coastal redwoods have inspired a “fog harp” that harvests several times more water than existing fog harvesters. Second, we demonstrate that synthetic trees are capable of passively pumping water against gravity on the same scale as natural trees. The beauty of freezing bubbles is explained by novel physical models. Finally, we show that simple scaling laws can rationalize the development of passive antifrosting surfaces
Organizer: Vinh Nguyen
Wednesday 4:00pm
304 Robeson Hall
(poster)
Classes end
(No CSB Discussion Meeting)
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
Exams Begin
(No CSB Discussion Meeting)
Organizer: Vinh Nguyen
Friday 4:00pm
304 Robeson Hall
(poster)
University Commencement and College and Department Ceremonies
(No CSB Discussion Meeting)
Organizer: Vinh Nguyen
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








Spring 2019
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 ClassesUniversity Offices Closed)
Host:
"Special Seminar"
Friday, 4:00pm
304 Robeson Hall
(poster)
Prof. Xinqi Gong (Renmin University of China)
"Mathematical Intelligence Applications for BioMarcromolecular Problems"
The intersection among mathematics, information and biology has becoming more and more interesting and important. Many studies in this direction have led to developments of theories, methods and applications. But the too fast advancing of nowadays forefront information technology and biology knowledge, have triggered two obviously emerging phenomena, tremendous brandnew peaks accessible by new kinds of efforts, randomly meaningless results by incorrect intersections. Here I will show some of our recent results in developing and distinguishing efficiently intelligent approaches and applications for computational molecular biology and medical problems, such as protein structurefunctioninteraction prediction and pancreas cancer CT image analysis using algorithms like Fast Fourier transform, Monte Carlo, and deep learning, and some new designed physical and geometrical features.
Host: Shengfeng Cheng
"Special Colloquium" Monday 4:00pm
190 Goodwin Halll
(poster)
Dr. Rui Zhang (University of Chicago)
"Structure and Dynamics of Topological Defects in Active Liquid Crystals"
Topological defects in nematic liquid crystals exhibit unique optical and physicochemical properties that have led to emerging applications in directed selfassembly of colloids and macromolecules. Recent experiments have demonstrated that active matter that consists of a dense collection of selfpropelled rods can form an active nematic liquid crystal in which defects bind and unbind in a chaoticlike manner. Abundant examples of active nematics are found in different animate and inanimate systems, including flocking animals, bacteria, tissue cells, biopolymer suspensions, and even vibrating granular rods. However, the material properties of and seemingly chaoticlike defect dynamics in these nonequilibrium systems are poorly understood, limiting their applications. In this talk, I will discuss our recent work on unraveling defect behavior in active nematics. Specifically, we have adopted a hydrodynamic model to explain how the morphology, structure and dynamics of defects are determined by the interplay between elasticity and activity. Our model predictions are successfully confirmed by actomyosinbased experiments, shedding light on understanding and further control of topological defects in active liquid crystals.
Host: Shengfeng Cheng
"Special Colloquium" Friday, 2:30pm
210 Robeson Hall
(poster)
Dr. Ting Ge (Duke University)
"Rheology and Nanorheology of Entangled Melts of NonConcatenated Ring Polymers"
Rheology is the branch of science that studies how matter deforms and flows. Rheology of polymers is characterized by viscoelasticity that exhibits both solidlike elastic and liquidlike viscous features depending on the relevant time scales. Understanding and controlling the macromolecular dynamics underlying the viscoelastic response of polymers is one of the great challenges in polymer science. Complementary to experiments, theoretical and computational approaches provide deep physical insight into polymer dynamics. We use scaling theory and molecular dynamics simulation to study the rheology of nonconcatenated ring polymers [1], a prominent example of polymers with nonlinear architectures. The study of ring polymers sheds light on the effects of polymer architecture on polymer dynamics and rheology. Additionally, the melt of nonconcatenated ring polymers serves as a good model for the deswollen polymer network that possesses superelasticity and for the organization of chromosomes critical to the gene expression and regulation in cell nucleus. Unlike their linear counterparts, ring polymers do not form longlived entanglement network. Their viscoelastic response is characterized by a powerlaw stress relaxation prior to terminal viscous flow. The theoretical description and simulation results agree with each other and are in consistence with experimental observations of ring polymers. We further use molecular simulations to study nanorheology, which employs embedded nanoparticles to explore local viscoelasticity of polymers [2]. Nanorheology is important to the processing of nanoparticle polymer composites and the design of particlebased drug delivery systems in living cells as well. We find that the motion of large nanoparticles is not strongly suppressed in ring polymers as in linear polymers, because there is no longlived entanglement network that traps nanoparticles. With increasing nanoparticle size, the local viscoelasticity experienced by the particle approaches the bulk viscoelasticity of ring polymers. We estimate that the bulk viscoelasticity is reached for nanoparticle size about twice the average spanning size of ring polymers. References [1]. “SelfSimilar Conformations and Dynamics in Entangled Melts and Solutions of Nonconcatenated Ring Polymers”, T. Ge, S. Panyukov, and M. Rubinstein, Macromolecules 49, 708 (2016) [2]. “Nanorheology of Entangled Polymer melts”, T. Ge, G. S. Grest, and M. Rubinstein, Phys. Rev. Lett. 120, 057801 (2018) .
Host: Shunsaku Horiuchi
Monday 4:00pm
304 Robeson Hall
(poster)
Bart Brown (Virginia Tech)
“Coarsening with nontrivial indomain dynamics and dynamically generated hierarchies in predatorprey games”
Spatial manyspecies predatorprey systems have been shown to yield very rich spacetime patterns. We study the effects of nontrivial indomain pattern formations in the context of a sixspecies predator prey game which exhibits growing domains composed of three species in a rockpaperscissors relationship. Through the investigation of different quantities, such as spacetime correlations and the characteristic length, and interface width we demonstrate that the nontrivial dynamics inside the domains affects the coarsening process as well as the properties of the interfaces separating the domains. A ninespecies game is also introduced characterized by the spontaneous formation of spirals within spirals. The properties of these nested spirals are investigated through similar quantities including the temporal Fourier analysis of species density.
Host: Michel Pleimling
"Special Colloquium" Friday, 2:30pm
210 Robeson Hall
(poster)
Dr. Daniel Sussman (Syracuse University)
"Anomalous Interfaces in Biological Matter"
What can we learn about dense biological tissue by viewing it as a soft, active matter system? The mechanical and dynamical properties of dense collections of cells help govern processes ranging from wound healing to embryonic development to cancer progression, and an outstanding challenge is developing tractable models that can predict and explain the amazing variety of complex phenomena that even simple cellular systems can exhibit. Recent experiments have shown, for example, that many tissues lie close to a collective rigidity transition, and I will briefly discuss how simple coarsegrained models of dense tissue can support unusual forms of mechanical integrity. I will then show that these models exhibit anomalous interfacial properties, with different measurements of the surface tension between two tissues types differing by orders of magnitude. This departure from equilibrium behavior can be understood as a generic consequence of certain topological features of the cellcell interactions, and I will discuss the potential relevance of this mechanism for both biological processes (such as cell sorting and compartmentalization) and for designing new materials with exotic bulk and boundary behavior.
Host: Michel Pleimling
Monday 4:00pm
304 Robeson Hall
Condensed Matter Seminar
(poster)
Sam Carter (Naval Research Lab)
"Spins in InAs quantum dots: qubits, sensors, and photon sources "
Over the past few decades a number of exciting applications of quantum coherence and entanglement have been developed that promise fundamental improvements in computing, secure communications, and sensing. A team of scientists at the Naval Research Laboratory are working to develop a physical implementation for these quantum information applications using semiconductor indium arsenide quantum dots (QDs). This system has the advantages of a robust solid state host, strong optical transitions, mature device fabrication, tunable properties, and a scalable, monolithic architecture. A single electron or hole spin within a QD acts as a stationary quantum bit that can be optically controlled on a picosecond timescale. In this presentation, I will discuss how a spin in a QD or in a pair of coupled QDs can also be used for sensing mechanical motion and for generating tunable single photons. To sense motion, QDs have been incorporated into mechanical resonators, which couple to the dots through strain. When mechanical resonators are driven, the optical transitions of QDs shift significantly [1], and the spin states shift as well [2]. In single QDs, the hole spin shows much stronger coupling to strain than electrons spins, due to the stronger spinorbit interaction. In coupled QDs, a pair of interacting electron spins can be made highly sensitive to strain gradients that change the relative QD energies. To generate photons, we make use of the Raman spinflip process, which has the advantage of generating photons with properties determined by the drive laser and the spin properties. In this way, we are able to demonstrate spectral and temporal control over single photon wavepackets [3], with very low two photon emission probability and high indistinguishability. Finally, I will briefly discuss efforts that combine these topics, in which highly localized strain is used to tune multiple QD photon emitters into resonance within nanophotonic waveguides [4]. This work is supported by the U.S. Office of Naval Research and the OSD Quantum Sciences and Engineering Program. [1] Carter, S. G. et al. Sensing flexural motion of a photonic crystal membrane with InGaAs quantum dots. Appl. Phys. Lett. 111, 183101 (2017). [2] Carter, S. G. et al. Spinmechanical coupling of an InAs quantum dot embedded in a mechanical resonator. Phys. Rev. Lett. 121, 246801 (2018). [3] Pursley, B. C., Carter, S. G., Yakes, M. K., Bracker, A. S. & Gammon, D. Picosecond pulse shaping of single photons using quantum dots. Nat. Commun. 9, 115 (2018). [4] Grim, J. Q. et al. Scalable in operando strain tuning of multiple quantum dots within a photonic waveguide architecture. arXiv 1810.05195 (2018).
Host: Sophia Economou
"Special Colloquium" Friday, 2:30pm
210 Robeson Hall
(poster)
Dr. Antonia Statt (Princeton University)
"Pathways to Structure Formation in Colloid and Polymer Mixtures"
Soft matter is important in technological applications, biology and everyday life. Its behavior on mesoscopic scales is challenging to predict because dominant energy scales are of the magnitude of thermal fluctuations. I will present simulation results for two examples of structure formation in soft matter: colloidal crystal nucleation and inverted stratification in drying polymer mixtures. We developed a novel method to determine nucleation barriers without calculating the an isotropic interracial tension or locating the interface precisely. By demonstrating the importance of hydrodynamic interactions during evaporation, we show that hydrodynamics need to be incorporated when predicting the structure of drying films.
Host: Uwe Tauber
"Special Colloquium" Monday 4:00pm
190 Goodwin Hall
(poster)
Dr. Cihan Nadir Kaplan (Harvard University)
"Morphing hard and soft matter by reactiontransport dynamics"
Engineering nextgeneration materials that can grow into efficient multitasking agents, move rapidly, or discern environmental cues greatly benefits from inspiration from biological systems. In the first part of my talk, I will present a geometrical theory that explains the biomineralizationinspired growth and form of carbonatesilica microarchitectures in a dynamic reactiondiffusion system. The theory predicts new selfassembly pathways of intricate morphologies and thereby guides the synthesis of lightguiding optical structures. The second part is dedicated to a soft matter analog of controlled actuation and complex sensing in living systems. Specifically, I will introduce a continuum framework of a simple hydrogel system that is activated upon transport and reaction of chemical stimuli. The hydrogel exhibits unique cascades of mechanical and optical responses, suggesting that common gels have a much larger sensing space than currently employed. The theoretical work presented in my talk is intimately connected to modern materials science. The effective convergence of theory and experiment paves the way for optimized hard or soft biomimetic materials for applications ranging from bottomup manufacturing to soft robotics.
Host: Rana Ashkar
"Special Colloquium" Monday 4:00pm
190 Goodwin Halll
(poster)
Dr. Trung Dac Nguyen (Northwestern University)
"Engineering materials from bottom up for bioremediation applications"
The accumulation of mismanaged organic waste and toxins in insecticides and warfare agents poses serious environmental threats to the ecosystem and human health. Bioremediation provides an effective means to address these issues, for instance, by using cleaning agents containing enzymes that break down the pollutants into more biofriendly products. The ability to maximize the catalytic activity of those enzymes out of their native media will enable industryscale bioremediation applications for a wider variety of pollutants. In this talk, I will discuss a recently proposed bottomup approach for stabilizing enzymes in organic solvents, where the enzymes are mixed with copolymers that are composed of hydrophobic and hydrophilic monomers arranged in disordered sequences. These socalled random heteropolymers possess a rich diversity in monomer sequences, which plays a vital role in reducing the enzymes’ exposure the unfavorable solvent. This helps explain why the proposed approach is superior to the oftenused reversemicelle techniques in retaining the activity of numerous types of enzymes in toluene. Furthermore, the collectivity in the interaction between the polymers and enzymes upon assembly is predicted to lead to the uniform size of the assembled complexes, which is important for their solubility and delivery. These outstanding features of the random heteropolymer approach suggests new possibilities for engineering proteinbased materials for applications much beyond bioremediation.
Host: Daniel Capelluto
"Special Seminar"
Tuesday, 5:00pm
304 Robeson Hall
(poster)
Prof. Nuno Araujo (University Lisbon, Portugal)
"Dynamics of Colloidal Particles on Surface and Interfaces"
Colloidal particles are considered ideal building blocks to produce materials with enhanced physical properties. The stateoftheart techniques for synthesizing these particles provide control over shape, size, and directionality of the interactions. In spite of these advances, there is still a huge gap between the synthesis of individual components and the management of their spontaneous organization towards the desired structures. The main challenge is the control over the dynamics of selforganization. In their kinetic route towards thermodynamically stable structures, colloidal particles selforganize into intermediate structures that are much larger than the individual particles and become the relevant units for the dynamics. To follow the dynamics and identify kinetically trapped structures, one needs to develop new theoretical and numerical tools. In this seminar, we will discuss the selforganization of colloidal particles under confinement. Experiments with suspensions of ellipsoidal colloidal particles suggest a transition in the statistical properties of the stain left by an evaporating drop, depending on the eccentricity of the particles [1]. We proposed a stochastic model to show that the verystrong anisotropic capillary attraction between particles stemming from the deformation of the interface can be responsible for such transition [2,3]. We will discuss the main mechanisms involved and compare the quantitative results with experiments. With the experimental groups of Erika Eiser (Univ. Cambridge) and Jasna Bruijc (New York University), we have shown that the longrange capillary attraction and consequent formation of kinetically trapped structures of colloidal particles at interfaces can be avoided using DNAcoated colloids on complementary functionalized interfaces (oil droplet) [4], where we keep the irreversible interfacial binding but suppress the strong attraction, resulting in a fully ergodic colloidal dynamics. We will discuss how the coverage of the oil droplet by colloidal particles and the selfassembled structures depend on different system parameters, such as, temperature and bulk concentration of colloidal particles. [1] P. J. Yunker et al. Physical Review Letters 110, 035501 (2013). [2] C. S. Dias et al., Soft Matter 14, 1903 (2018). [3] C. S. Dias, N. A. M. Araújo, M. M. Telo da Gama, EPL 107, 56002 (2014). [4] D. Joshi et al. Science Advances 2, e1600881 (2016). * with M. M. Telo da Gama, C. S. Dias, A. S. Nunes, and D. Pinto
Host: Uwe Tauber
Monday 4:00pm
304 Robeson Hall
(poster)
APS Meeting (No Seminar)
Host:
Monday, 4:00pm
304 Robeson Hall
(poster)
Spring Break
Host:
Monday 4:00pm
304 Robeson Hall
(poster)
Prof. So Takei (Queens College, NY)
“Macroscopic Quantum Spintronics Devices”
We propose two platforms for realizing macroscopic spintronics qubits. The first prototype magnetic quantum information processing device, based on spin superfluidity and spin Hall phenomena, realizes the spinsupercurrent analog of the superconducting phase qubit, and allows for full electrical control and readout. The second device stores a quantum state in a topological defect of a magnetic insulator and realizes the magnetic analog of the threelevel rfSQUID qubit. We propose noninvasive methods to coherently control and readout the quantum state using ac magnetic fields and magnetic force microscopy, respectively. Various physical estimates for both devices, e.g., operational temperatures and decoherence times, will be made and discussed.
Host: Satoru Emori
Monday 4:00pm
304 Robeson Hall
(poster)
Prof. Shawn Cui (Math Dept. , Virginia Tech)
“4dimensional topological quantum field theories from fusion categories ”
We give a construction of a family of 4D topological quantum field theories (TQFT). By the AtiyahSegal axiomatic formulation, a 4D TQFT assigns vector spaces to 3D space manifolds and linear maps to 4D spacetime manifolds (cobordisms) satisfying some compatibility conditions. If the spacetime manifold has no boundary, then the corresponding linear map is a scalar, called the partition function. We will focus on the construction of partition functions. The data used in the construction involves some interplay between group actions and fusion categories. The resulting TQFTs generalize simultaneously most known ones in the literature such as DijkgraafWitten TQFT, CraneYetter/WalkerWang TQFT, etc. Further generalizations using higher categories will also be discussed briefly.
Host: Djordje Minic
Monday 4:00pm
304 Robeson Hall
Condensed Matter Seminar
(poster)
Prof. Herbert Fotso (University at Albany)
“Taming the Solid State Environment: Spin Qubits and Quantum Optics”
A variety of solid state systems are promising candidates for implementation of quantum bits (qubit) in quantum information processing (QIP). These include the NitrogenVacancy centers and other color centers as well as quantum dots. For such systems, stationarytoflying qubit conversions are of central importance. Furthermore, the ability to generate distributed entanglement across distant quantum nodes is essential for the construction of scalable quantum networks and for many fundamental QIP operations (quantum teleportation, Bell inequality tests...). Solid state quantum emitters are subject to fluctuations of different types in the surrounding bath (charge, spin, strain) and these fluctuations can in turn modify their optical properties and adversely affect QIP operations. For instance, entangling two qubits can be achieved through photon interference on a beam splitter. However, this process will see its efficiency drastically diminished by fluctuations in the uncorrelated environments of the respective qubits. We will show that appropriate control protocols can be employed to mitigate the effects of the environment on quantum emitters and enhance the efficiency of fundamental quantum information processing operations.
Host: Ed Barnes
Monday 4:00pm
304 Robeson Hall
Joint CM Seminar
(poster)
Prof. Chih Kuan Tung (North Carolina A&T State University)
"What Do Bull Sperm Know about Emergent Behaviors?"
In a complex system, some patterns or orders only emerge when the objects interact with the environment or each other. In a dynamical system, the description of how the environmental stress induces the new order can often be described by a bifurcation. In a manybody system, the interaction between individual objects often results in a phase transition or phase separation. These are arguably the most universal descriptions you can find in physics, covering phenomena from Higgs mechanism in high energy, superconductivity in condensed matter, to thermal convection in nonlinear dynamics. Biology provides vast number of different complex systems, which provide a fertile ground to explore universality through their emergent behaviors. In this talk, I will focus on two emergent behaviors discovered by using microfluidics to model the physical environment of the mammalian female reproductive tract for sperm. By modeling the outward going fluid flow in the female tract, we showed that sperm swimming against a flow can be described by a bifurcation theory, such that the upstream orientation order only emerges when the flow rate exceeds a critical level, and the emergence follows a ½ power law, which is known for a mean field theory. By adding polymer into the sperm medium to model the viscoelasticity naturally found in the mucus, we found that sperm start to form dynamic clusters, so that the cells dynamically join or dissociate from the clusters, similar to a liquid/gas phase separation. Further, by modeling the pulsatile flow generated by muscular contraction, we saw hundreds of sperm forming a large flock after the flow dissipated. Interestingly, the direction of the large flock can be either with or against the flow direction. I will discuss the implications in both physics and biology.
Host: Shengfeng Cheng
Monday 4:00pm
304 Robeson Hall
(poster)
Dr. Ulrich Dobramysl (University of Cambridge)
"How do cells sense direction?"
In many biological processes, in particular embryonic and brain development, cells need to follow chemical gradients to arrive at a precise location. To this end, they need to be able to determine the direction and position of a source releasing diffusing guidance cues from information gathered by receptor clusters located on the cell membrane. We develop an analytical model and an efficient numerical simulation procedure to calculate the particle fluxes to receptors and determine the limits of direction sensing in different environments. We find that the cell needs three or more receptor clusters to enable reconstruction of the exact source position from measured particle fluxes. We develop simulations of cells navigating in a chemical gradient based on these findings and show that cells are indeed able to find their targets.
Host: Uwe Tauber
Monday 4:00pm
304 Robeson Hall
(poster)
Prof. Peter Schauss (University of Virginia)
“Quantum gas microscopy of manybody dynamics in FermiHubbard and Ising systems”
The ability to probe and manipulate cold atoms in optical lattices at the atomic level using quantum gas microscopes enables quantitative studies of quantum manybody dynamics. While there are many welldeveloped theoretical tools to study manybody quantum systems in equilibrium, gaining insight into dynamics is challenging with available techniques. Approximate methods need to be benchmarked, creating an urgent need for measurements in experimental model systems. In this talk, I will discuss two such measurements. First, I will present a study that probes the relaxation of density modulations in the doped FermiHubbard model. This leads to a hydrodynamic description that allows us to determine the conductivity. We observe bad metallic behavior that we compare to predictions from finitetemperature Lanczos calculations and dynamical mean field theory. Second, I introduce a new platform to study the 2D quantum Ising model. Via optical coupling of atoms in an optical lattice to a lowlying Rydberg state, we observe quench dynamics in the resulting Ising model and prepare states with antiferromagnetic correlations
Host: Ed Barnes
Monday 4:00pm
304 Robeson Hall
(poster)
Prof. Sebastian Deffner (University of Maryland)
“Quantum thermodynamics: An introduction to the thermodynamics of quantum computers”
We are the verge of a technological revolution. Over the last couple of years the first computational devices have become commercially available that promise to exploit socalled quantum supremacy. Even though the thermodynamic cost for processing classical information has been known since the 1960s, the thermodynamic description of quantum computers is still at its infancy. This is due to the fact that many notions of classical thermodynamics, such as work, do not readily generalize to quantum systems in the presence of thermal and quantum noise. In this colloquium, we will outline a novel conceptual framework of an emerging theory, Quantum Thermodynamics, and its application to quantum computers.
Host: Sophia Economou
Monday 4:00pm
304 Robeson Hall
(poster)
Host:
Thursday, 1:00pm
"Special Seminar"
304 Robeson Hall
(poster)
Prof. Wei Li (Central China Normal University, Wuhan)
"Reinforcement learning in complementarity game and population dynamics"
I will systematically test and compare different reinforcement learning schemes in a complementarity game played between members of two populations. In our setting an optimized learning scheme, which beats all the rest, can be identified. I also compare these reinforcement learning strategies with evolutionary schemes. This gives insight into aspects like the issue of quick adaptation as opposed to systematic exploration or the role of learning rates.
Host: Uwe Tauber
Wednesday,
210 Robeson Hall
(poster)
Final Exam Week
(No Seminars)
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.







