2019 Discussion Meetings

No Discussion Meeting

Host:

No Discussion Meeting

Host:

No Discussion Meeting

Host:

Ahmadreza Azizi (Physics, Virginia Tech)

"Critical Phenomena in Presence of Symmetric Absorbing States"

Phase transitions in non-equilibrium systems have been the subject of many studies in the area of statistical physics. In this talk, I will discuss standard steady-state and time-dependent quantities for models which exhibit a voter-like phase transition, a symmetry-breaking phase transition in absence of bulk noise, and an absorbing phase transition belonging to the Directed Percolation universality class. The Langevin description of systems with two symmetric absorbing states will be introduced and the resulting phase diagram with three different phases (disordered and active, ordered and active, absorbing) separated by critical lines belonging to three different universality classes (generalized voter, Ising, and directed percolation) will be discussed. I also present a proposed microscopic model with two symmetric absorbing states that possesses all the features of the Langevin description.

Advisor: Michel Pleimling

No Discussion Meeting

Host:

No Discussion Meeting

"TBD"

Host:

No Discussion Meeting

Host:

Thanksgiving Holiday (No Classes)

Host:

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 two-dimensional flows, bifurcations, Lorenz equations, one-dimensional 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

Shengfeng Deng (Physics, Virginia Tech)

"Phase Portraits of Two-Dimensional Flows"

Higher-dimensional 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 long-term behaviors of many physical systems.

Host: Uwe Tauber

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

Ruslan Mukhamadiarov (Physics, Virginia Tech)

"Nonlinear Dynamics and Chaos: Limit Cycles and 2-D Bifurcations"

Ubiquitous in nature, limit cycles are inherently nonlinear phenomena that can model systems with self-sustained 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

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 protein-ligand 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 end-point binding free energy methods, in which binding affinity is predicted based on molecular simulations of the bound complex.

Host: Igor Tolokh

Austin Warren
(Physics, Virginia Tech)

"Nonlinear Dynamics and Chaos: Lorenz System and 1-D 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 well-known systems: the continuous Lorenz system and the discrete logistic map.

Host: Uwe Tauber

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 reaction-diffusion 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

"TBD"

Host:

"Exams Begins for Summer Session II

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

"No CSB Discussion Meeting Scheduled"

Organizer: Vinh Nguyen

"No CSB Discussion Meeting Scheduled"

Organizer: Vinh Nguyen

Center for Soft Matter and Biological Physics Workshop (Location Blacksburg Community Center)

"No Discussion Meeting"

Organizers: William Ducker & Katrina Loan

Student discussion with Dr. Rui Zhang (University of Chicago)

Host: Shengfeng Cheng

Student discussion with Dr. Daniel Sussman (Syracuse University)

Organizer: Michel Pleimling

Student discussion with Dr. Antonia Statt (Princeton University)

Organizer: Uwe Tauber

Student discussion with Dr. Cihan Nadir Kaplan (Harvard University)

Host: Rana Ashkar

"APS March Meeting Practice"

Organizer: Vinh Nguyen

Student discussion with Dr. Trung Dac Nguyen (Northwestern University)

Host: Daniel Capelluto

"APS March Meeting" (Boston)

"No discussion meeting scheduled"

Organizer: Vinh Nguyen

"Spring Break continuing ends March 17"

"No CSB Discussion Meeting Scheduled"

Organizer: Vinh Nguyen

Organizer: Vinh Nguyen

"Faculty Meeting"

"No Discussion Meeting"

Organizer: Vinh Nguyen

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 so-called active sense, which relies on a self-generated 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 well-studied model which shows collective behavior, the Vicsek model. The Vicsek model enforces a local alignment rule in groups of self-propelled 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

"Faculty Meeting"

"No Discussion Meeting"

Organizer: Vinh Nguyen

Dr. Saptarshi Chakraborty (Physics, Virginia Tech)

"Polymer-Stabilized 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