Events Archive

Jan
28
2011
"Invariant solutions and state-space dynamics in wall-bounded flows" by Predrag Cvitanovic
It has recently become possible to compute precise equilibrium, traveling wave, and periodic orbit solutions to pipe and plane Couette flow at Reynolds numbers above the onset of turbulence. These invariant solutions capture the complex dynamics of unstable coherent structures in wall-bounded flows and provide a framework for understanding turbulent flows as dynamical systems. We present a number of weakly unstable equilibria, traveling waves, and periodic orbits of plane Couette flow and visualizations of their physical and state-space dynamics. What emerges is a picture of low-Reynolds turbulence as a walk among a set of weakly unstable invariant solutions...
Jan
26
2011
"Directed Cell Migration and Signal Relay" by Erin Rericha
Directed Migration of cells is vital to a wide array of biological processes: from the coordinated migration of cells during embryo development to the uncontrollable migration of a metastatic cancer.  We investigate directed cell migration in the model organism Dictyostelium aiming to understand the underlying biophysics of their motion, their direction, and the coordination among cell groups.   The problem of directed cell migration is often broken into three independent modules: a compass,...
Jan
25
2011
"Transport in Graphene" by Alex Wiener
The non-equilibrium current fluctuations, or shot noise, in ballistic graphene have received much attention since the seminal 2006 paper by Tworzydlo et al.  In that work, it was shown that shot noise can be generated even in a completely impurity-free sheet of graphene.  This result is surprising, as shot noise is expected to vanish in conductors without electron scattering.  The unexpected noise has been attributed to evanescent, that is, exponentially damped waves that backscatter electrons, even in clean graphene.  The predicted shot noise has been verified experimentally, but...
Jan
24
2011
"Phase transitions and information processing in the brain" by Woodrow Shew
The cerebral cortex is a highly complex network comprised of billions of excitable nerve cells.  The coordinated dynamic interactions of these cells underlie our thoughts, memories, and sensory perceptions.  A healthy brain carefully regulates its neural excitability to optimize information processing and avoid brain disorders.  If excitability is too low, neural interactions are too weak and signals fail to propagate through the brain network.  On the other hand, high excitability can result in excessively strong interactions and, in some cases, epileptic seizures.  While it is commonly supposed that healthy neural excitability must...
Jan
21
2011
Soft Matter 'Odes': from the life and death of viscoelastic jets to the structural color of jeweled beetles
12:15 in the M-Building, room 3201 A, for details see www.softmatter.gatech.edu/baglunch.html Most industrial and biological materials exist, or are processed, in the form of multicomponent, microstructured fluids, and their ultimate function and use is dependent on the understanding of the optics,...
Dec
16
2010
"Neutrino Oscillations Between Near and Far" by David Wark
The discovery of neutrino oscillations has been one of the major advances in our understanding of particle physics in recent times, and we are still trying to fully understand them and the insights they may give to physics at very high energies and perhaps even into the matter-anti-matter asymmetry of the universe.  Long Baseline Neutrino Oscillation Experiments are becoming one of the main tools for the study of neutrinos.  The talk will briefly outline the history and physics of neutrino oscillations and long baseline experiments, and then discuss results from the current round of experiments - OPERA, MINOS, and now T2K.  I will then discuss...
Dec
09
2010
"2D turbulence - Where do we stand?" by Robert Ecke
Two dimensional turbulence is an idealization of real 3D systems with anisotropy caused by geometric confinement or body forcing.  I will review the current state of understanding of 2D turbulent flows including specific theoretical predictions, numerical simulation results and experimental realizations of quasi-2D turbulent systems.  Relevance to geophysical systems will be discussed.
Dec
08
2010
"Nearly perfect flows" by Wendy Zhang
In school, we learned that fluid flow becomes simple in two limits.  Over long lengthscales and at high speeds, inertia dominates and the motion can approach that of a perfect fluid with zero viscosity.  On short lengthscales and at slow speeds, viscous dissipation is important.  Fluid flows that correspond to the formation of a finite-time singularity in the continuum description involve both a vanishing characteristic lengthscale and a diverging velocity scale.  These flows can therefore evolve into final limits that defy expectations derived from properties of their initial states.  This talk focuses on 3 familiar processes that...
Dec
06
2010
"Multiple Time Scale Dynamics in Chemical Oscillators" by Chris Scheper
(Nonlinear Science Webinar) Dynamical systems with multiple time scales have invariant geometric objects that organize the dynamics in phase space. The slow-fast structure of the dynamical system leads to phenomena such as canards, mixed-mode oscillations, and bifurcation delay. We'll discuss two projects involving chemical oscillators. The first is the analysis of a simple chemical model that exhibits complex oscillations. Its bifurcations are studied using a geometric reduction  of the system to a one-dimensional induced map. The second investigates the slow-fast mechanisms generating mixed-mode oscillations in a model of the Belousov-Zhabotinsky (BZ)...
Dec
01
2010
"Condensed Matter Physics Opportunities in Semiconductor Nanowires from Bottom-up" by Xuan Gao
Semiconductor nanowires synthesized in the bottom up approach have shown promising potential for a variety of applications in nanoelectronics, optoelectronics, biosensing and energy conversion. With the small length scale and a variety of material choices, nanowires also offer a versatile playground to explore mesoscopic and quantum physics. I will discuss our recent studies of magneto-transport phenomena in InAs and Bi2Se3 nanowires. In nanowires of InAs, a conventional low band-gap semiconductor, quantum interference and spin-orbit coupling lead to one-dimensional localization or anti-localization of electrons. For nanowires/ribbons of Bi2Se3, a 3D...

Pages