Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session H29: Glassy Dynamics and Jamming I |
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Sponsoring Units: DFD Chair: Karin Dahmen, University of Illinois at Urbana-Champaign Room: Colorado Convention Center 303 |
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H29.00001: Dynamic Facilitation in Colloidal Glasses Scott V. Franklin, Eric R. Weeks Dense colloidal suspensions share many characteristics with molecular glasses and, because easily visualized, are a model system for investigating the transition to the glassy state. An important feature of glasses is the presence of spatially heterogeneous dynamics; at any given time only a small subset of particles (clusters) are significantly mobile. To explain the origin and spatiotemporal correlation of clusters, Garrahan and Chandler proposed {\it dynamic facilitation}, in which motion at one location facilitates subsequent (in time) motion at adjacent regions. We use confocal microscopy to investigate dynamic facilitation in binary mixtures of micron-sized PMMA spheres in two and three dimensions. Dynamic facilitation is identified with spatial correlations between the most mobile particles at two subsequent time intervals, a measure used previously by Vogel and Glotzer to analyze simulations of glass-forming liquids. This provides a critical test of how mobility propagates through the sample in space-time and the spatial and temporal correlation of mobile clusters. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H29.00002: Excess Vibrational Modes in Model Glasses Ning Xu, Matthieu Wyart, Andrea Liu, Sidney Nagel We performed both theoretical analysis and computer simulations to study the excess low-frequency normal modes (boson peak) for two widely-used model glasses at zero temperature. The onset frequencies for the anomalous modes from the simulations agree very well with predictions from variational calculations based on minimizing the vibrational energy cost of the lowest-frequency anomalous mode. This energy cost originates from the excess interactions per particle over $z_c$, where $z_c=2d$ is the minimum number required for mechanical stability in $d$ dimensions. The total $z$ interactions per particle are divided into two classes: $z_1$ ``stiff'' interactions determine the structure of the anomalous mode by adding extra nodes; the remaining $z-z_1$ interactions act as a perturbation and increase the vibrational energy of the mode by increasing the restoring force for displacements. Even though both glasses studied have a high number of interactions per particle, $(z_1-z_c)/z_c$ is always smaller than $0.6$, which indicates that the physics of jamming is relevant to the study of the excess normal modes in glasses. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H29.00003: Combining Coarse-Graining and Density of States Monte Carlo: Application to Ortho-terphenyl Jayeeta Ghosh, Roland Faller The non equilibrium transition from liquid to glass is a challenging problem in condensed matter physics. Various techniques have been applied to elucidate the nature of transition without reaching consensus. The relevant time scales near the glass transition are so long that Molecular Dynamics fails. There is much debate whether standard Monte Carlo succeeds can sample phase space near or below the glass transition temperature. We therefore combine advanced techniques to study the system near the glass transition temperature. Based on atomistic models of the small organic glass former Ortho-terphenyl (OTP) we develop a mesoscale model in which each phenyl ring is replaced by a single interaction center. We obtain a structurally coarse-grained model based on Boltzmann inversion of atomistic radial distribution function at various temperatures. As atomistic radial distribution functions are only weakly temperature dependant, the optimization can be performed at any temperature and can be used for a range of temperatures. It turned out that in the glassy range we need to optimize the potential below the glass transition temperature. Once we have a valid mesoscale model we apply the Wang-Landau Density of States Monte Carlo technique to find the density of states for the system. This novel Monte Carlo technique has already been applied to model glass forming materials but not yet to a chemically explicit model. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H29.00004: Geodesic path picture for slow dynamics in supercooled liquids Chengju Wang, Richard M. Stratt How does dynamics dramatically slow down with decreasing temperature in supercooled liquids? We suggest that the answer can be deduced from the geometry of the potential energy landscape. Instead of looking at real dynamical processes associated with barriers hoping, the landscape is characterized by the geodesic (shortest) paths in the \textit{energy landscape ensemble}, which was defined to include all the configurations with a potential energy less than a given value. Within our geodesic path theory, the diffusion constants depend on the typical ratio of the Euclidean distance to the geodesic path length. Computer simulations show that using only this geometric property of the landscape, one can reproduce the dramatic slow down in diffusion constants for the Kob-Andersen model, a typical glassy system. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H29.00005: Random Field Ising Model In and Out of Equilibrium Yang Liu, Karin Dahmen We present numerical studies of zero-temperature Gaussian random-field Ising model (zt-GRFIM) in both equilibrium and non-equilibrium. We compare the no-passing rule, mean-field exponents and universal quantities in 3D (avalanche critical exponents, fractal dimensions, scaling functions and anisotropy measures) for the equilibrium and non-equilibrium disorder-induced phase transitions. We show compelling evidence that the two transitions belong to the same universality class. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H29.00006: Probing large length scale behavior of spin glasses with patchwork dynamics Creighton Thomas, Olivia White, Alan Middleton Glassy systems equilibrate on timescales that are difficult to reach with direct simulation of dynamics. The characteristic length scale over which fluctuations occur grows sub-logarithmically in time, so examining aging and rejuvenation effects is problematic. In order to probe large length scale dynamics, we use ``patchwork dynamics,'' in which a patch of size $a$ is selected out of the spin glass and optimized subject to fixed boundaries provided by the neighboring spins to the patch. Using this method, we investigate the large length scale dynamics of the 2D Ising spin glass with Gaussian $J_{ij}$, as well as the random bond Ising ferromagnet, where equilibration is found on scales longer than $a$, and the critical point between these two states. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H29.00007: Aging and non-Gaussian dynamics in a colloidal glass Gianguido C. Cianci, Eric R. Weeks As a hallmark of the glassy state of matter, aging has attracted substantial attention, yet it remains a poorly understood phenomenon. It manifests itself by a dependence of the dynamical properties of the sample on the time elapsed since vitrification. The glassy state is also marked by dynamics that are heterogeneous in both time and space, and that exhibit non-Gaussian statistics over moderate to long timescales. We use a density and refractive index matched suspension of micron sized PMMA colloids as a model glassy material. At these length scales, laser scanning confocal microscopy allows us to follow the motion of a few thousand particles in real time and real space. We study the interplay between the timescales at which the dynamics are non-Gaussian and the age of the sample. We also analyze the spatial extent over which the dynamics are heterogeneous and examine the age dependence of this length scale. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H29.00008: Replica theory for fluctuations of the activation barriers in glassy systems Joerg Schmalian, Maxim Dzero, Peter Wolynes Using an effective potential approach to self generated glasses we determine the nucleation of entropic droplets in systems with random first order transition and entropy crisis. We demonstrate that fluctuations of the configurational entropy and of the liquid glass surface tension are crucial for an understanding of the barrier fluctuations in glassy systems and thus are responsible for the broad spectrum of excitations and heterogeneous dynamics in glasses. In particular we derive a relation between the length scale for dynamic heterogeneity and the related barrier fluctuations. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H29.00009: ABSTRACT WITHDRAWN |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H29.00010: Transport in Disordered Reaction-Diffusion Systems Andrew Missel, Karin Dahmen The effects of quenched spatial disorder in the reaction rates on the behavior of reaction-diffusion (RD) models have been difficult to discern, but understanding these effects is essential for predicting the behavior of any real system reasonably well-described by such a model. We present here a step towards an understanding of these effects on transport in RD systems, taking as our model a $1$D system in which particles compete ($2A\to A$) and diffuse with spatially homogeneous rates, reproduce ($A\to2A$) on certain sites (``oases''), and die ($A\to0$) on all others. We show that predictions from a simplified linear model for the first passage properties between two oases match the results of Monte Carlo simulations; these results, along with some ideas from percolation theory, can be used to make some predictions about the nature of transport across a disordered (many oases) system in higher dimensions. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H29.00011: Is microscopic description of inherent structures possible? Valentin A. Levashov, Takeshi Egami, Rachel S. Aga, James R. Morris Description of relaxation in a supercooled liquid of N particles using 3N dimensional potential energy landscape (PEL) implicitly favors the idea that the structure is too complex to be described by any microscopic local structural parameters. We addressed this issue by using atomic level stresses (ALS) introduced a while ago as local structural parameters. With MD simulations and the steepest decent method on a model of liquid iron we studied how the distributions of ALS in inherent structures (IS) depend on the original temperature. We found that the ALS of the IS clearly show the crossover and glass transition temperatures. Thus we conclude that relaxation in inherent structures could be described not only by macroscopic, but also by microscopic topological quantities (MTQ). We also found a way to relate the character of fluctuations in MTQ at real temperatures to the energies of the corresponding IS. We found that the mode-coupling temperature is located significantly below the crossover temperature, closer to the glass transition temperature. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H29.00012: Self-Organized Criticality Below The Glass Transition: A Computer Simulation Katharina Vollmayr-Lee, Elizabeth A. Baker We obtain evidence that the dynamics of a glassy system below the glass transition is characterized by self-organized criticality. To investigate the dynamics of a binary Lennard-Jones system we use molecular dynamics simulations. To study cooperative motion we define single particle jump events via single particle trajectories and identify clusters of jump events which are correlated in space and time. We find string-like clusters whose size is power-law distributed not only close to $T_{{\mbox{\scriptsize {c}}}}$ but for {\em all} temperatures below $T_{{\mbox{\scriptsize {c}}}}$, indicating self-organized criticality which is suggestive of a freezing in of critical behavior. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H29.00013: Dynamics and effective temperature for a steady-state sheared glass Thomas Haxton, Andrea Liu In a model sheared glass, the slow dynamics near the onset of jamming are shown to be controlled by a well-defined effective temperature $T_{\rm eff}$. We conduct two-dimensional nonequilibrium molecular dynamics simulations of steadily-sheared, densely-packed, bidisperse disks with soft repulsive pairwise interactions in contact with a heat reservoir. We calculate the viscosity and $T_{\rm eff}$ as functions of shear rate $\dot \gamma$ and bath temperature $T_{\rm bath}$. At $\dot \gamma=0$, the system undergoes a glass transition at $T_{\rm bath}=T_g$. We study the steady state at $\dot \gamma \ne 0$ and $T_{\rm bath} < T_g$. At low $\dot \gamma$, $T_{\rm eff}$ decreases extremely slowly with $\dot \gamma$ and is nearly independent of $T_{\rm bath}$, while the viscosity continues to increase rapidly. The dramatic change in dynamics with a gradual change in effective temperature is reminiscent of the behavior of the quiescent system as temperature is lowered towards $T_g$. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H29.00014: Observations of shear-induced clusters seen near the colloidal glass transition Dandan Chen, Denis Semwogerere, Eric R. Weeks Many studies of the glass transition focus on structural relaxation arising from thermally induced dynamics. Several of these studies observed isolated clusters of fast-moving particles. We present experimental work that finds similar heterogeneities from mechanically-induced motion applied to a dense colloidal suspension. We study micron-sized PMMA spheres with a volume fraction of approximately 50\%. The sample is subjected to shear while simultaneously the dynamics are recorded using fast confocal microscopy. From the resulting 3D movie the trajectories of the individual particles are tracked and the macroscopically imposed shear is subtracted to study the mechanically-induced non-affine particle motion. We find fast-moving cooperative clusters with pronounced motion in the shear direction. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H29.00015: System-size dependence of dynamical heterogeneity in a glass-forming liquid Chandan Dasgupta, Smarajit Karmakar Dynamical heterogeneity in supercooled liquids is often characterized by a space- and time-dependent higher-order correlation function of local density fluctuations and the corresponding susceptibility (the so-called four-point susceptibility). If the growth of this susceptibility as the temperature is decreased towards the ideal glass transition temperature of mode-coupling theory is a consequence of a growing dynamical correlation length, the dependence of this quantity on the system size should exhibit finite-size scaling behavior. We have used constant-temperature molecular dynamics simulations to study the temperature and sample-size dependence of this quantity for a well-known glass-forming liquid (the Kob-Anderson mixture). Our results show the expected finite-size scaling behavior of the four-point susceptibility in the temperature range over which it exhibits a power-law growth. However, the sample-size dependence of the time scale at which the susceptibility peaks does not exhibit conventional finite-size scaling, possibly indicating the presence of effects not captured in mode-coupling theory. [Preview Abstract] |
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