Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session P8: Glassy Dynamics |
Hide Abstracts |
Sponsoring Units: DFD Chair: Itai Cohen, Cornell University Room: Morial Convention Center RO6 |
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P8.00001: Nonlinear Dynamics Near the Jamming Transition Edward J Banigan, David A Egolf How dynamical behaviors and static measures are related near the jamming transition remains an open question. In simulations of a two-dimensional sheared granular cell, we have calculated mathematical quantities that characterize the underlying nonlinear dynamics near the jamming transition. We find that the Lyapunov exponents and vectors characterizing the most important dynamical modes correlate well in space and time to localized events that alter the physical characteristics of the system. For example, the Lyapunov exponents and vectors highlight areas in which particles are involved in cooperative rearrangement or the formation or destruction of stress chains. In at least some cases, the behavior of the dynamical quantities appears to indicate future position or stress rearrangements. In addition, we report measurements of a dynamical time scale and a dynamical length scale that diverge as the system jams, suggesting an intriguing connection between the jamming transition and a transition between chaotic and non-chaotic dynamical states. [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P8.00002: Probing Cooperative Motion in Super-Cooled Colloidal Suspensions Prasad Sarangapani, Y. Elaine Zhu The physics of the glass transition remains inadequately understood despite its broad technological relevance. The anomalous divergence of viscosity without apparent structural change as a liquid is cooled has been attributed to the existence of growing dynamic length scales of ``cooperatively rearranging regions'' (CRR). In this work, we use ultra-fast fluorescence correlation spectroscopy (FCS) combined with high-speed imaging to determine the CRR sizes by measuring single-particle dynamics of tracer nano-particle embedded in super-cooled ``hard-sphere'' colloidal suspensions. Fluorescent poly-(methyl methacrylate) (PMMA) tracer particles of radii ranging from $r=$0.1-0.4 $\mu $m, mixed with plain PMMA particles of radius, $r=$0.6 $\mu $m and bulk volume fraction, \textit{$\phi $ =} 0.38-0.58, serve as excellent probes for changes in the energy barrier landscape of the suspensions of increasing volume fraction and are sensitive to the creation and annihilation of icosahedral order in metastable colloidal fluids. We also find that the correlation length, determined by fluctuation-dissipation relations from the measured auto-correlation functions, shows a dramatic increase in the super-cooled regime until it diverges at \textit{$\phi $}=0.58. [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P8.00003: Structural relaxation in sheared two-dimensional foams Matthias Mobius, Gijs Katgert, Martin van Hecke Athermal and disordered systems at rest, such as foams and granular media, are stuck in a meta-stable configuration. Upon shear the system unjams and complex vortex-like rearrangements ensue that are correlated in time and space. In our experiment we investigate what the typical time scales of these structural relaxations are as a function of the local shear rate in a two-dimensional, disordered foam that is linearly sheared. After an initial super-diffusive regime, the bubbles become diffusive at later times. This transition is reflected in the statistics of the bubble displacements, which are initially strongly correlated and non-Gaussian but eventually become Gaussian. We find that the relaxation time decreases with shear rate. For large shear rates the dependence follows a power law with an exponent significantly different from -1. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P8.00004: From spontaneous to induced dynamic fluctuations: Granular packings as an experimental probe Frederic Lechenault, Olivier Dauchot, Giulio Biroli, Jean-Philippe Bouchaud We track the motion of a horizontally vibrated amorphous assembly of bidisperse hard disks, for densities ranging across the jamming transition. The spatial extension of dynamical heterogeneities and the associated relaxation time are found to exhibit critical behavior. Moreover, a dynamical fluctuation inequality relating the dynamical susceptibility $\chi_4$ and the response of the dynamics to a change in density is tested. As the diffusion length is found to rescale these quantities, the dependencies of the inequality on length and time scales as well as density can be evaluated independently. Surprisingly, the lower bound is found to reproduce the non-monotonic behavior of $\chi_4$ in time, which reveals an intimate link between dynamical heterogeneity and marginal super diffusion. Finally, the bound is shown to be tight and to mimic the anomalous features of the dynamical susceptibility across the transition. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P8.00005: Influence of the microstructure on jammed packings of spheres Eric Corwin, Maxime Clusel, Alexander Siemens, Jasna Brujic Jammed matter is by definition impenetrable to light, such that
little is known about the geometry of jammed systems. Using
confocal microscopy to image an emulsion in 3D, we use the
enhanced fluorescence at the droplet contacts to determine the
contact network inside this model frictionless system. This
enables the experimental determination of the average
coordination number $ |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P8.00006: Mode-coupling and generalized mode-coupling theory: a diagrammatic approach Grzegorz Szamel We present a diagrammatic approach to the dynamics of interacting Brownian particles. Within this approach, the time-dependent density correlation function is represented by a series of diagrams with three and four leg vertices. We analyze the structure of this series and obtain a diagrammatic interpretation of reducible and irreducible memory functions. The one-loop self-consistent approximation for the latter function coincides with mode-coupling approximation for Brownian systems that was derived previously using a projection operator approach. Finally, we investigate the diagrammatic interpretation of a generalized mode-coupling theory. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P8.00007: Percolating Clusters in Systems of Gapped Rigid Rings. Christopher LaSota, Ariel Helfer We have examined the behavior of kinetically agitated collections of rigid rings with angular gaps in them. For small gap angles, large clusters form readily and are sufficiently tangled so that they may be raised vertically under gravitational stresses without decomposing. Using gravity as a stressor under semi-static conditions, we have measured average cluster size as a function of the gap angle and witness what appears to be a second order percolation phase transition. The critical gap angle depends somewhat on the relative thickness of the ring material compared to the ring diameter. Although friction is necessary for the formation of clusters, it appears that cluster formation is dominated by geometry effects. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P8.00008: Jamming transition in a temperature-sensitive 2D colloidal suspension Zexin Zhang, Daniel T. N. Chen, Arjun G. Yodh, Kevin B. Aptowicz, Piotr Habdas We experimentally investigate the jamming transition of a 2D colloidal system. The system consists of a bidisperse mixture of thermoresponsive microgel particles confined between two glass slides, with a thickness of roughly the diameter of the larger particle. The packing density of the system is tuned by changing the temperature. A range of packing densities, both below and above the jamming transition is studied. We use video microscopy and particle tracking techniques to characterize the motion of the particles. On approaching the jamming transition the motion becomes slower and more heterogeneous. We characterize the jamming transition in terms of both structure (pair correlation function) and dynamics (mean square displacement, non-Gaussian parameter, four-point susceptibility). To our knowledge this study provides the first experimental evidence for the jamming transition in a 2D colloidal system. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P8.00009: Exact Enumeration of Jammed States for Confined Hard Discs S.S. Ashwin, Richard K. Bowles Enumeration of jammed states of particle systems interacting with hard potentials such as hard discs and hard spheres is a long- standing problem which holds the key to understanding the nature of glassy dynamics and the question of the possibility of an ideal glass transition in these systems. A simple model consisting of hard discs (of diameter $\sigma$) trapped between two hard lines separated by a distance $H$ exhibits slow relaxation and heterogeneous dynamics characteristic of glassy systems. We map the locally jammed structures in this model to tiles and pose the problem of enumeration of jammed states for the case $H<2\sigma$, as a tiling problem on a subset of a plane. Further on applying constraints for collective jamming on the arrangement of the tiles, we exactly enumerate the entire jamming landscape of the system and explore how this landscape is connected to the thermodynamics and dynamics of the glassy system. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P8.00010: Heterogeneities in granular dynamics Anita Mehta The absence of Brownian motion in granular media is a source of much complexity; among these is the presence of heterogeneity, whether static or dynamic, within a given system. Such strong heterogeneities can exist as a function of depth in a box of grains; this is the system we study here. We present results from three-dimensional, cooperative and stochastic Monte Carlo shaking simulations of spheres on heterogeneous density fluctuations. These are justaposed with results obtained from a theoretical model of a column of grains under gravity; frustrations via competing local fields is included in our model, while the effect of gravity is to slow down the dynamics of successively deeper layers. The combined conclusions suggest that the dynamics of a real granular column can be divided into different phases -- {\it ballistic}, {\it logarithmic}, {\it activated} and {\it glassy} -- as a function of depth. The nature of the ground states and their retrieval, in the glassy phase, shows clear evidence of {\it intrinsic} states, which lie below a band of approximately degenerate ground states. In the other three phases, by contrast, the system jams into a state chosen randomly from this upper band of metastable states. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P8.00011: Confocal Microscopy of Shear-Induced Dynamics in Jammed Emulsions Joaquim Clara-Rahola, Eric R. Weeks Emulsions are liquid droplets suspended in a second continuous fluid. We study polydisperse decane-in-water emulsions at droplet volume fractions of about 0.8. At such concentrations emulsions are jammed and the system exhibits the properties of a solid. Droplet rearrangements due to Brownian motion are limited in this jammed material. Thus, to induce droplet displacements at length scales above a particle diameter, an oscillatory strain is applied. We use confocal microscopy to track the trajectories of the droplets in real time and space. By taking advantage of this technique we quantify the affine and non-affine motion of the droplets due to the shear. Moreover, we study elastic and plastic droplet reconfigurations as well as the spatial extent of the rearrangements when the droplet volume fraction and polydispersity are varied. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P8.00012: Vacancy diffusion in a triangular lattice model M. Jeng, M. Bowick, W. Krauth, J. M. Schwarz, X. Xing We study vacancy diffusion in the classical triangular lattice dimer model, subject to the kinetic constraint that dimers can only translate, but not rotate. A single vacancy---i.e. a monomer---in an otherwise fully packed lattice, is always localized in a tree-like structure. The distribution of tree sizes is asymptotically exponential and has an average of $8.16\pm 0.01$. A connected pair of monomers has a finite probability of being delocalized. When delocalized, the diffusion is anomalous: $\langle \vec{r}^2 \rangle \propto t^{\beta}$, with $\beta=0.46\pm 0.05$. The same diffusion law is also exhibited by clusters of three or four monomers. It is found that both swap motions (translations of dimers transverse to their axes) and glide motions (translations of dimers parallel to their axes) are essential for the large-scale diffusion of monomers. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P8.00013: Studying microscopic rearrangements in a sheared supercooled colloidal liquid Dandan Chen, Denis Semwogerere, Joaquim Clara-Rahola, Eric R. Weeks Shearing induces complex micro-structure changes inside an amorphous material, which is related to interesting phenomena like shear thicking and shear thinning. We use a colloidal suspension to simulate amorphous materials, and we study how shearing changes this structure using fast confocal microscopy. Many experiments and simulations have found macro-stress flucatuations in sheared dense jammed suspensions. However, the micro-rearrangements of the particles while being sheared are not very clear. We study the non-affine motion of the colloids, finding the particles move in groups, and characterize these groups for different shearing rates. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P8.00014: Temperature control of attractive interactions in colloids Peter Schall, Zhibing Hu Attractive colloidal systems have attracted increasing interest recently: They exhibit phase behavior with solid, liquid, and gas phases, and various metastable states, ranging from gel-like to glassy. These colloidal systems offer a convenient way to investigate important phenomena such as phase formation and kinetic arrest. The most prominent colloidal systems are colloid-polymer mixtures, in which the attractive strength is fixed by the concentration of the added polymer. We present a colloidal system that allows variation of the attractive potential with external control: a binary liquid solvent gives rise to temperature-dependant particle attractions close to the demixing temperature of the liquid mixture. This allows us to use temperature control to induce transitions from gas to liquid to solid, or to form metastable gel-like or glassy states. Variation of the heating rate allows us to investigate the kinetics of these transitions. In this talk, I will focus on a novel system, in which close index- and density matching of the solvent and the particles is possible; this enables us to study bulk processes with temperature control. [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P8.00015: A thermodynamic equation of jamming Kevin Lu, H. Pirouz Kavehpour Materials ranging from sand to fire-retardant to toothpaste are considered fragile, able to exhibit both solid and fluid-like properties across the jamming transition. Guided by granular flow experiments, our equation of jammed states is path-dependent, definable at different athermal equilibrium states. The non-equilibrium thermodynamics based on a structural temperature incorporate physical ageing to address the non-exponential, non-Arrhenious relaxation of granular flows. In short, jamming is simply viewed as a thermodynamic transition that occurs to preserve a positive configurational entropy above absolute zero. Without any free parameters, the proposed equation-of-state governs the mechanism of shear-banding and the associated features of shear-softening and thickness-invariance. [Preview Abstract] |
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