Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session S29: Glassy Dynamics and Jamming II |
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Sponsoring Units: DFD Chair: Eric Weeks, Emory University Room: Colorado Convention Center 303 |
Wednesday, March 7, 2007 2:30PM - 2:42PM |
S29.00001: Correlations of spatial structure and particle motion in shear-induced clusters of a near-glassy colloidal suspension. Denis Semwogerere, Dandan Chen, Eric R. Weeks We study fast-moving particle clusters formed as a result of shear applied to a colloidal suspension near the glass transition. The suspension is of micron-sized PMMA spheres in an index-matched fluid that allows visualization of the individual particles using high-speed confocal microscopy. The particles are tracked and their individual 3D trajectories determined. The shear-induced non-affine motion of each particle is extracted from its trajectory by subtracting the macroscopically imposed shear motion. Fast-moving particles are observed to move cooperatively as a group. We examine correlations between local spatial structure and the non-affine motion of the particles. [Preview Abstract] |
Wednesday, March 7, 2007 2:42PM - 2:54PM |
S29.00002: Shear-driven dynamic clusters in a colloidal glass Christoph Eisenmann, Chanjoong Kim, Johan Mattsson, David Weitz We investigate the effect of shear applied to a colloidal glass on a microscopic level using a shear device that can be mounted on top of a confocal microscope. We find that the glass yields at a critical strain of about 10\%, independently of the shear rate. Surprisingly, the yielding is accompanied by an increase of cooperative particle movements and a formation of dynamic clusters which is in contrast to the normal glass transition where one typically finds heterogeneity increasing whilst moving towards the glass transition. [Preview Abstract] |
Wednesday, March 7, 2007 2:54PM - 3:06PM |
S29.00003: Reversibility and self-organization in non-Brownian suspensions Laurent Cort\'{e}, Paul Chaikin, Jerry Gollub, David Pine Many-body systems often exhibit irreversible behavior even though the governing equations of motion are reversible. Nevertheless, it is unusual to encounter a physical system in which the transition from reversible to irreversible behavior can be explored experimentally. Recent experiments in our lab on periodically sheared non-Brownian suspensions show a sharp transition from reversible to irreversible chaotic behavior above a concentration-dependent threshold strain amplitude.\footnote{D. J. Pine, J. P. Gollub, J. F. Brady \& A.M. Leshansky, Nature, \textbf{438}, 997-1000 (2005).} The observation of a sharp threshold is puzzling as the initial distribution of particles is random, with no obvious length scale for the onset of irreversibility. We develop a simple model, explored through simulation and mean field theory, that captures the salient behavior of the experiments. For small strain amplitude, the model reveals that random displacements of colliding particles can cause the system to self-organize into a reversible state that avoid further collisions. The model provides new insights into how microstructure can spontaneously develop and how random encounters can help a system evolve towards a stable fixed point. [Preview Abstract] |
Wednesday, March 7, 2007 3:06PM - 3:18PM |
S29.00004: Ultraslow relaxation in aqueous glucose solutions near the glass transition David Sidebottom We report the results of both a dynamic and static light scattering study of the viscoelastic relaxation of aqueous glucose solutions. Photon correlation spectroscopy of samples with varying glucose concentration was conducted in both the polarized and depolarized scattering geometries. In addition to the usual, non-hydrodynamic, non-exponential, wavevector-independent viscoelastic (alpha) relaxation whose relaxation time approaches 100 seconds at the glass transition temperature, an even slower component of structural relaxation is observed. This ultraslow (exponential) relaxation is present only in the polarized scattering geometry and exhibits a relaxation time that varies as nearly the inverse square of the scattering wavevector. Static light scattering on these same solutions indicate the prescence of clusters with a size of order 50 nm. We speculate (1) that these clusters result from hydrogen bonding of water between glucose molecules and (2) that motion of these clusters within the fluid is the source for the ultraslow relaxation mode seen in the dynamic light scattering. [Preview Abstract] |
Wednesday, March 7, 2007 3:18PM - 3:30PM |
S29.00005: Anisotropic spatially heterogeneous dynamics in a model glass-forming binary mixture Grzegorz Szamel, Elijah Flenner We used computer simulations to calculate, for a model binary mixture, a four-point correlation function which measures the spatial correlations of the relaxation of different particles and the corresponding structure factor. We found that these four-point functions are anisotropic. The anisotropy is the strongest for times somewhat longer than the $\beta$ relaxation time, but it is quite pronounced even for times comparable to the $\alpha$ relaxation time. At the lowest temperatures the four-point structure factor is strongly anisotropic even for the smallest wavevector $q$ accessible in our simulation. [Preview Abstract] |
Wednesday, March 7, 2007 3:30PM - 3:42PM |
S29.00006: Isothermal compressibility effects in glass-forming liquids and polymers Wenjuan Liu, Ralph Colby, Jane Lipson We develop a simple model to account for the effects of density fluctuations in the dynamics of glass-forming liquids and polymers. The magnitude of the density fluctuation of any liquid is proportional to isothermal compressibility. As the isothermal compressibility at the glass transition increases, the (segmental) relaxation time distribution measured by dielectric spectroscopy broadens and the fragility of the glass-former diminishes. Exceptions to these rules are interesting and will be discussed in detail. [Preview Abstract] |
Wednesday, March 7, 2007 3:42PM - 3:54PM |
S29.00007: Localization Transition of the Three-Dimensional Lorentz Model and Continuum Percolation Felix Hoefling, Thomas Franosch, Erwin Frey The localization transition and the critical properties of the Lorentz model in three dimensions are investigated by computer simulations. We give a coherent and quantitative explanation of the dynamics in terms of continuum percolation theory, an excellent matching of both the critical density and exponents is obtained. Upon exploiting a dynamic scaling Ansatz employing two divergent length scales we find data collapse for the mean-square displacements and identify the leading-order corrections to scaling. Our data corroborate a hyperscaling relation that connects dynamic and geometric critical exponents. The non-Gaussian parameter is predicted to diverge at the transition. [F. Hoefling, T. Franosch, and E. Frey, Phys. Rev. Lett. 96, 165901 (2006)] [Preview Abstract] |
Wednesday, March 7, 2007 3:54PM - 4:06PM |
S29.00008: Dynamics during a transient gelation process studied by XPCS Andrei Fluerasu, Abdellatif Moussaid, Andrew Schofield, Anders Madsen Photon correlation spectroscopy with partially coherent X-ray beams (XPCS) available at third generation synchrotron sources is an experimental technique that allows the direct measurement of the low frequency microstructural dynamics that are often present in a large class of soft-condensed matter systems. In many such systems and in particular in concentrated disordered systems, at least two distinct relaxation mechanisms can usually be found. The fast(er) ones correspond to the ``trapped'' motion of individual particles or aggregates in ``cages'' created by other particles/aggregates. The slow relaxation modes correspond to the structural re-arrangements of the ``cages''. In this work we report the XPCS study of the structural dynamics associated with the slow collapse of transient gels consisting of mixtures of sterically-stablised polymethylmethacrylate (PMMA) particles and random-coil polystyrene (PS) dispersed in cis-decalin. The intermediate scattering functions change during the process from stretched to compressed exponential decays indicating a jamming of the system in the full aging regime. A complex aging behavior towards the final collapse of the gel is observed and we propose that large scale network deformations trigger an un-jamming process leading to the collapse. [Preview Abstract] |
Wednesday, March 7, 2007 4:06PM - 4:18PM |
S29.00009: Changing the Packing Fraction by Changing the Geometry: A Hyperbolic Approach to Jamming Carl D. Modes, Randall D. Kamien The jamming transition is an important and active area of current research in condensed matter physics, touching on phenomena from granular matter to supercooled liquids to glasses. Underlying the problem is the need to fully understand the properties of geometrically disordered configurations and their relation to ordered crystalline states, especially in systems where the effect of entropy dominates over that of energy. Of particular interest are systems for which the densities of isostaticity and crystallization are grossly separated, for example, in higher dimensions. In order to probe these systems with the Virial expansion, however, we must require that the onset of isostatic configurations occurs for sufficiently low numbers of simultaneously interacting particles. This leads us to the study of a hard disc fluid on the hyperbolic plane as a function of the curvature. [Preview Abstract] |
Wednesday, March 7, 2007 4:18PM - 4:30PM |
S29.00010: Packing of Tetrahedral and other Dice Paul Chaikin, Stacy Wang, Alexander Jaoshvili The densest packing of tetrahedra remains an unsolved problem. Recently J. H. Conway, and S. Torquato$^{1}$, presented the densest packing yet found for tetrahedral, a structure which is a modification of packing tetrahedra in an approximation to an icosahedron and then packing the icoshedra. The best packing was under 0.72, considerably less than the (exact) densest sphere packing of 0.7405{\ldots} We have measured the random packing of tetrahedral dice in different sized spherical and cylindrical containers, and extrapolated the results to obtain the packing fraction in the limit of no boundaries. We have also measured their density toward the center of a spherical container away from the walls. Both measurements are similar to previous studies of ellipsoids. We find that the dice pack to better than 0.75. But the dice have very slightly rounded vertices and edges. While the total volume change due to the rounding is less than 0.03 (and in the direction to make a larger difference between crystal and random packing), it is difficult to approximate the effect of the rounding. We discuss the relative packings and the nature of the inter-di contacts. 1. J. H. Conway, and S. Torquato, PNAS, \textbf{103}, 10612-10617, (2006) [Preview Abstract] |
Wednesday, March 7, 2007 4:30PM - 4:42PM |
S29.00011: Elastic heterogeneity of soft random solids Xiaoming Mao, Paul M. Goldbart, Xiangjun Xing, Annette Zippelius The spatial heterogeneity of amorphous solids, which records the randomness present at the solidification transition, confers heterogeneity on elastic properties. Especially for soft random solids, which have exceptionally small shear modulus due to the large thermal fluctuations in the positions of the particles, this elastic heterogeneity exhibits interesting long-range correlations. We examine elastic heterogeneity in soft random solids via a two-pronged approach [1]. First, we examine a phenomenological elastic free energy, featuring a quenched random kernel, which induces randomness in the residual stress and Lame coefficients. Second, we explore a semi-microscopic model network using replica statistical mechanics. This model has a vulcanization transition, and the associated Goldstone fluctuations characterize shear deformations and can reproduce the phenomenological model. Via this correspondence we infer the statistical properties of the elastic heterogeneity, finding that correlations involving the residual stress are long-ranged and governed by a universal parameter that also gives the mean shear modulus. This statistical characterization allows the construction of the statistics of non-affine deformations in soft random solids. [1] Xiaoming Mao, Paul M. Goldbart, Xiangjun Xing and Annette Zippelius, cond-mat/0610407. [Preview Abstract] |
Wednesday, March 7, 2007 4:42PM - 4:54PM |
S29.00012: The jamming transition and beyond: Density dependence of the relevant length and time scales in a horizontally vibrated granular monolayer Frederic Lechenault, Olivier Dauchot A dense amorphous monolayer of hard disks is horizontally driven by a glass plate oscillating underneath while confined in a fixed rectangular cell. As the packing fraction is decreased, the system exhibits a transition between a totally jammed state in which the pressure is driven by the contact network and a ``supercooled'' regime in which the kinetic contribution becomes dominant. We characterize the diffusion properties of such packing across the transition. Furthermore, we compute the self- intermediate scattering function $F_{s}(\tau,\bf{k})$ and the so- called dynamical susceptibility $\chi_4(\tau,\bf{k})$. First we show that the former scales with the diffusive length. Then we find that the cooperative scale associated to the latter increases as the packing is increased toward the transition and then drops abruptly as a certain critical density $\phi_c$ is crossed. Finally we uncover a relationship between $F_{s}$ and $\chi_4$ and discuss its link with a dynamical fluctuation dissipation relation. [Preview Abstract] |
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