Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session V2: Relaxation and Dynamic Heterogeneity and Glass |
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Sponsoring Units: DCMP Chair: Zexin Zhang, University of Pennsylvania Room: Oregon Ballroom 202 |
Thursday, March 18, 2010 8:00AM - 8:36AM |
V2.00001: Irreversible reorganization in a supercooled liquid originates from localized soft modes Invited Speaker: |
Thursday, March 18, 2010 8:36AM - 9:12AM |
V2.00002: Dynamic Heterogeneity and Relaxation Time Very Close to Dynamic Arrest Invited Speaker: In supercooled molecular fluids or concentrated colloids and grains, the dynamics slow down markedly with no distinct structural changes as the glass and jamming transitions are approached. There is now ample evidence that structural relaxation in glassy systems can only occur through correlated rearrangements of particles, leading to dynamics that are heterogeneous both in time and in space. On approaching these transitions, the size of these rearranging domains grows in glass-formers, colloids, and driven granular materials alike, providing a possible explanation for kinetic slowing. However, little is known yet on the behavior of dynamical heterogeneity and relaxation time very close to dynamical arrest. In this talk, I'll present recent results from our group for a simples model sustem --colloidal hard spheres--, as well as for other glassy and jammed soft materials. By extending previous data [1] by at least 2 orders of magnitude in time, we established that the volume fraction dependence of relaxation time and dynamic heterogeneity in colloidal hard spheres follow mode coupling theory (MCT) [2] predictions only in a restricted density range [3]. Unlike previous studies, we provide equilibrium measurements above the MCT critical packing fraction, thereby proving unambiguously that in our samples the algebraic divergence is absent at the predicted packing fraction. The behavior of dynamical heterogeneity is even more surprising. While in supercooled hard spheres the size of domains undergoing cooperative rearrangements is limited to a few particles at most, closer to jamming the correlation length of the dynamics increases dramatically, approaching the system size in a variety of systems [4,5]. In this regime, spatial and temporal fluctuations of the dynamics may decouple, as observed for near hard sphere particles: while the range of spatial correlations continuously increase on approaching jamming, the temporal fluctuations of the dynamics initially increase with particle volume fraction, but drop markedly very close to jamming [5], unveiling a richer-than-expected scenario. \\[4pt] [1] W. van Megen et al., Phys. Rev. E 58, 6073 (1998); \\[0pt] [2] W. G\"otze, J. Phys. Condens. Matter 11, A1 (1999); \\[0pt] [3] G. Brambilla et al., ``Probing the Equilibrium Dynamics of Colloidal Hard Spheres above the Mode-Coupling Glass Transition,'' Phys. Rev. Lett. 102, 085703 (2009); \\[0pt] [4] A. Duri et al., ``Resolving long-range spatial correlations in jammed colloidal systems using photon correlation imaging,'' Phys. Rev. Lett. 102 085702 (2009); \\[0pt] [5] Pierre Ballesta, Agn\`es Duri, \& Luca Cipelletti, ``Unexpected drop of dynamical heterogeneities in colloidal suspensions approaching the jamming transition,'' Nature Physics 4, 550 - 554 (2008) [Preview Abstract] |
Thursday, March 18, 2010 9:12AM - 9:48AM |
V2.00003: Dynamic Heterogeneity and the Colloidal Glass Transition in Confinement Invited Speaker: We study concentrated colloidal suspensions, a model system which has a glass transition. These are suspensions of small solid particles in a liquid, and exhibit glassy behavior when the particle concentration is high; the particles are roughly analogous to individual molecules in a traditional glass. We view the motion of these colloidal particles in three dimensions by using an optical confocal microscope. This allows us to directly study the microscopic behavior responsible for the macroscopic viscosity divergence of glasses. In particular, we study the influence of confinement, which in our experiments slows the particle motion. This suggests that confinement causes the onset of the colloidal glass transition to happen ``sooner,'' at particle concentrations which are not normally glassy. We examine confinement between parallel glass plates, in thin cylindrical capillary tubes, and in emulsion droplets, finding similar behavior in these geometries. The particles exhibit spatially heterogeneous dynamics, although the character of this behavior depends on the geometry and boundary conditions. Work done in collaboration with Kazem Edmond, Gary Hunter, Carolyn Nugent, and Nabiha Saklayen. [Preview Abstract] |
Thursday, March 18, 2010 9:48AM - 10:24AM |
V2.00004: Locally Favoured Structures and Dynamic Arrest Invited Speaker: The mechanism by which a liquid may become arrested, forming a glass or gel, is a long standing problem of condensed matter physics. While possible dynamic mechanisms have received considerable attention, the prevailing view is that ``the arrangement of atoms and molecules in glass is indistinguishable from that of a liquid.'' On the contrary, here we present direct experimental evidence of a structural mechanism for dynamical arrest. In particular, long-lived (energetically) locally favoured structures (LFS), whose geometry may prevent the system relaxing to its equilibrium state, have long been thought to play a key role in dynamical arrest. Here we propose a general definition of LFS which we identify with a novel topological method and combine these with experiments on colloidal liquid-gel, and glass-liquid-glass transitions. In these systems, the equilibrium state is crystal-fluid coexistence, and a crystal respectively: in both colloidal gels and glasses, suppression of crystallisation forms a key part of dynamical arrest. The population of LFS is a strong function of (effective) temperature in the ergodic liquid phase, rising sharply approaching dynamical arrest. We show that the ``arms'' of the colloidal gel are entirely comprised of LFS, which we argue form on shorter timescales than crystallisation. Gelation and dynamical arrest are identified with the formation of a percolating network of LFS, which we show are intimately related to dynamical heterogeneities. We seek to demonstrate that LFS can provide a structural order parameter for dynamical arrest. In the case of the glass-liquid-glass transition exhibited by ``sticky spheres'' at high density, like the colloidal gel, the LFS provide a clear structural signature of dynamical arrest. [Preview Abstract] |
Thursday, March 18, 2010 10:24AM - 11:00AM |
V2.00005: Experimental studies on the local structure, dynamics, and dynamic heterogeneity in colloidal glasses Invited Speaker: As the temperature of a glass-forming liquid is lowered, its relaxation time diverges. It is believed that this divergence of relaxation time during the glass transition is related to dynamic heterogeneity, the spatio-temporally heterogeneous, highly correlated motion of constituents. However, the structural causes of dynamic heterogeneity and dynamic arrest remain elusive. Colloidal glasses exhibit much of the same phenomenology as molecular glasses. In this talk I will present two colloidal experiments to study glassy dynamics, and hope to shed some light on the relationship between local structure, dynamics, and dynamic heterogeneity. First, we employ a unique optical heating/quenching technique to study the non-equilibrium aging process in glasses. We found that during aging dynamical heterogeneity grows. The size of locally correlated rearranging domains associated with so-called irreversible rearrangements increases, leading to the slowing dynamics characteristic of aging in glasses. Second, we utilize a novel colloidal system with tunable attractive potential. We are able to change the inter-particle potential form repulsive to attractive \textit{in situ}, which allows a direct comparison between glassy dynamics in repulsive and attractive glasses. We find that more particles are involved in cooperative rearrangements in attractive glasses than their repulsive counterparts. As a result, dynamics in attractive glasses are heterogeneous over more length scales and time scales. [Preview Abstract] |
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