Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session G34: Glassy Dynamics and Jamming |
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Sponsoring Units: DFD Chair: Eric Weeks, Emory University Room: Baltimore Convention Center 337 |
Tuesday, March 14, 2006 8:00AM - 8:12AM |
G34.00001: On the Lower Critical Dimension of the Edwards-Anderson Spin Glass Stefan Boettcher The Edwards-Anderson model of Spin Glasses is studied on dilute hyper-cubic lattices in dimensions $d=2,3,\ldots,7$. Accurate predictions for the stiffness exponent $y_d$ are obtained that describes low-energy excitations. Continuing $y_d$ off the integers shows that its zero is located at $d=5/2$ to within 0.1\%, a prediction that is corroborated by other numerical and theoretical work. \\ Related Publication: Phys. Rev. Lett. {\bf 95}, 197205 (2005). \\ Related Webpage: http://www.physics.emory.edu/faculty/boettcher/ . [Preview Abstract] |
Tuesday, March 14, 2006 8:12AM - 8:24AM |
G34.00002: Direct measurement of the distinct part of van Hove correlation function $G_{d}(r$,$t)$ in colloidal gels and glasses Yongxiang Gao, Maria Kilfoil Great effort has been put into understanding the mechanism and dynamics of glass formation, and progress has been made. It is now known that the cage effect causes the dynamical arrest, and thus the observed glass transition. This has been proven by direct observation by microscopy, by light scattering and rheology experiments of hard sphere systems near the glass transition, and by computer simulation. However, no real-space direct three dimensional measurements of large-length-scale dynamics have been done in glass samples. The distinct part of the van Hove correlation function $G_{d}(r$,$t)$ provides direct access to the dynamics in glassy systems at all length scales. We use confocal microscopy to measure this function in both colloidal gels and glasses. Since both glass and gel formation are kinetic and not thermodynamic phenomena, we may expect that they share similarities. By looking at their dynamics, we may gain more insights into the unified jamming picture of liquid-to-disordered-solid transition. [Preview Abstract] |
Tuesday, March 14, 2006 8:24AM - 8:36AM |
G34.00003: Fluctuations of Structure and Dynamics in an Aging Colloidal Glass Gianguido C. Cianci, Eric R. Weeks When a liquid is quenched to form a glass it becomes trapped in a non-equilibrium state. The non-ergodicity of the system is most clearly highlighted by the dependence of the glass' properties on the time elapsed since the quench. This phenomenon is known as aging. Dense colloidal suspensions have been shown to be a good model for the glassy state. We use fast laser scanning confocal microscopy to image sterically stabilized, micron-sized, PMMA spheres in three dimensions and track their positions over time with sub-pixel accuracy. While aging is most commonly detected by measuring the evolution of variables (such as the mean squared displacement or the intermediate scattering function) averaged over the entire system and over time these quantities cannot yield information about the detailed, structural changes that occur during aging. Confocal microscopy, on the other hand, allows us to intimately study the aging process with minimal or no averaging. We study the statistics of the fluctuations of geometrical and dynamical quantities over time as the sample ages. In particular, we find that the aging process is heterogeneous in time and space and compare the fluctuations of the aging process to intermittent, record induced dynamics models. [Preview Abstract] |
Tuesday, March 14, 2006 8:36AM - 8:48AM |
G34.00004: Dynamic correlations and correlation length in the aging of a simple structural glass Azita Parsaeian, Horacio Castillo We present the results of a detailed molecular dynamics simulation of the spatial correlations of fluctuations in a simple binary Lennard-Jones glass former in the aging regime. We study the 4-point generalized density susceptibility $\chi_4(t, t_w)$ and the dynamic correlation length $\xi_4(t, t_w)$ associated with it, where $t_w$ is the waiting time. We find that these data are consistent with the scalings $\chi_4(t,t_w)=F(t_w) {\chi_4}^{0}(C(t, t_w))$ and $\xi_4(t,t_w)=G(t_w) {\xi_4}^{0}(C(t, t_w))$, where $C(t, t_w)$ is the incoherent part of the intermediate scattering function. We also find that $F(t_w)$ can be fit as a power of the waiting time, and $G(t_w)$ can be fit as a logarithm of the waiting time. These differing time dependencies, plus the dramatically different behavior of the scaling functions $\chi_4^0(C)$ and $\xi_4^0(C)$ for small $C$ lead to the conclusion that the time evolution of $\chi_4(t,t_w)$ {\em is not} controlled by its associated dynamic correlation length $\xi_4(t,t_w)$. [Preview Abstract] |
Tuesday, March 14, 2006 8:48AM - 9:00AM |
G34.00005: Dynamic fluctuations of elastic lines in three-dimensional random enviroments Sebastian Bustingorry, Leticia Cugliandolo, Daniel Dominguez Elastic lines embedded in three-dimensional random environments present a low temperature glassy regime, with aging characterized by multiplicative scaling. We studied the scaling properties of the distribution functions of different dynamical observables: the roughness, the mean-squared-displacement and its associated response function. Following the multiplicative scaling hypothesis, we numerically show that the distribution functions depend only on the mean scaled value of the variables, and not on the different time scales involved. These results could be extended to the scaling of distribution functions in critical-like systems. [Preview Abstract] |
Tuesday, March 14, 2006 9:00AM - 9:12AM |
G34.00006: Microstructure of a polymer glass overaged by application of instantaneous shear strains Bela Joos, Matthew L. Wallace When applying a transient shear on jammed colloidal
suspensions, Viassnoff and Lequeux ({\it Phys. Rev. Lett.} {\bf
89}, 065701 (2002)) observed both rejuvenation and overaging in
the system, as the relaxation times are altered in a non-
trivial way. Application of instantaneous, one-time shear
deformations on a polymer glass by molecular dynamics
simulations produces a similar behavior (M.L. Wallace and B.
Jo\'os, {\it Phys. Rev. Lett.}, in press). Two regimes are
observed corresponding to elastic and plastic strains. Of
particular interest are deformations in the plastic regime,
above the yield strain ($\epsilon >0.1$), where the
characteristic relaxation times $\tau_{1/2}$ increase
exponentially with $\epsilon$, after a long waiting time $t_w$
following the deformation. We are in the process of
understanding the nature of this state of the glass. There is a
slight increase in the average energy of the inherent structures
$ |
Tuesday, March 14, 2006 9:12AM - 9:24AM |
G34.00007: Observed Effects of Confinement on Colloidal Glasses Carolyn Nugent, Hetal Patel, Joe Saldana, Eric R. Weeks We used a binary colloidal suspension to model glass molecules. By increasing the concentration of colloids, a glass transition is induced. Previous studies of colloidal suspensions show that as the glass transition is approached, colloids tend to move in groups of increasing size. In order to examine this phenomenon more closely, a colloidal suspension was confined between two parallel glass plates with a narrow gap of the order of a few colloid diameters in thickness. Confocal microscopy was used to observe the particles and follow their motion over time. We observed that motion in confined regions was much slower than motion in non-confined regions of the same sample. In addition, the motion perpendicular to the walls was slower than motion parallel to the walls. The arrangement of colloids into layers was also observed, which further influenced the motion. [Preview Abstract] |
Tuesday, March 14, 2006 9:24AM - 9:36AM |
G34.00008: The effective temperature and the universal scaling behavior of aging colloidal glass system Ping Wang, Chaoming Song, Hernan Makse Mostly due to the enormous practical importance of glassy systems there has been a vast literature describing different theoretical frameworks for glasses, yet without a common theory applicable to the diverse range of systems undergoing a glass transition. Here we present experimental results on a simple glassy system pointing to a unifying view of out-of-equilibrium systems. We investigate correlation and response functions to external fields to monitor the aging of a colloidal glass composed of highly concentrated hard spherical particles suspended in an refraction index matched fluid for visualization. Our analysis reveals that even though the system is aging far from equilibrium, it behaves as it is equilibrated at a constant temperature, independent of the age. This temperature is larger than the bath, and can be rationalized by the cage dynamics in the system. Global and local fluctuations in the observables are also studied showing a common scaling behavior with the age of the system. While these results can be interpreted within the framework of unifying formulations of aging, the observed scaling forms of the correlation functions do not conform to the theoretical predictions. [Preview Abstract] |
Tuesday, March 14, 2006 9:36AM - 9:48AM |
G34.00009: Aging of Johari-Goldstein Relaxation in Structural Glasses Hasan Yardimci, Robert L. Leheny Using frequency-dependent dielectric susceptibility measurements we characterize the aging in two supercooled liquids, sorbitol and xylitol, below their calorimetric glass transition temperatures, T$_{g}$.~ In addition to the alpha relaxation that tracks the structural dynamics, the susceptibilities of both liquids possess a secondary Johari-Goldstein relaxation at higher frequencies.~ Following a quench below T$_{g}$, the susceptibility slowly approaches equilibrium behavior.~ For both liquids, features of the Johari-Goldstein relaxation display a dependence on the time since the quench, or aging time, that is very similar to the age dependence of the alpha peak. However, one can not assign a single fictive temperature to both the alpha and Johari-Goldstein relaxations.~ For example, the peak frequency of the Johari-Goldstein relaxation remains constant during aging for sorbitol while it increases with age for xylitol, inconsistent with a decreasing fictive temperature.~ This behavior contrasts with that of the high frequency tail of the alpha peak whose shape and position track the aging of the main part of the peak. [Preview Abstract] |
Tuesday, March 14, 2006 9:48AM - 10:00AM |
G34.00010: Glassy Dynamics in Suspensions of Non-Spherical Colloids Galina Yatsenko, Kenneth Schweizer Glassy dynamics of isotropic fluids of hard symmetric dumbbells (diatomics), rods, cylinders and disks are studied using a simple form of mode coupling theory and its generalization to predict barriers and activated hopping transport. Orientational degrees of freedom dynamically enter in a pre-averaged manner, and structural correlations are quantified based on a center-of-mass version of the site-site Reference Interaction Site Model. A length-to-diameter (aspect) ratio serves as a principal parameter describing shape anisotropy. The ideal glass transition volume fraction of dumbbells is predicted to be a nonmonotonic function of aspect ratio. For continuous cylinders, ellipsoids and disks power law dependencies of glass transition volume fraction and localization length on aspect ratio are found in the highly anisotropic limit. The consequences of discotic liquid crystalline order for infinitely thin disks have also been studied. Multiple scaling behaviors with volume fraction and aspect ratio are predicted for the localization length, barrier height, elastic shear modulus and yield stress. [Preview Abstract] |
Tuesday, March 14, 2006 10:00AM - 10:12AM |
G34.00011: Microrheology of an aging colloidal glass S. Jabbari-Farouji, D. Mizuno, M. Atakhorrami, E. Eiser, C. Schmidt, F. MacKintosh, G. Wegdam, D. Bonn Laponite is a synthetic clay which after mixing with water, spontaneously evolves from an initially liquid and ergodic state to a non-ergodic glassy state that exhibits elastic behavior. We provide a direct experimental test of the Stokes-Einstein relation as a special case of the fluctuation-dissipation theorem (FDT) in this aging colloidal glass. The use of combined active and passive microrheology allows us to independently measure both the correlation and response functions in this non-equilibrium situation. Contrary to previous reports, we find no deviations from the FDT over several decades in frequency (1 Hz-10 kHz) and for all the observed aging times. Our measurements also demonstrate the applicability of fluctuation-based (passive) microrheology in a non-equilibrium glassy system. This method allows obtaining the viscoelastic properties over a very wide frequency range. Our striking observation is that there is a cross over in frequency behavior of complex shear modulus of system from single power law at early stages of aging to two power laws at later stages. This suggests the existence of two distinct viscoelastic contributions in the aging glass: (i) a high-frequency viscoelastic response in which the shear modulus increases rapidly with frequency; and (ii) a predominantly elastic (weakly frequency-dependent) response at lower frequencies, which becomes increasingly important as the system ages. [Preview Abstract] |
Tuesday, March 14, 2006 10:12AM - 10:24AM |
G34.00012: Activated Hopping and Dynamic Heterogeneity in Glassy Colloidal Suspensions Erica Saltzman, Kenneth Schweizer A microscopic statistical dynamical theory of barriers and activated transport in dense colloidal suspensions has been developed by combining and extending methods of mode coupling, density functional and particle hopping theories. Quasi-analytic results for the mean relaxation time and ensemble-averaged transport coefficients agree well with experiment. However, a full determination of the dynamics requires numerical integration of the nonlinear overdamped stochastic equation of motion, i.e. Brownian dynamics simulation. This enables calculation of average quantities including the mean-square displacement, incoherent dynamic scattering function and alpha relaxation time, as well as trajectory-based quantities such as displacement and relaxation time distributions. Dynamic heterogeneity effects are explored by studying various decoupling factors, the non-gaussian parameter, the bimodality of the displacement distribution and the non-Fickian wavevector dependence of the structural relaxation time. Comparisons to mode-coupling theory, simulation and experimental results are performed. [Preview Abstract] |
Tuesday, March 14, 2006 10:24AM - 10:36AM |
G34.00013: Dependence of relaxation time on effective temperature in driven glasses Thomas Haxton, Ajay Gopinathan, Andrea Liu Relaxation times of a class of driven glassy systems are shown to depend on a well-defined effective temperature in much the same way that they depend on temperature in quiescent systems. Molecular dynamics simulations were run for two-dimensional systems of bi-disperse spheres interacting via soft repulsive pair potentials. At high density the systems undergo a glass transition as temperature is lowered. We study low-temperature systems driven by an imposed shear gradient in steady state at a fixed high density. Effective temperatures can be defined from fluctuation-dissipation relations by the long-time limit of the ratio of correlation to response. Throughout a range of bath temperatures and shear rates, relaxation times are found to depend only on the bath temperature $T$ and the effective temperature $T_{\rm eff}$. In particular, the relaxation time of the driven system as a function of $T_{\rm eff}$ can be mapped on to the relaxation time of the quiescent system as a function of $T$, using a scale factor that varies only weakly with the ratio $T/T_{\rm eff}$. This suggests that shear unjams the system because it gives rise to a $T_{\rm eff}$ that is higher than the glass transition temperature. [Preview Abstract] |
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