Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S24: Focus Session: Jamming: Effective temperature and Aging |
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Sponsoring Units: GSNP Chair: Daniel Blair, Harvard University Room: LACC 411 |
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S24.00001: Measuring effective temperatures in sheared, athermal systems at fixed normal load Ning Xu, Corey O'Hern We perform molecular dynamics simulations of repulsive athermal systems sheared at fixed normal load to study the effective temperature $T_L$ defined from time-dependent fluctuation- dissipation relations for density. We show that these systems possess two distinct regimes as a function of the ratio $T_S/V$ of the granular temperature to the potential energy per particle. At small $T_S/V$, these systems are pressure-controlled and $T_L$ is set by the normal load. In contrast, they behave as quasi-equilibrium systems with $T_L \approx T_S$ that increases with shear rate at large $T_S/V$. The fact that $T_L$ is slaved to the pressure at small $T_S/V$ indicates that the variables $T_L$, pressure, and density are not sufficient to describe dense, slowly-sheared athermal systems. Another important implication for systems at small $T_S/V$ is that $T_L$ for two systems placed in contact will not equilibrate when a pressure gradient is maintained between them. Thus, $T_L$ does not behave as a thermodynamic temperature variable in the pressure-controlled regime and new definitions of effective temperature should be explored. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S24.00002: Off-equilibrium fluctuation dissipation relation in polymer glasses Hassan Oukris, Nathan Israeloff Aging dynamics in thin films of polyvinyl acetate (PVAc) is investigated using a rapid quench process by applying local joule heating to a thin layer capacitor. We study the validity of fluctuation dissipation relation (FDR) during aging in a glassy polymer. The time evolution of noise and dielectric susceptibility is analyzed in the frequency range 0.1 Hz- 10$^{3}$ Hz following quenches to below and near the glass transition temperature (T$_{g})$. It is found that eliminating extraneous noise spikes due to differential thermal contraction is important for accurately determining effective temperature. We find a large violation of FDR which extends to ft$_{w}$=300, much larger than for spin glasses. The results of these studies are important in understanding the relaxation dynamics of structural glasses and in testing the validity and relevance of the effective temperature concepts. Experimental study of FDR in aging systems can give new insight into the interpretation of the aging dynamics. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S24.00003: Experimental measurement of an effective temperature for densely packed granular materials Ping Wang, Chaoming Song, Hernan Makse A densely packed granular system is an example of an out-of- equilibrium system in the jammed state. It has been a longstanding problem to determine whether this class of systems can be described by concepts arising from equilibrium statistical mechanics, such as an ``effective temperature'' and ``compactivity.'' The measurement of the effective temperature is realized in the laboratory by slowly shearing a closely-packed ensemble of spherical beads confined by an external pressure in a Couette geometry. All the probe particles, independent of their characteristic features, equilibrate at the same temperature, given by the packing density of the system. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S24.00004: Energy landscape picture of overaging and rejuvenation in a sheared glass Daniel Lacks, Mark Osborne Molecular simulations and an energy landscape analysis are used to investigate the effects of shear on aging in a glass. Shear beyond the yield point is shown to change the state of a glass such that it resembles (but is not identical to) a different stage in the aging process. A cycle of large strain rejuvenates the glass by relocating the system to shallower energy minima, while a cycle of small strain overages the glass by relocating the system to deeper energy minima. The balance between overaging and rejuvenation is controlled by how well the glass was initially annealed. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S24.00005: Overaging and rejuvenation in a polymer glass subjected to shear deformation Matthew L. Wallace, Bela Joos When applying a transient shear on a jammed substance, one can induce both rejuvenation and overaging in the system, as the relaxation times are altered in a non-trivial way. We induce the overaging of a polymer glass by instantaneous, one-time shear deformations, and follow its evolution for long waiting times $t_w$. After each deformation, the characteristic relaxation time of the system $\tau_{1/2}$ increases. We find in our molecular dynamics simulations two distinct regimes, primarily based on how the system evolves after a waiting time $t_w$ following one deformation. In the low-shear regime, both the energy of the inherent structure of the system and $\tau_ {1/2}$ change very little with $t_w$, and there are no apparent structural changes in the system. In the high-shear regime, we see an initial combination of rejuvenation and overaging in the system and $\tau_{1/2}$ has a well-defined logarithmic dependence on $t_w$. Furthermore, it is found that when successive deformations are applied, no memory effect or directional preference arises from the previous shearing. Finally, we investigate the signatures of the overaged state. The polymer glass is obtained by compressing isothermally a melt of short freely jointed chains interacting with van der Waals interactions. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S24.00006: Time evolution of local fluctuations in the aging of a simple glass Horacio Castillo We report results of molecular dynamics simulations of the out of equilibrium dynamics of a simple glass former, a binary Lennard-Jones system, after a quench to low temperatures. We explore in detail the fluctuations in the mean square displacement and local correlator describing small, coarse grained regions of the system. We evaluate the probability distribution funcion (PDF) of those local quantities, as a function of waiting time $t_w$ and final time $t$. We find that both probability distribution functions show very good scaling when the waiting time and final time are chosen to keep the global incoherent scattering function $C_q(t,t_w)$ at a constant value. In fact, the global incoherent scattering function is a better predictor for both probability distribution functions than the ratio of the final and waiting times $t/t_w$ or the value of the global mean square displacement of the particles $\Delta(t,t_w)$. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 4:18PM |
S24.00007: Shearing glassy materials: insight from computer simulations Invited Speaker: I will discuss some aspects of flow in glassy systems that have been recently investigated using computer simulations. I will first describe some statistical properties of homogeneous flow using the ``effective temperature'' concept. Then I will show that complex phenomena such as yield stress, shear banding, complex stress strain curves, commonly observed in metallic glasses, amorphous polymers or complex fluids, can be observed in a very simple ``computer glass'' model. The existence of shear bands can be understood from the presence of a nonzero yield stress, larger than the small shear limit of homogeneous flow curves. This in turn implies that the flow curve is effectively nonmonotonic, and allows shear band formation in a restricted domain of shear rates. Finally, I will describe some results obtained in athermal systems, in which it is possible to decompose the deformation in terms of elementary plastic events. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S24.00008: Aging effects in shear yielding of glassy solids Joerg Rottler, Mark O. Robbins The shear yield stress of amorphous glassy materials is usually taken to be a function of the loading state, the temperature and the strain rate. Since glasses are out of equilibrium systems, yielding is additionally influenced by the material's own intrinsic aging dynamics. We study the role of aging in the well-known generic Lennard-Jones glasses through molecular dynamics simulations. For temperatures not too far below the glass transition temperature, the (transient peak) yield stress increases logarithmically with the waiting time in the glassy regime. The rate dependence is directly related to the age of the system. If the waiting time is much larger than the time to reach the yield point, the yield stress follows a universal logarithmic rate dependence, but in the opposite limit the system behaves like a system in steady shear that is constantly ``rejuvenated.'' These effects disappear at very low temperatures where the aging dynamics is frozen out. Implications for phenomenological models of plasticity in glassy materials are pointed out. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S24.00009: Local aging effects in glassy polymers probed by 1/f noise Koneswaran Sinnathamby, Nathan Israeloff Local aging effects in glassy system have been investigated by nanometer scale probing of polarization noise fluctuations in a glassy polymer polyvinyl-acetate (PVAc) near to the primary relaxation region. Using ultra high vacuum (UHV) capacitance scanning probe microscopy (SPM) techniques, nanometer scale polarization fluctuations were probed. Time dependent changes in the noise spectrum, and high order statistical variations of the noise time series were studied and analyzed with varying temperature. Local aging was studied by analyzing the variance of the noise spectrum following a temperature quench. The temperature dependent variance is larger than the expectations of Gaussian noise, and this effect decreases with aging. Moreover, the experimental 1/f noise and computer simulations of noise from a simple model were compared in order to better understand the cooperativity and heterogeneity found in glassy polymers. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S24.00010: SPM investigation of local aging effects in glassy polymers Philip Crider, Nathan Israeloff We investigate the cooperative and heterogeneous nature of glassy dynamics by nanometer-scale probing in a glassy polymer, Polyvinyl-Actetate (PVAc), with a Scanning Force Microscope (SFM). Using ultra-high-vacuum (UHV) Scanning Capacitive Force Microscopy techniques, nanometer-scale capacitive responses are probed. Dielectric relaxation near the glass transition is investigated, and scanning capabilities are utilized to analyze spatial response on a nanometer scale. The results of these studies may yield insight into the understanding of temperature-dependent cooperative length scales, local aging properties, and energy landscape properties of evolving dipole clusters on a mesoscopic scale. Results are used to test the validity and relevance of current models of glassy dynamics. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S24.00011: Local correlations and local responses in the aging of a sample Allen Dahili, Horacio Castillo We investigate the relationship between local fluctuations in the correlation and the response in a three dimensional strucural glass model. We perform molecular dynamics simulations of a binary Lennard-Jones mixture and measure local correlations and responses in small regions of the system, after a quench into the glass regime. We compare the joint probability distribution of these local quantities with the out-of-equilibrium fluctuation-dissipation relation of the bulk system. We also compare our results with the results of analogous simulations in short range spin glass systems, which support the presence of a soft mode in the aging dynamics. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S24.00012: Structural Details of Aging in Colloidal Glasses Gianguido C. Cianci, Eric R. Weeks Dense colloidal suspensions are good model glass formers. We use fast laser-scanning confocal microscopy, which allows real-time tracking of the trajectories of thousands of colloidal particles in 3-D, to study non-equilibrium phenomena in colloidal glasses. In particular we are interested in aging, the dependence of physical properties on the time elapsed since the creation of the sample. We investigate this non-equilibrium behavior in terms of colloidal packing. Tetrahedra, or triangular based pyramids, represent the ideal packing of 4 spheres in three dimensions, however they do not tile 3-D space. This frustration between local and global packing optimization has been invoked as a possible origin for the glass transition. We therefore study how these tetrahedra evolve as the sample ages. We are particularly interested in how the characteristics of tetrahedra, such as irregularity (the normalized standard deviation of the edge lengths of the tetrahedra), influence their dynamics. Furthermore, we study the correlations between the structure of tetrahedra and the dynamics of the particles that form them. [Preview Abstract] |
Wednesday, March 23, 2005 5:18PM - 5:30PM |
S24.00013: Growth of spatial correlations during the aging of a simple structural glass Azita Parsaeian, Horacio Castillo We investigate dynamical heterogenities in a binary Lennard-Jones system below the glass transition temperature, by computing spatial correlations of fluctuations in the aging regime. A theoretical framework based on the existence of a soft mode in the nonequilibrium dynamics predicts the presence of a dynamical correlation length which grows with time. In our simulations we find that correlations grow with growing waiting time, as expected from this picture. [Preview Abstract] |
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S24.00014: Aging at criticality in models with absobing states Jose J. Ramasco, Malte Henkel, Maria Augusta Santos, Constantino A. da Silva Santos The long-time dynamics of the critical models with absobing states which are brought suddenly out of an uncorrelated initial situation undergoes ageing in close analogy with quenched magnetic systems. In particular, we show that time-translation invariance is broken and that dynamical scaling holds. We find that the autocorrelation and autoresponse exponents $\lambda_{\Gamma}$ and $\lambda_R$ are equal but, in contrast to systems relaxing to equilibrium, the aging exponents $a$ and $b$ are distinct. A recent proposal to define a non-equilibrium temperature through the short-time limit of the fluctuation-dissipation ratio is therefore not applicable. [Preview Abstract] |
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