Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Q9: Focus Session: Dynamics of Glassy Systems |
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Sponsoring Units: GSNP Chair: David Grier, New York University Room: 303 |
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q9.00001: Geometric interpretation of pre-vitrification in hard sphere glasses Invited Speaker: Matthieu Wyart I will derive a geometric condition for the stability of hard sphere configurations, and I will show empirically that this condition is saturated in the glass phase. The marginal stability observed explains the presence of slow modes in the short term dynamics of the glass, and supports a microscopic interpretation for the rapid initial rise of the viscosity, or pre-vitrification, when the packing fraction increases. This observation also suggests a possible cause for the collective nature of the structural relaxation near the glass transition. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q9.00002: Aging of a Colloidal Suspension of Thermosensitive Particles Kyaw Win, Gregory McKenna, Tetsuharu Narita, Francois Lequeux, Srinivas Pullela, Zhengdong Cheng We have studied the aging behavior of a concentrated suspension of thermosensitive particles (PNIPAM).~ We found that the characteristic time of the dynamics of the system as a function of waiting time obeys the usual power law with exponent $\sim $ 1 both after shear melting the system and after a jump in temperature which is equivalent to a jump in concentration.~ We also report for the first time the observation of an extreme asymmetry of approach in the aging behavior with respect to down jump and up jump in temperature.~ While an asymmetry effect is already known in conventional glass formers such as polymers and small molecule liquids, it is both much more pronounced in the colloidal PNIPAM suspension and of a fundamentally different character. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q9.00003: Rejuvenation and memory in a 2D colloidal glass Jennifer M. Lynch, Zexin Zhang, Peter Yunker, Arjun G. Yodh, Eric R. Weeks We work with a 2D colloidal system that has a glass transition and use this system to experimentally observe memory and rejuvenation effects as the sample ages. In particular, we study a system of colloidal particles made of thermosensitive poly(N-isopropylacrylamide) (NIPA) polymer. The sample is confined in a narrow quasi-2D gap between parallel glass plates, which allows easy observation and rapid temperature response. Lowering temperature increases the size of the colloidal particles, which can induce the glassy state due to the crowding of the particles. When our colloidal sample is quenched into the glassy state, particle motion slows over time; this is aging. In molecular glasses, prior experiments studied how aging is modified when the temperature is changed while the sample ages, finding that aging at a first temperature $T_1$ and aging at a later time with second temperature $T_2$ are mostly independent. ``Memory'' relates to the case $T_1 < T_2$ and "rejuvenation" to the opposite case. Our colloidal system allows us to observe both of these effects. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q9.00004: A continuous time random walk description of the hopping dynamics in an aging polymer glass Mya Warren, Joerg Rottler Due to the non-equilibrium nature of the glassy state, structural relaxation becomes increasingly sluggish with the wait time $t_w$ since vitrification. As a result, dynamical correlation functions age, and often obey a simple rescaling with $t_w$: $C(t,t_w) = C_0(t) + C_{age}(t/t_w^{\mu})$. It has recently been shown that, to first order, scaling also applies to the distributions of local correlations and displacements (the van Hove function). In this study, we use molecular dynamics simulations to measure the statistics of the discontinuous hopping events that characterize structural relaxations during aging. This allows us to map the particle dynamics onto a continuous time random walk, which successfully reproduces the entire distribution of displacements. Our results bear a striking resemblance to the popular trap model of aging. We find that the hop time distribution takes the form of a power law which is independent of $t_w$, whereas the time to the first hop shifts to longer times with $t_w$. This two-timescale behavior explains not only the scaling of the distribution functions for times $t\sim t_w$, but also small deviations from perfect scaling that have been observed at longer times. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q9.00005: The Defect Diffusion Model and Isochoric Energy and Isobaric Enthalpy for Glass Formers Michael Shlesinger, John Bendler, John Fontanella, Mary Wintersgill The defect diffusion model produces stretched exponential relaxation, in supercooled liquids, through the sub-diffusive motion of defects. The aggregation of the defects produces a Vogel-Fulcher type law for the divergence of the time scale at a critical temperature. The model is employed to calculate the ratio of the apparent isochoric activation energy to the isobaric activation enthalpy, $E_{V}$*/$H*$ or $E_{V}$/$E_{P}$,. This ratio measures the relative sensitivity of kinetic processes to changes in volume and temperature respectively. This ratio equation is tested using dielectric relaxation data for poly(vinyl acetate), viscosity data for glycerol and ionic conductivity data for poly(propylene glycol) containing LiCF$_{3}$SO$_{3}$. Good agreement between theory and experiment is found using model parameters previously published. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q9.00006: Fluctuations in the relaxation of a strong glass Azita Parsaeian, Horacio E. Castillo, Katharina Vollmayr-Lee We present results of molecular dynamics simulations of amorphous silica, carried out by using the BKS inter-atomic potential. We quantify the evolution of fluctuations by studying the probability distributions of local observables such as individual particle displacements $\Delta x$ and local coarse grained intermediate scattering functions $C_r$. We test for universality by comparing the probability distributions with those in small molecule glasses and in polymer glasses. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q9.00007: A percolation model for dynamics in glass-forming materials Gregg Lois, Jerzy Blawzdziewicz, Corey O'Hern We characterize the glass to liquid transition as a percolation of mobile regions in configuration space. We find that many hallmarks of glassy dynamics, for example stretched-exponential response functions and a diverging structural relaxation time, result from critical properties of mean-field percolation. Specific predictions of the percolation model include the range of possible stretching exponents $1/3 \leq \beta \leq 1$ and the functional dependence of the alpha relaxation time and stretching exponent on temperature, density, wave number, and entropy. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q9.00008: Finite Temperature Simulations of Glassy Models with Patchwork Dynamics Creighton Thomas, Alan Middleton We present simulation results on aging effects in the late time dynamics of two glassy models: the Edwards-Anderson Ising spin glass and a disordered lattice dimer model. As these models have glassy dynamics, direct simulations take prohibitively long times. We use patchwork dynamics, in which we replace local Monte Carlo updates with efficient exact finite temperature equilibration of subsystems, or patches. We scale the simulation data and find a collapse to relate the dynamics for different patch sizes, with larger patches evolving the system more rapidly. We investigate the use of this technique to study rejuvenation and memory effects. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q9.00009: Connecting microscopic and phenomenological approaches to glassy dynamics Malcolm Kennett, Matthew Downton Kinetically constrained spin models are known to exhibit dynamical behavior mimicking that of glass forming systems. They are often understood as coarse-grained models of glass formers, in terms of some ``mobility'' field. The identity of this ``mobility'' field has remained elusive due to the lack of coarse-graining procedures to obtain these models from a more microscopic point of view. Here we exhibit a scheme to map the dynamics of a two-dimensional soft disc glass former obtained from Molecular Dynamics simulations onto a kinetically constrained spin model, providing an attempt at bridging these two approaches. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q9.00010: Glassy Dynamics in Systems of Ellipse-shaped Particles Carl Schreck, Mitch Mailman, Bulbul Chakraborty, Corey O'Hern Glass-forming materials possess a critical cooling rate r*; for thermal quench rates r $>$ r*, these systems form disordered solids; for r $<$ r*, they form (poly) crystalline materials. ~We investigate the influence of particle shape (or anisotropic interactions) on the critical cooling rate. In particular, we perform molecular dynamics (MD) simulations of ellipsoidal particles in 2D as a function of aspect ratio of the major to minor axes to optimize the local packing efficiency and the critical cooling rate to improve glass-forming ability. Also, previous mode-coupling theoretical studies have predicted that over a wide range of aspect ratios, the rotational and translational degrees of freedom undergo dynamical arrest at the same temperature. We will perform MD simulations as a function of the cooling rate, packing fraction, and aspect ratio to determine whether novel glass phases also exist in which the rotational and transitional degrees of freedom freeze at different temperatures. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q9.00011: Self-Assembly of Spherical Colloidal Particles at Low N Natalie Arkus, Vinothan Manoharan, Michael Brenner The number of rigid structures that a system of N particles can form grows exponentially with N. Stabilizing any one structure over all others is thus a challenging problem. We consider a system of N spherical colloidal particles that cannot deform or overlap, and which exhibit a short-range attractive force. We present a method, using graph theory and geometry, that solves for all possible rigid packings of N particles - the resultant set of packings is provably complete. We then present a mechanism that is capable of stabilizing any one structure over all others (in the zero temperature limit), and which is experimentally realizable - thereby, potentially allowing us to direct the self-assembly of a desired structure. We compare to preliminary experimental results. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q9.00012: Aging in the shear-transformation-zone theory of plastic deformation Joerg Rottler, Philipp Maass Aging phenomena in the plastic response of glassy solids are studied within the theory of shear transformation zones (STZs), which describes the kinetic rearrangement of localized defects in response to external stress. To account for the slow, non-equilibrium dynamics after a quench below the glass transition temperature, two possible models are considered. In the first model, transition rates between the internal states of STZs decrease with time, while in the second model aging occurs due to the relaxation of an effective temperature that determines the number density of STZs and other out-of-equilibium degrees of freedom. We show that for reasonable choices of parameters, both models capture qualitatively typical aging features seen in computer simulations and experiments on polymer and other soft glasses: (i) Compliance curves measured for different waiting times after the quench can be superimposed when the observation times are rescaled with relaxation times, and (ii) stress-strain curves show a stationary plateau stress independent of waiting time and a peak stress that increases logarithmically with both waiting time and the strain rate. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q9.00013: Stable glass transformation to supercooled liquid via surface-initiated growth front Mark Ediger, Stephen Swallen, Katherine Traynor, Robert McMahon, Thomas Mates Recently it has been established that vapor deposition onto substrates at 0.85 T$_{g}$ can produce high density, high stability, low enthalpy glasses. These glasses may be the most stable ever produced in a laboratory (using the glass formed by cooling the liquid as the reference state). Here we use SIMS to observe the transformation of isotopically layered stable glasses of trisnaphthylbenzene into a liquid during annealing above T$_{g}$. In contrast to the predictions of standard models, the observed transformation is spatially heterogeneous. The liquid grows into the stable glass with sharp growth fronts initiated at the free surface and at the interface with the substrate. For the free surface, the growth velocity is constant in time and has the same temperature dependence as self-diffusion in the equilibrium supercooled liquid. These stable glasses are packed so efficiently that defects such as surfaces and interfaces are required to initiate the transformation to the liquid even well above T$_{g}$. [Preview Abstract] |
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