Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session H8: Focus Session: Glassy Dynamics in Colloids |
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Sponsoring Units: DFD Chair: David Weitz, Harvard University Room: Morial Convention Center RO6 |
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H8.00001: Are colloidal and molecular glass formation related? Invited Speaker: Understanding why and how a glass is formed on a microscopic level remains an outstanding problem in condensed matter physics. A molecular glass is normally formed by cooling of a liquid. Upon entering the supercooled state, the structural dynamics slows down dramatically and eventually the liquid enters the non- equilibrium glassy state. On route towards the glass, the behaviour shows a range of highly general, near universal characteristics, such as stretched exponential behaviour of dynamic correlation functions and cooperative dynamics. Such generalities exist even though molecular glasses can be formed from liquids encompassing a wide range of molecular structures and interactions. Glass formation also occurs in altogether very different systems. One of the most interesting, both from a fundamental and an applications point of view, is that of colloidal suspensions. The high degree of control that can be achieved regarding colloidal particle size, shape and interactions makes this a fantastic model system in learning about glass-formation. We know that a range of properties observed during dynamic arrest in molecular systems for decreasing temperature are indeed mirrored in the arrest of a colloidal suspension upon increase of particle volume fraction. However, the richness in phenomenology observed for liquids has generally not been observed for colloids. We will discuss to what extent colloidal glass formation can be viewed as equivalent to molecular glass formation and present recent experimental work that suggests a remarkably direct connection. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H8.00002: Applications of patchwork dynamics for glassy systems Creighton Thomas, Alan Middleton, Olivia White We present work on ``patchwork dynamics'' as a technique for studying the nonequilibrium properties of glassy systems. In patchwork dynamics, we replace local Monte Carlo simulations, which require exponentially long times to equilibrate at a given length scale, with exact equilibration on patches at a given length scale, which can be done rapidly in models such as the 2D Ising spin glass and disordered dimer models. We have demonstrated some interesting applications of patchwork dynamics to such systems: 1) as a heuristic ground state algorithm for the 2D Ising spin glass on a torus (for which there are no known fast exact algorithms) and the 3D Ising spin glass; 2) as a method to study aging effects, persistence, and memory in 2D and 3D Ising spin glasses; 3) as a sampling procedure to study the nonequilibrium properties of disordered dimer models at finite temperatures. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H8.00003: Aging of a Binary Colloidal Glass Jennifer M. Lynch, Gianguido C. Cianci, Eric R. Weeks After having undergone a glass transition, a glass is in a non-equilibrium state, and its properties depend on the time elapsed since vitrification. We study this phenomenon, known as aging. In particular, we study a colloidal suspension consisting of micron-sized particles in a liquid --- a good model system for studying the glass transition. In this system, the glass transition is approached by increasing the particle concentration, instead of decreasing the temperature. We observe samples composed of particles of two sizes ($d_{1} = 1.0\mu m$ and $d_{2} = 2.0\mu m$) using fast laser scanning confocal microscopy, which yields real-time, three-dimensional movies deep inside the colloidal glass. We then analyze the trajectories of several thousand particles as the glassy suspension ages. Specifically, we look at how the size, motion and structural organization of the particles relate to the overall aging of the glass. We find that areas richer in small particles are more mobile and therefore contribute more to the structural changes found in aging glasses. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H8.00004: Structural signatures of dynamical heterogeneity in supercooled liquids Heidi Perry, David Reichman The underlying mechanism of the transition from liquid to glass is a long-standing open question in condensed matter physics. One long sought after clue to understanding the glass transition is a link between the structure and dynamics of a vitrifying fluid. The dynamics of a supercooled liquid near the glass transition have proven to be collective and heterogeneous, with the length scale of the dynamic regions increasing as the glass transition temperature is approached. Using computer simulations and a normal mode analysis, we demonstrate a link between the structural properties of a super cooled liquids and the collective dynamical regions. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H8.00005: ABSTRACT WITHDRAWN |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H8.00006: Fluctuations in the aging regime of a polymer glass Azita Parsaeian, Horacio E. Castillo We perform numerical simulations to investigate the fluctuations in the aging regime of a system of polymers which are interacting via the Lennard-Jones potential. We characterize how the fluctuations evolve by studying (i) probability distributions of local observables such as individual particle displacements $\Delta x$ and intermediate scattering functions $C_r$ associated with small regions and (ii) dynamic correlation functions such as the four-point density correlation $g_4({\bf r}, t, t_w)$. We find that, similar to small molecule glasses, the probability distributions of local observables approximately collapse when the global two-time correlation $C_{\mbox{global}}(t,t_w)$ is held fixed. We test for universality by comparing the probability distributions in the small molecule glass with those in the polymer glass. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H8.00007: Equipartition theorem in glasses and liquids Valentin A. Levashov, Takeshi Egami , Rachel S. Aga , James R. Morris In glasses and liquids phonons have very short life-time, whereas the total potential energy is not linear with temperature, but follows the T**(3/5) law. Thus it may appear that atomic vibrations in liquids cannot be described by the harmonic oscillator model that follows the equipartition theorem for the kinetic energy and potential energy. We show that the description of the nearest neighbor oscillation in terms of the atomic level stresses indeed provide such a description. The model was tested for various pair-wise potentials, including the Lennard-Jones potential, the Johnson potentials, and only the repulsive part of the Johnson potential. In all cases each of the local elastic energies of the six independent components of the stress tensor is equal to kT/4, thus the total potential energy is equal to (3/2)kT. Thus this model provides the basis for discussing the thermodynamic properties of glasses and liquids based on atomistic excitations. An example of this model leading to the description of the glass transition temperature in metallic glasses is discussed [1]. \newline [1] T. Egami, et al., Phys. Rev. B 76, 024203 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H8.00008: Exceptionally Stable Organic Glasses with Low Enthalpy and High Kinetic Stability Prepared by Vapor Deposition Kenneth L. Kearns, Stephen F. Swallen, M.D. Ediger, Ye Sun, Tian Wu, Lian Yu Vapor deposition can be used to prepare glasses of 1,3,5-(tris)naphthylbenzene (TNB) and indomethacin (IMC) that are much more stable than those created by cooling from the liquid. By controlling the temperature of the substrate and the deposition rate, the stability of the glass can be tuned. Glasses can be deposited with enthalpies as much as 10 J/g lower than the glass made by cooling the liquid. Vapor-deposited glasses can also be superheated well above the conventional T$_{g}$. The slow evolution from the low energy glass to the supercooled liquid is observed and can take tens of hours to evolve at temperatures near T$_{g }$+ 5 K. Trends in stability support an enhanced surface dynamics mechanism where we estimate the dynamics in the top 1 nm to be about 7 orders of magnitude faster than the bulk at T$_{g}$ -- 25 K. Vapor deposition has also allowed us to progress more than 40{\%} towards the bottom of the amorphous potential energy landscape. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H8.00009: Magnetic Analogies for the Dynamics of Glass Forming Liquids Jacob Stevenson, Peter Wolynes We present a direct mapping between the dynamics of glass forming liquids and a general random field / random coupling Ising model using the replica effective potential approach. Using the overlap between two structural states of a supercooled liquid we construct a constrained overlap free energy that can be mapped directly onto that of an Ising Hamiltonian. For a Lennard-Jones glass the fluctuations and mean values of the random fields and interactions place it within the universality group of the random field Ising magnet, not the Ising spin glass. This corresponds with the explanation for a random first order transition. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H8.00010: Direct imaging of particle dynamics in attractive colloidal glasses Piotr Habdas, Andrzej Latka, Yilong Han, Ahmed Alsayed, Arjun G. Yodh We use confocal and fluorescent microscopy to study the dynamics of glassy colloidal suspensions. The suspensions are composed of PMMA colloidal particles in density and index-of-refraction matched liquid and stained with a fluorescent rhodamine dye. A controllable depletion attraction is induced between hard-sphere PMMA particles by adding different amounts of polystyrene polymer to the suspension. Our dynamical measurements focus on jumps experienced by PMMA particles that escape the cage formed by its neighbors. We track these particles over time and correlate particle fluctuations with its changes in average position. We find that as the strength of the attractive potential increases, and the system enters an ``attractive liquid'' phase, the number of jumping particles increases. We calculate the distribution of particle jump sizes, time between jumps, and spatial distribution of particle jumps; these observations are compared to predictions of molecular dynamics simulations. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H8.00011: Shear-induced ordering and vitrification of concentrated emulsions Jung-Ren Huang, Thomas G. Mason Using time resolved light scattering, we investigate the degree of droplet deformation and ordering within concentrated oil-in-water emulsions subjected to oscillatory shear between parallel glass plates. We create uniform microscale droplets between the plates by rupturing a premixed emulsion of larger droplets at a fixed strain amplitude and frequency. Subsequently, by independently adjusting the strain amplitude and frequency and recording videos of the dynamic scattering pattern, we examine how the instantaneous applied shear and prior shear history influence the positional structure of the droplets. We also explore how the ordering of the emulsion droplets depends on the oil volume fraction, both above and below that associated with maximally random jamming of uniform hard spheres. The short-range stabilizing repulsion between oil droplets enforces ordering in the shear direction; yet, by contrast to sheared colloidal hard spheres, the deformability of the oil droplets allows concentrated emulsions to un-jam at sufficiently high shear rates. We propose a real-space model, based on the form factor of ellipsoidal droplets and structure factor of ordered, jammed, and un-jammed configurations, that is consistent with the observed light scattering patterns. This technique can be used to transform the structure of a uniform emulsion between ordered and disordered droplet configurations. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H8.00012: Accentuated shear thinning of soft sphere suspensions Hans M. Wyss, Johan Mattsson, Alberto Fernandez-Nieves, Giovanni Romeo, Melaku Muluneh, Zhibing Hu, David A. Weitz Suspensions of soft colloidal particles exhibit highly unusual rheological behaviors; surprisingly, despite the importance of these materials in a wide range of applications, the underlying physical mechanisms remain poorly understood. Experiments show that suspensions of soft particles exhibit a highly pronounced shear thinning; this decrease in viscosity with increasing shear rate far exceeds the shear thinning observed in suspensions of solid particles. We use soft microgel particles as a model system to elucidate this behavior. Our experimental system allows us to study the mechanical behavior both macrosopically and locally, at the scale of the colloidal particles themselves. We combine data obtained at different length scales to arrive at a simple picture of the observed accentuated shear thinning. [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H8.00013: Investigation of Rejuvenation and Overaging in Glassy Energy Landscapes Mya Warren, Joerg Rottler Many glassy systems experience a change in their aging dynamics under the influence of mechanical load. It has long been known that large stresses can cause an apparent decrease in relaxation times (rejuvenation) in polymer glasses, but in colloidal glasses an increase (overaging) has also been observed depending on the strain amplitude. The conditions under which rejuvenation or overaging occur are not yet fully understood. Additionally, there is still considerable controversy over the nature of the resultant states. In order to gain intuition on these outstanding questions, we investigate the aging dynamics under load though stochastic simulations of the Soft Glassy Rheology (SGR) model. For both stress controlled and strain controlled loading, the SGR model exhibits clear regions of overaging and rejuvenation in a parameter space defined by the noise temperature, the quench history, and the strain. Additionally, results show that the states produced under loading are distinct from those that would naturally be visited during aging, and this has effects on the subsequent aging trajectory. Results from the energy landscape picture are compared to pertinent molecular dynamics studies. [Preview Abstract] |
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