Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session A18: Soft Glassy Materials |
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Sponsoring Units: DCMP GSNP DPOLY Chair: Gary Hunter, New York University Room: 403 |
Monday, March 3, 2014 8:00AM - 8:12AM |
A18.00001: A Long-Lived Oscillatory Space-Time Correlation Function of Two Dimensional Colloids Jeongmin Kim, Bong June Sung Diffusion of a colloid in solution has drawn significant attention for a century. A well-known behavior of the colloid is called Brownian motion : the particle displacement probability distribution (PDPD) is Gaussian and the mean-square displacement (MSD) is linear with time. However, recent simulation and experimental studies revealed the heterogeneous dynamics of colloids near glass transitions or in complex environments such as entangled actin, PDPD exhibited the exponential tail at a large length instead of being Gaussian at all length scales. More interestingly, PDPD is still exponential even when MSD was still linear with time. It requires a refreshing insight on the colloidal diffusion in the complex environments. In this work, we study heterogeneous dynamics of two dimensional (2D) colloids using molecular dynamics simulations. Unlike in three dimensions, 2D solids do not follow the Lindemann melting criterion. The Kosterlitz-Thouless-Halperin-Nelson-Young theory predicts two-step phase transitions with an intermediate phase, \textit{the hexatic phase} between isotropic liquids and solids. Near solid-hexatic transition, PDPD shows interesting oscillatory behavior between a central Gaussian part and an exponential tail. Until 12 times longer than translational relaxation time, the oscillatory behavior still persists even after entering the Fickian regime. We also show that multi-layered kinetic clusters account for heterogeneous dynamics of 2D colloids with the long-lived anomalous oscillatory PDPD. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A18.00002: Categorizing Dense Attractive 2D Colloidal Packings using Vibrational Modes and Local Structure Matthew Lohr, Tim Still, Kevin Aptowicz, Ye Xu, Matthew Gratale, Arjun Yodh In this work, we investigate the microscopic dynamics of quasi-2D dense attractive colloidal systems. We confine bidisperse polystyrene spheres between glass coverslips in a suspension of water and 2,6-lutidine; as we increase the temperature of the sample into a critical regime, lutidine wets the colloids, creating a strong attractive interaction (greater than 4kT). We track the particle locations via bright-field video microscopy and analyze the dynamics of packings at various packing fractions. Subsequent calculations of the vibrational modes of the systems demonstrate a hallmark of ``glassy'' vs. ``gel-like'' behavior at low frequencies. Specifically, we observe a sudden increase in the density of low-frequency modes with decreasing packing fraction. These modes appear to be coupled to collective motion of large groups of particles. Additionally, there is evidence that this change in vibrational behavior is tied to a packing's local structure. By investigating the correlation between collective vibrations and local packing, we take a significant step towards delineating ``gel'' and ``attractive glass'' states in attractive, dense disordered systems. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A18.00003: Microscopic and mechanical roles of disorder in a jammed polycrystal Nathan Keim, Paulo Arratia We present experiments on the relationship between the microscopic structure of a polycrystalline 2D solid and its response to deformation. The material is a monolayer of mutually repulsive particles adsorbed at an oil-water interface, for which we simultaneously measure bulk mechanical response (oscillatory shear rheology) and image the motion of many individual particles. Crystallinity is varied through changes to materials and preparation. We investigate the role of less-ordered regions (e.g. grain boundaries) in the system's response to deformation, including the locations of particle rearrangements as deformation amplitude is increased. We consider the extent to which even a small amount of disorder makes elastic deformation, energy dissipation, and yielding behaviors similar to those of highly disordered materials. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A18.00004: Flowing properties of quasi-2D emulsions in Couette geometry Carlos Orellana, Eric Weeks We study the flow of dense emulsion in a quasi-two-dimensional Couette geometry. Our samples are oil-in-water emulsions confined between two close-spaced parallel plates, so that the droplets are deformed into pancake shapes.~By means of microscopy, we measure the droplet positions and their deformation,~which is related to the stress on the individual droplet. In this system without static coulomb friction, we observe a continuous transition from affine motion to topological rearrangements that span the whole system as the area fraction is increased. We compare our results to granular experiments and simulations. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A18.00005: Equilibrium and non-equilibrium aggregation in two dimensional systems with competing interactions Mahesh Bandi, Tamoghna Das A two dimensional system of mono-disperse particles with competing short range attraction and long range repulsion is numerically investigated. Keeping the competing interaction strength fixed at low temperature and density, a dynamical transition from an equilibrium to a non-equilibrium state can be achieved by tuning the repulsion length alone. This is accompanied by a structural transition from non-compact (equilibrium) to compact (non-equilibrium) aggregates. Whereas strong bonding is responsible for non-compact cluster formation, caging dynamics in compact clusters result in non-exponential relaxation characteristic of glassy behaviour. With increasing temperature, the non-equilibrium aggregation gives way to an ergodic liquid. With increasing density, the system undergoes a geometric transition into a percolating gel state, independent of temperature and repulsion length. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A18.00006: Experimental signature of the self-caging in quiescent colloidal glasses Minh Triet Dang, Sanne Loenen, Katharine Jensen, Rojman Zargar, Daniel Bonn, Peter Schall Glasses have liquid-like structure, but solid-like properties. Here, we use colloidal glasses to directly visualize particle configurations in glasses and supercooled liquids. In hard-sphere systems, the particle configurations provide a unique route to the free energy of the system, determined by geometry only. We determine the free volumes of the particles, and directly relate to their free energy changes. We observe two different length scales of free volume distribution in space and a long-range correlation of local free energy of colloidal glasses. This is the first experimental signature self-caging of colloidal glasses, which indicates the first-order phase transition in glasses. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A18.00007: Soft Spots in Aging Colloidal Glasses Moyosore Odunsi, Craig Maloney, Eric Weeks We track the movement of particles in colloidal glasses using confocal microscopy. Colloidal glasses are out of equilibrium and so they age: the motion of the particles slows down over time. The aging is initiated by stirring the sample. In our aging samples, we examine the motion of the particles on small time scales to infer positions of soft spots (localized regions of low-frequency vibrational modes). We then look at correlations between these soft spots and subsequent particle rearrangements on larger time scales. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A18.00008: 3D confinement effect on diffusive behaviors of dense colloidal suspensions Bo Zhang, Xiang Cheng We design an experimental system to investigate three-dimensional (3D) confinement effect on dense colloidal suspensions. By solidifying the aqueous phase of an oil-in-water emulsion, we achieve a 3D confinement with no-slip boundary conditions. Fast confocal microscopy is used to image dynamics of colloidal particles at different volume fractions and confinement lengths. We systematically measure particles' mean square displacement (MSD) and the system's overlap factor proposed in the random first order transition theory. Based on these measurements, a confinement ``phase diagram'' is constructed. We find a strong confinement effect for suspensions at moderate volume fractions with the confinement length smaller than 10 particle diameters. Finally, we also compare our results with other 3D confined systems with different boundary conditions including systems with slip boundaries and with fixed particle boundaries. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A18.00009: Mechanical response of a colloidal glass undergoing repeated local perturbation Tim Still, Ye Xu, Kevin Aptowicz, Arjun Yodh If an amorphous solid is deformed beyond a certain threshold, it undergoes rearrangements on a microscopic level. Often these rearrangements are irreversible and the glassy material finds a new minimum in the energy landscape. However, if the glass is repeatedly perturbed with a moderate cyclical deformation, the mechanical response of the glass can evolve from irreversible to reversible. In our experiments, we utilize colloidal particles with strong thermophoretic properties and local laser heating to generate singular and periodic local non-homogeneous perturbations in quasi-two-dimensional colloidal glasses. The individual particles are soft and deformable, and the elasticity of the material induces mechanical recovery when laser heating ceases. Optical microscopy and particle tracking allow us to follow the path of each individual particle and determine the reversibility and affinity of the mechanical response on a single particle level. This enables us to investigate the microscopic mechanisms of energy dissipation in model glasses and sheds light on the onset of mechanical failure in disordered materials. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A18.00010: Stretched-exponential relaxation in sheared non-Brownian suspensions Joseph Paulsen, Sidney Nagel Relaxations in glasses are often approximated by a stretched exponential. Many models to explain this behavior posit a heterogeneous spread of single exponential processes with a wide distribution of relaxation times. Here, we study the approximately stretched-exponential relaxation that we find in a model, developed by Cort\'{e} et al.~[1], of sheared non-Brownian suspensions. Using a one-dimensional version with a variety of interaction rules [2], we investigate how the wide spectrum of relaxation timescales originates from density fluctuations in the initial (random) configuration of particles. Our theoretical arguments are in good agreement with numerical simulations and reveal a functional form for the relaxation that is distinct from, but well-approximated by, a stretched exponential. \\[4pt] [1] L. Cort\'{e}, P. M. Chaikin, J. P. Gollub, and D. J. Pine, Nature Phys. 4, 420 (2008). \newline [2] N. C. Keim, J. D. Paulsen, and S. R. Nagel, Phys. Rev. E 88, 032306 (2013). [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A18.00011: Yielding of glasses -- a dynamic first-order transition? Peter Schall, Triet Dang, Bernd Struth, Dmitry Denisov We use a new combination of x-ray scattering and rheology to elucidate the yielding of glasses. By combining dynamic structure factor measurements with oscillatory rheology, we can resolve structural changes during the yielding of colloidal glasses. Surprisingly, we find a sharp symmetry change in the structure factor upon yielding, signaling a first order transition of the glass. This symmetry change is accompanied by a sharp change of fluctuations from non-Gaussian to Gaussian distributions of the scattered intensity. We interpret these observations as a new dynamically induced first order transition from a solid- to a liquid-like state of the glass. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A18.00012: Emergence of cooperativity in plasticity of soft glassy materials J\'er\^ome Crassous, Antoine Le Bouil, Axelle Amon, Sean McNamara The elastic coupling between plastic events is generally invoked to interpret plastic properties and failure of amorphous soft glassy materials. We report an experiment where we observe that plastic zones form structures of growing size as the system approaches failure. For this we impose a homogeneous stress on a granular material, and measure local deformations for very small strain increments using a light scattering setup. We observe non-homogeneous strains that appear as small line segments of mesoscopic size that lengthen as the system approaches failure. The line segments have a well defined orientation clearly distinct from macroscopic shear band that appears at failure. The presence and the orientation of those localized deformations may be understood by considering how a localized plastic reorganizations redistribute stresses in a surrounding continuous elastic medium. The mesostructure of the plastic deformation before failure and the presence of plastic events are confirmed by numerical simulations. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A18.00013: Two step yielding of core-shell microgels: An investigation of ``hard'' and ``soft'' cage yielding mechanisms via Rheo-SANS Javoris Hollingsworth, Zhi Zhou, Song Hong, Guangmin Wei, He Cheng, Charles Han The yielding mechanism of hybrid microgels composed of a polystyrene (PS) core and thermosensitive poly(N-isopropylacrylamide) (PNIPAM) shell were studied via small-angle neutron scattering with rheological measurements (rheo-SANS). While the PS core attributes to the hard sphere properties of the microgels, the soft sphere properties are due to their PNIPAM shell; softness increases as a function of temperature and decreases with PNIPAM shell thickness. By varying the core-shell ratio or temperature, a series of particles ranging from near-hard sphere to typical soft microgels were obtained. Generally, the first yielding event occurs when short range interactions (bonds between interconnected or local clusters) are broken, whereas the second yielding event is due to the breaking of long range interactions (nearest-neighbor ``cages''). According to the results, near-hard sphere suspensions exhibit single-step yielding; however, suspensions at intermediate core-shell ratios display two-step yielding characteristics. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A18.00014: Effects of particle softness on shear thickening of microgel suspensions He Cheng, Zhi Zhou, Charles Han A series of microgel particles composed of a polystyrene (PS) core and thermo-sensitive poly (N-isopropylacrylamide) (PNIPAM) shell with different shell thicknesses were investigated to elucidate the effect of microgel softness on its shear thickening behavior. Since the softness of the microgels increases with decreasing temperature through the volume phase transition effect of PNIPAM shell, the measured softness parameter, n, which is derived from the Zwanzig-Mountain equation, was used to measure and describe the combined influences of temperature and shell thickness. According to our results, the softness parameter is able to estimate the shear thickening behavior of microgel suspensions at least semi-quantitatively. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A18.00015: ABSTRACT WITHDRAWN |
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