Session V36: Disordered and Glassy Systems (Non-Polymeric)

Injecting a droplet into a quasi-2D jammed emulsion: Fluctuations and rearrangements

We experimentally study the dynamic response of a quasi-two-dimensional emulsion to a slowly growing injected droplet. Our area fractions range from $\phi = 0.77 - 0.99$, such that the droplets are in most cases in contact with one another and are in many cases highly deformed. There is no dependence of the average flow behavior on distance to the inflation droplet, or on polydispersity or packing fraction of the emulsions. However, the fluctuations of velocity increase as the packing fraction increases. The magnitude of the fluctuations appears similar in both monodisperse, moderately ordered samples and bidisperse, disordered samples.   

Densest packings of hard spheres in a cylinder

Densely packing hard spheres (HS) within a cylinder is remarkably complex. Little is known about the densest achievable packings when the cylinder diameter, D, is larger than 2.85 times the sphere diameter, s. Here, we extend the identification of the densest packings up to D = 4.00s by adapting Torquato-Jiao’s adaptive-shrinking-cell formulation and sequential-linear-programming technique to this geometry. We identify 17 new structures, almost all of them chiral. Beyond D, approx2.85s , most of the structures consist of an outer shell and of an inner core that compete for being close packed. In some cases the shell adopts a periodic configuration that is optimal and the stacking of core spheres within it is quasiperiodic, while in other cases a direct interplay between the two layers is observed. For some packings the very distinction between the core and shell vanishes, which results in exotic geometries, including some that are a three-dimensional extension of packing hard disks in a circle. In order to connect our results with experiments on comparable systems, we also consider the ease with which these structures assemble. Using kinetic Monte Carlo simulations, we find that some of the structures promtply assemble while others simply do not.   

Labyrinthine phase and slow dynamics in a driven magnetic granular medium

Labyrinthine patterns arise in two-dimensional physical systems submitted to competing interactions, ranging from the fields of solid-state physics to hydrodynamics. Here we experimentally investigate a labyrinthine phase in an out-of-equilibrium system constituted of vibrated granular particles. Once sufficiently magnetized, they self-organize into short chains of particles in contact and randomly orientated. We quantitatively characterize the transition from a granular gas state to a labyrinthine phase, and we explain the formation of these chains using a simple model. Interestingly, the labyrinthine phase does not display any steady state: its morphology evolves with the aging time on very long timescales. Experiments suggest that here, slow dynamics involves strong structural rearrangements and therefore is comparable to slow dynamics in structural glasses. We characterize this aging process and evaluate to what extent this analogy holds.   

From Gelation and Glass Transition of Colloidal Systems to Polymers

Charles C. Han, Guangcui Yuan and He Cheng Joint Laboratory of Polymer Science and Materials, ICCAS, Beijing, China and Institute for Advanced Study, Shenzhen University, Shenzhen, China Aggregation and gelation behavior of mixed suspensions of polystyrene microspheres and poly(N-isopropylacrylamide) microgels have been studied. In dilute microsphere suspensions, with increasing concentration of microgel (MG), microspheres (MS) first aggregated with each other through the bridging of the microgels, then dispersed individually when saturated adsorption was achieved, and finally depletion clusters formed at even higher concentrations of microgel. In concentrated microsphere suspensions, with saturated MG adsorption, a state transition from attractive glass to repulsive glass can be observed. This type of system can be viewed as a molecular model system which has a long range repulsive interaction potential and a short range attractive potential. A comparison between the glass transition of the colloidal systems and the glass transition of polymeric systems can be made.   

Relaxation and self-diffusion of supercooled liquids derived from picosecond timescale dynamics

We use neutron scattering and nonlinear optical measurements to investigate ps-ns timescale dynamics in liquid, supercooled liquid, and glassy states. The experimental observables show evidence of dynamic heterogeneity on this timescale that supports a facilitated dynamics picture. We obtain a direct measure of the concentration of molecular excitations, or mobile regions, as a function of time and temperature. Using a model [1] broadly consistent with that proposed by Chandler and co-workers [2], we are able to quantitatively predict self-diffusion rates and Stokes Einstein violation deep in the supercooled regime directly from ps timescale and Angstrom - nanometer length scale measurements for all systems we have investigated. The model we employ also provides a clear physical mechanism for the Johari-Goldstein relaxation process. [1] M.T. Cicerone, Q. Zhong & M. Tyagi, PRL 113 117801 (2014). [2] J. P. Garrahan & D. Chandler, Coarse-grained microscopic model of glass formers, PNAS 100, 9710 (2003).   

In search of a Corresponding state description of the thermodynamics and dynamics of complex fluids.

Long ago, Pitzar introduced a scheme for characterizing the relative "complexity" of fluids based on a consideration of the temperature dependence of the second virial coefficient at low temperature, where this property is sensitive to the form of the inter-molecular potential. "Simple" fluids, in this classification scheme, are those that satisfy a common "corresponding states" in which the properties (eg. shape of the phase boundary, surface tension etc.) obey a universal reduced variable scaling description. This idea was pioneered by van der Waals based on his equation of states describing the pressure of gases. Many real fluids are not "simple" in the sense that the molecules have complex shapes and interact with a combination of short-range and long-range directional interactions having different spatial interaction ranges. These features lead to these fluids being classified as "complex". We show that an effective reduced variable description for a model complex fluid can be achieved by defining a reduced variable involving the second virial coefficient. We further show that a recent reduced variable description of the dynamics and thermodynamics of glass-forming liquids derives from the same principle.   

Short-Time Glassy-like Dynamics Observed in Viscous Protein Solutions with Competing Potential Features.

Structures in concentrated protein solutions caused by the combination of short-range attraction (SA) and long-range repulsion (LR) have been extensively studied due to their importance in understanding therapeutic protein formulations and the phase behavior in general. Despite extensive studies of kinetically arrested states in colloidal systems with short-range attraction, less is understood for the effect of an additional longer-range repulsion on model colloidal systems with a SA interaction. Highly purified lysozyme is used a model experimental system due to its stable globular structure and SALR interactions at low ionic strength that can be quantitatively modeled. The fluid microstructure and protein short time self diffusion are measured across a broad range of conditions by small angle neutron scattering (SANS) and neutron spin echo (NSE), respectively. Newtonian liquid behavior is observed at all concentrations, even with an increase of zero shear viscosity by almost four orders of magnitude with increasing concentration. However, dynamic measurements demonstrate a sub-diffusive regime at relatively short time scales for concentrated samples at low temperature. The formation of a heterogeneous density distribution is shown to produce localized regions of high density that reduce protein motion, giving it a glassy-like behavior at the short time scale. This heterogeneity occurs at the length scale associated with the intermediate range order driven by the competing potential features, distinguishable from heterogeneous colloidal gels.   

Combined Effects of Media Disorderedness and Tracer Shape on the Trend of Translation-Rotation Decoupling in Two-Dimensional Binary Colloids

While translational diffusion of tracers often violates the Stokes-Einstein relation, the rotational diffusion of tracers follows the Debye-Stokes-Einstein relation faithfully in the glass-forming materials. A previous study revealed that in two-dimensional (2D) monodisperse colloids, as the dynamics of media became heterogeneous in 2D hexatic phase, the tracer shape and the local media structure affected the translation-rotation decoupling trend significantly [1]: the rotation of tracers was enhanced compared to the translation for square tracers but was rather suppressed for diamond tracers. The shape dependency of rotation originated from the similarity in structure between the local hexagonal media structure and the tracer shape. Unlike in 2D monodisperse colloids where the liquid-to-hexatic phase transition takes place, in 2D binary colloids, a phase transition from the liquid to either solid or glass depends on the disorderedness that is controlled by size and number ratios. We present simulation results on the combined effects of the tracer shape and the local media disorderedness on the translation-rotation decoupling, which relates closely to the nature of glass transition. [1] J. Kim and B. J. Sung, Phys. Rev. Lett. 115, 158302 (2015)   

Spatio-temporal correlations in Coulomb clusters

Dynamical response of Coulomb-particles in nanoclusters are investigated at different temperatures characterizing their solid-like (Wigner molecule) and liquid-like behavior. The density correlations probe spatio-temporal relaxation, uncovering distinct behavior at multiple time scales in these systems. They show a stretched-Gaussian or stretched-exponential spatial decay at long times in circular and irregular traps. Interplay of confinement and long-range nature of interactions yields spatially correlated motion of the particles in string-like paths, leaving the system heterogeneous even at long times. While particles in a `solid' flow producing dynamic heterogeneities, their random motion in `liquid' defies central limit theorem. Distinguishing the two confinements, temperature dependent motional signatures serve as a criterion for the crossover between `solid' and `liquid'. The irregular Wigner molecule turns into a nearly homogeneous liquid over a much wider temperature window compared to the circular case. The temperature dependence of different relaxation time scales builds crucial insights. A phenomenological model, relating the unusual dynamics to the heterogeneous nature of the diffusivities in the system, captures much of the subtleties of our numerical simulations.   

Glassy Spin Dynamics in Buckled Colloidal Crystal

Geometric frustration arises when lattice structure prevents simultaneous minimization of local interaction energies. It leads to highly degenerate ground states and complex behaviors in frustrated magnetic materials. Here we experimentally studied buckled 1.5-layer colloidal NIPA microgel crystals confined between parallel plates. Spheres buckled up and down are analogous to antiferromagnetic Ising spins. These spins on the distorted triangular lattice exhibit glassy dynamics at low temperatures. In particular, a spin only has 13 nearest-neighbor configurations, which enables to reveal the correlation between structures and dynamical heterogeneity. Soft modes also localize at high-energy regions. Further, we compared the colloidal spin system with kinetic constrained models (KCMs) and observed dynamical facilitation behaviors including excitations lines in space-time. Similar structures and glassy dynamics are also observed in our simulation of Coulomb charges on a triangular lattice.   

Experiments reveal different dynamics in two and three dimensions near the colloidal glass transition

We use microscopy to study both 3D and quasi-2D colloidal systems as they approach their glass transitions. We use two different bidisperse 2D systems, one of which has hard particles and the other which has particles interacting with long range dipolar interactions. The 3D system also has hard interactions (3D data obtained from Narumi, et al. Soft Matter 2011). In the 3D data, we observe significant plateaus in the mean square displacement curves, in contrast to 2D. This indicates stronger transient localization in 3D. In both 2D systems, as we approach the glass transition, we observe decoupling between translational time scales and time scales for structural reorientation. In 3D, these time scales always remain coupled. Finally, in 2D we observe large clusters of particles moving in parallel directions, but similar clusters are markedly smaller in 3D. In both 2D systems, these clusters become larger on approaching the glass transition. We attribute the observed decoupling of translational and bond-orientational times in 2D to the presence of these large directional clusters. Overall, our results are in good qualitative agreement with recent simulation results [Flenner and Szamel, Nature Communications 2015].   

Measuring heterogenous stress fields in a 3D colloidal glass

Glass in our common experience is hard and fragile. But it still bends, yields, and flows slowly under loads. The yielding of glass, a well documented yet not fully understood flow behavior, is governed by the heterogenous local stresses in the material. While resolving stresses at the atomic scale is not feasible, measurements of stresses at the single particle level in colloidal glasses, a widely used model system for atomic glasses, has recently been made possible using Stress Assessment from Local Structural Anisotropy (SALSA). In this work, we use SALSA to visualize the three dimensional stress network in a hard-sphere glass during start-up shear. By measuring the evolution of this stress network we identify local-yielding. We find that these local-yielding events often require only minimal structural rearrangement and as such have most likely been ignored in previous analyses. We then relate these micro-scale yielding events to the macro-scale flow behavior observed using bulk measurements.   

Mean-field description of plastic flow in amorphous solids

Failure and flow of amorphous materials are central to various phenomena including earthquakes and landslides. There is accumulating evidence that the yielding transition between a flowing and an arrested phase is a critical phenomenon, but the associated exponents are not understood, even at a mean-field level where the validity of popular models is debated. Here we solve a mean-field model that captures the broad distribution of the mechanical noise generated by plasticity, whose behavior is related to biased L\'evy flights near an absorbing boundary. We compute the exponent $\theta$ characterizing the density of shear transformation $P(x)\sim x^{\theta}$, where $x$ is the stress increment beyond which they yield. We find that after an isotropic thermal quench, $\theta=1/2$. However, $\theta$ depends continuously on the applied shear stress, this dependence is not monotonic, and its value at the yield stress is not universal. The model rationalizes previously unexplained observations, and captures reasonably well the value of exponents in three dimensions. These results support that it is the true mean-field model that applies in large dimension, and raise fundamental questions on the nature of the yielding transition.   

Rearrangement dynamics in colloidal particle packings identified through local structure and machine-learning

We explore the connection between measures of local structure and particle rearrangements in soft thermal quasi-two-dimensional colloidal systems employing a machine learning approach. Local structure is characterized by two and three point structure functions that measure radial and angular distributions of particles, and rearrangements are identified by a measure of change in average colloidal particle position. By generating labeled training data, we can extract the features of these functions that contribute to the likelihood of a rearrangement. In particular, we use a machine-learning algorithm to construct a decision function in the form of a scalar field we call softness that with high accuracy labels regions of particles more likely to rearrange. Thus, we can predict dynamic rearrangements from the instantaneous local structure. The softness field remains a good predictor when we vary the packing fraction between training and test data sets. In glassy samples, the softness field can identify aging as particles become less likely to undergo cage rearrangements.   

A Generic Microscopic Theory for the Universality of TTLS Meissner-Berret Ratio of Amorphous Solid

Tunneling-two-level-system (TTLS) has successfully explained several experimental results for amorphous solid which do not exist in crystalline counterparts. However longituinal and transverse phonon-TTLS coupling constants' ratio $\gamma_l/\gamma_t$ has been found to lie between $1.5$ and $1.6$ for 13 different amorphous solids which cannot be explained within TTLS model. In this paper by developing an interacting generic block model with random stress tensors, we show the universality essentially comes from interaction between generic blocks, independent of the material's microscopic structure. In the appendix we also give a detailed correction for non-elastic stress-stress interaction coefficient $\Lambda_{ijkl}^{(ss')}$ derived by Joffrin and Levelut.