Session Q13: Glassy Systems and Jamming I

The Influence of Boundary Roughness on the Dynamics of Confined Colloidal Suspensions near the Glass Transition

We study the relationship between boundary conditions and particle motion in confined, concentrated colloidal suspensions. Studies of polymer fluids in confinement have shown that changes in mobility are strongly dependent upon the polymer/surface interaction. We model this interaction by observing the effects of textured surfaces on colloidal particle mobility in confined dense suspensions (near the glass transition). We use high-speed confocal microscopy to directly image and track the colloidal particles in thin, wedge-shaped sample chambers made from textured glass. We texture the glass in a controlled, reproducible manner by spincoating and sintering colloidal suspensions onto glass slides. We expect the texturing to frustrate the formation of layers seen in smooth-walled confinement, resulting in decreased translational diffusion as compared to the smooth wall case. By studying these dynamics we gain a better understanding of the glass transition and its dependence on interfacial dynamics versus finite size effects.   

Structural Rearrangements in Confined Colloidal Liquids under Oscillatory Shear

We have investigated the dynamics of confined suspensions under oscillatory shear using a micron-gap rheometer interfaced with confocal microscopy. Our system consists of sterically stabilized poly-(methyl methacrylate) (PMMA) particles suspended in density and refractive index matched solvents at particle volume fractions, \textit{$\phi $ }= 0.40-0.43, confined between two solid surfaces with gaps ranging from $\sim $10-30 particle layers. Above a threshold strain of $\sim $6{\%} where an applied deformation is sufficient to induce plastic behavior, we find that structural rearrangements are highly anisotropic. Non-affine motion, determined by subtracting the globally uniform strain from the bare particle coordinates, reveals that particles move as cooperatively rearranging groups with a preferred orientation transverse to the flow direction. Measures which probe cooperative dynamics all reveal a strong amplitude, thickness, and directional dependence on the characteristic sizes of cooperatively rearranging regions. Interestingly, we find that medium range orientational order has a significant influence on shear-induced dynamics, particularly the shapes of rearranging regions.   

Application of Edwards' statistical mechanics to polydisperse and high-dimensional jammed sphere packings

The Edward's statistical mechanics of jammed sphere packings [Song et al., Nature (London) 453, 629 (2008)] is generalized to different systems: polydisperse sphere packings in three dimensions, and high-dimensional monodisperse sphere packings. The theory predicts the density of random close packing and random loose packing of polydisperse systems for a given distribution of particle size and describes packings for any interparticle friction coefficient. In the high-dimensional limit, an asymptotic solution of the self-consistent relation is obtained by saddle-point evaluation and checked numerically. The resulting random close packing density scaling is consistent with that of other approaches, such as replica theory and density-functional theory. The theory could serve as a starting point to solve more difficult problems: such as predicting the optimal density of non-spherical packings, and understanding the higher-order correlations present in amorphous jammed packings.   

Cyclic simple shear in a two-dimensional granular system

We study the evolution of a 2D granular system consisting of frictional photo-elastic disks under large numbers of small-amplitude cyclic shear cycles. We are particularly interested in the reversibility of the system under cyclic shear. The experiments are carried out on a specially designed apparatus which can create quasi-static, nearly uniform simple shear. By using photo-elastic particles and a fluorescent labelling technique, we obtain information about displacement, rotation and contact forces for each particle following each small strain. We also obtain the system-level behaviour over many shear cycles. To better understand the nature of jamming, we have carried out shearing runs that explore various initial states which are initially unjammed, isotropically jammed or anisotropically jammed, and we compare the results for different initial states.   

Microscopic Dynamics of Quasi-2D Dense Colloidal Gels

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 ( $>$ 4kT). We specifically study suspensions in the ``dense gel'' regime, i.e., at a volume fraction high enough that the attractive particles form a spanning cluster, yet just low enough that there exists some structural heterogeneity larger than the individual particle size. We track the particle locations via bright-field video microscopy and analyze the dynamics of the system in order to compare them to lower-volume-fraction gel states and higher-volume-fraction glassy states. In doing so, we pinpoint the similarities and differences in the mechanisms for dynamic arrest in low-density colloidal gels and high-density colloidal glasses.   

Structural Correlations in Glass-Forming Hard Spheres Fluids

Recent studies have detected the presence of a growing static length scale associated with the glassy dynamical slowdown. Yet no fully satisfying microscopic description of such a length scale has yet been formulated. We critically evaluate the hypothesis that correlated structural defects could underlie the growing relaxation time in deeply supersaturated fluid. Though a clear structural signature of a developing order in these systems is found, the resulting defect geometry does not lead quite match the Frank- Kasper defect scenario. The dimensionally generalizable nature of the defects, however, make them promising options for defining static observables.   

Study of experimental protocols for producing random close packed colloids

A collection of spheres can be packed tightly into an amorphous state known as ``random close packing.'' In our experiment, colloidal particles are allowed to slowly sediment forming a random close packed state. By adjusting the solvent's density we finely control the rate at which the sedimentation occurs. We then use confocal microscopy to image the sample. By imaging overlapping regions we determine the positions of hundreds of thousands of particles. From this data, we measure the distribution of Voronoi volumes and the contact number distribution, and examine how these distributions depend on the sedimentation rate.   

Rotational and Translational Phonon Modes in Glasses Composed of Ellipsoidal Particles

The effects of particle shape on the vibrational properties of colloidal glasses are studied experimentally. `Ellipsoidal glasses' are created by stretching polystyrene spheres to different aspect ratios and suspending the resulting ellipsoidal particles at high packing fraction. By measuring displacement correlations between particles, we extract vibrational properties of the ellipsoidal glass. Low frequency modes in glasses composed of ellipsoidal particles with major/minor axis aspect ratios $\sim$1.1 are observed to have predominantly rotational character. By contrast, low frequency modes in glasses of ellipsoidal particles with larger aspect ratios ($\sim$3.0) exhibit a mix of rotational and translational character. All glass samples were characterized by a distribution of particles with different aspect ratios. Interestingly, even within the same sample it was found that small- aspect-ratio particles participate relatively more in rotational modes, while large-aspect-ratio particles tend to participate relatively more in translational modes.   

Dynamical heterogeneities and fluctuations of the time variables in structural glasses

The existence of dynamical heterogeneities in disordered materials is considered now as a crucial element in explaining many observed features of their dynamical behavior. In this work, we investigate a possible hypothesis for their origin, which assumes that they emerge from soft (Goldstone) modes associated with a broken continuous symmetry under time reparametrizations. To test this hypothesis, we construct coarse grained observables from data obtained in simulations of four models of structural glasses. The fluctuations of these observables are decomposed into transverse components associated with the postulated time-fluctuation soft modes and a longitudinal component unrelated to them. We find that as temperature is lowered and timescales are increased, the time reparametrization fluctuations become increasingly dominant and their correlation volumes grow together with the correlation volumes of the dynamical heterogeneities, while the correlation volumes for longitudinal fluctuations remain small.   

Experimental observation of deformation and structural defects in hard-sphere colloid glasses

We performed experiments on a 1.55-$\mu$m-diameter monodisperse, hard-sphere colloid glass under simple shear at various strain rates, while simultaneously tracking real-time individual positions of roughly 100,000 particles by confocal microscopy. We probe the elastic, anelastic, and plastic responses of the system to applied strain, with particular focus on identifying the local mechanisms of deformation. In plastic deformation, we observe thermally activated rearrangements of groups of particles, the nature and concentration of which are correlated with local parameters such as strain, Voronoi volume, and free volume.   

Aging dynamics of a colloidal glass - time resolved viscoelastic properties and the role of flow history

Many colloidal suspensions are inherently out of equilibrium and display a slow evolution of their dynamics over time. However, many features of the glass transition as encountered in polymer and molecular glasses are not conserved. This phenomenon is still not completely understood and little is known of the connection between flow history, as a determinant of the initial system state, and subsequent aging dynamics. Further, the changes in the energy landscape during aging can be understood from the frequency and strain dependence of the shear modulus but the non-stationary nature of these systems frustrates investigation of their instantaneous underlying properties. Here we discuss the use of stress jump experiments that investigate the role of flow history on aging, and the systematic reconstruction of the frequency and strain dependence as a function of age for a repulsive colloidal glass undergoing structural arrest and aging. We uncover a connection between the aging behavior and the rate of flow cessation that is additionally reflected in the dynamics of residual stress relaxation. Strikingly, the frequency dependence at fixed times can be rescaled onto a master curve, implying a simple connection between the aging of the system and the change in the frequency dependent modulus.   

Jamming, Clogging, and Fragility in Frictionless Disk Systems with Quenched Disorder

We consider a two-dimensional simulation model of binary frictionless disks which have a well defined jamming density of $\phi_{j} \approx 0.84$ in the absence of quenched disorder. When quenched disorder is added in the form of impenetrable immobile disks, the jamming density is reduced. As the density of the quenched disorder sites increases, we observe a crossover from a jamming transition to a clogging transition. The clogged state is defined as a highly heterogeneous granular packing that resists shear along one direction and that is composed of a combination of high density patches at the clean jamming density and very low density patches or voids. These clogged states are fragile in the sense that they are only clogged in the direction of an externally applied drive. After a clogged state has formed, if a new drive is applied in a different direction the disks can flow freely for a period of time before reorganizing into a new clogged state. In contrast, jammed systems are jammed in all directions simultaneously.   

Coupling Between Translational and Orientational Order in Fiber Suspensions

Suspensions of non-Brownian fibers under a small oscillatory shear flow find a random but completely reversible state, called ``random organization'': at each period, the non-hydrodynamic interactions modify both the orientation and positions of fibers, until a reversible configuration is found. As observed in sphere suspensions, there is a nonequilibrium absorbing phase transition when the strain is increased above a concentration-dependant threshold. The transient time, during which the activity decays algebraically, has a diverging duration; critical exponents are consistent with Manna universality class. Above the threshold, fibers get progressively aligned towards the vorticity and a reversible steady state is eventually found for a range of strain. This behavior is specific to fiber suspensions. We study whether or not these oriented reversible states are critical states. We experimentally evaluate the angles distribution of fibers in both vertical and horizontal planes and discuss the relation between these distributions and the existence of a reversible state.   

Force correlations near point J in a lattice model of jamming

We have constructed a lattice model of a jammed system in $d=2$ dimensions near the isostatic point (Point J). Adapting the Tighe model, we represent a jammed pack of particles as a regular hexagonal array, with repulsive forces between nearest neighbors. We generate near-isostatic jammed configurations by carrying out a Monte Carlo simulation with Tighe ``wheel moves'', which rearrange forces locally while preserving force balance on every particle. (Wheel moves correspond to a small dilation of a given particle.) The MC simulation is progressively biased towards the creation of ``missing contacts'', bonds which bear zero force. We reveal long-range correlations in the force network near Point J by determining for each particle the smallest ``collective move'' --- a set of wheel moves that taken together dilates the given particle, while preserving the existing missing contacts. The size of these collective moves diverges as Point J is approached.   

Time and volume fraction dependence of dynamic heterogeneity in a glass-forming binary hard-sphere mixture

We examined dynamic heterogeneity in a glass-forming binary hard-sphere mixture for volume fractions up to and including the so-called mode-coupling transition. We calculated the dynamic susceptibility $\chi_4(t)$, the four-point structure factor $S_4(q;t)$ and the dynamic correlation length $\xi(t)$. We find that the correlation length increases with time as $\xi(t) \sim \ln(t)$ and is independent of $\phi$ for times approximately between the $\beta$ and $\alpha$ relaxation time. The dynamic length plateaus at a $\phi$ dependent value $\xi_{\mathrm{max}}(\phi)$. We find that $\xi_{\mathrm{max}}(\phi)$ is proportional to the dynamic length at the $\alpha$ relaxation time, $\xi(\tau_\alpha)$. Finally, while for a limited range of volume fractions $\xi(\tau_\alpha) \sim \tau_\alpha^{1/z}$ with $1/z \approx 0.2$, we find that $\xi(\tau_\alpha) \sim \ln(\tau_\alpha)$ describes our data well for all $\phi$.