Single-particle fluctuations and directional correlations in driven hard-sphere glasses (original) (raw)

Heterogeneous Shear in Hard Sphere Glasses

There is growing evidence that the flow of driven amorphous solids is not homogeneous, even if the macroscopic stress is constant across the system. Via event-driven molecular dynamics simulations of a hard sphere glass, we provide the first direct evidence for a correlation between the fluctuations of the local volume fraction and the fluctuations of the local shear rate. Higher shear rates do preferentially occur at regions of lower density and vice versa. The temporal behavior of fluctuations is governed by a characteristic time scale, which, when measured in units of strain, is independent of shear rate in the investigated range. Interestingly, the correlation volume is also roughly constant for the same range of shear rates. A possible connection between these two observations is discussed.

Shear-induced anisotropic decay of correlations in hard-sphere colloidal glasses

Spatial correlations of microscopic fluctuations are investigated via real-space experiments and computer simulations of colloidal glasses under steady shear. It is shown that while the distribution of one-particle fluctuations is always isotropic regardless of the relative importance of shear as compared to thermal fluctuations, their spatial correlations show a marked sensitivity to the competition between shear-induced and thermally activated relaxation. Correlations are isotropic in the thermally dominated regime, but develop strong anisotropy as shear dominates the dynamics of microscopic fluctuations. We discuss the relevance of this observation for a better understanding of flow heterogeneity in sheared amorphous solids.

Long-Range Strain Correlations in Sheared Colloidal Glasses

Physical Review Letters, 2011

Glasses behave as solids on experimental time scales due to their slow relaxation. Growing dynamic length scales due to cooperative motion of particles are believed to be central to this slow response. For quiescent glasses, however, the size of the cooperatively rearranging regions has never been observed to exceed a few particle diameters, and the observation of long-range correlations that are signatures of an elastic solid has remained elusive. Here, we provide direct experimental evidence of long-range correlations during the deformation of a dense colloidal glass. By imposing an external stress, we force structural rearrangements that make the glass flow, and we identify long-range correlations in the fluctuations of microscopic strain, and elucidate their scaling and spatial symmetry. The applied shear induces a transition from homogeneous to inhomogeneous flow at a critical shear rate, and we investigate the role of strain correlations in this transition.

Particle dynamics predicts shear rheology of soft particle glasses

Journal of Rheology, 2020

Soft particle glasses are amorphous materials made of soft and deformable particles that are jammed above close-packing. They behave like weak solids at rest but they yield and flow under external mechanical constraints. Although soft particle glasses are widely used in applications, little is known about how the particle softness and microscopic dynamics determine the macroscopic rheology. Here we use three-dimensional particle dynamic simulations to analyze the dynamical properties of soft particle glasses at different scales. We demonstrate how the dynamics is determined by the persistence time and the magnitude of the fluctuating elastic forces that develop at contact in the flow. The shear-induced diffusion coefficient, the local structural relaxation times, the shear stress, and the normal stress differences are interconnected through simple relationships that allow the prediction of the macroscopic rheology from the microscopic dynamics.

Avalanches mediate crystallization in a hard-sphere glass

Proceedings of the National Academy of Sciences

By molecular dynamics simulations we have studied the devitrification (or crystallization) of aged hard-sphere glasses. First we find that the dynamics of the particles are intermittent: quiescent periods, when the particles simply "rattle" in their nearest-neighbor cages, are interrupted by abrupt "avalanches" where a subset of particles undergo large rearrangements. Second, we find that crystallization is associated with these avalanches but that the connection is not straightforward. The amount of crystal in the system increases during an avalanche but most of the particles that become crystalline are different from those involved in the avalanche. Third, the occurrence of the avalanches is a largely stochastic process. Randomizing the velocities of the particles at any time during the simulation leads to a different subsequent series of avalanches. The spatial distribution of avalanching particles appears random, although correlations are found among avalanche initiation events. By contrast, we find that crystallization tends to take place in regions that already show incipient local order.

Shear Induced Orientational Ordering in Active Glass

arXiv (Cornell University), 2021

Dense assemblies of self propelled particles, also known as active or living glasses are abundant around us, covering different length and time scales: from the cytoplasm to tissues, from bacterial bio-films to vehicular traffic jams, from Janus colloids to animal herds. Being structurally disordered as well as strongly out of equilibrium, these systems show fascinating dynamical and mechanical properties. Using extensive molecular dynamics simulation and a number of different dynamical and mechanical order parameters we differentiate three dynamical steady states in a sheared model active glassy system: (a) a disordered phase, (b) a propulsion-induced ordered phase, and (c) a shear-induced ordered phase. We supplement these observations with an analytical theory based on an effective single particle Fokker-Planck description to rationalise the existence of the novel shearinduced orientational ordering behaviour in our model active glassy system that has no explicit aligning interactions, e.g. of Vicsek-type. This ordering phenomenon occurs in the large persistence time limit and is made possible only by the applied steady shear. Using a Fokker-Planck description we make testable predictions without any fit parameters for the joint distribution of single particle position and orientation. These predictions match well with the joint distribution measured from direct numerical simulation. Our results are of relevance for experiments exploring the rheological response of dense active colloids and jammed active granular matter systems.

Glass dynamics at high strain rates

Physical Review E, 2012

We present a shear-transformation-zone (STZ) theoretical analysis of molecular-dynamics simulations of a rapidly sheared metallic glass. These simulations are especially revealing because, although they are limited to high strain rates, they span temperatures ranging from well below to well above the glass transition. With one important discrepancy, the simplified STZ theory used here reproduces the simulation data, including the way in which those data can be made to collapse approximately onto simple curves by a scaling transformation. The STZ analysis implies that the system's behavior at high strain rates is controlled primarily by effective-temperature thermodynamics, as opposed to system-specific details of the molecular interactions. The discrepancy between theory and simulations occurs at the lower strain rates for temperatures near the glass transition. We argue that this discrepancy can be resolved by the same multi-species generalization of STZ theory that has been proposed recently for understanding frequency-dependent viscoelastic responses, Stokes-Einstein violations, and stretched-exponential relaxation in equilibrated glassy materials.

Glassy dynamics in asymmetric binary mixtures of hard spheres

Physical Review E

We perform a systematic and detailed study of the glass transition in highly asymmetric binary mixtures of colloidal hard-spheres, combining differential dynamic microscopy experiments, event-driven molecular dynamics simulations and theoretical calculations, exploring the whole state diagram and determining the self and collective dynamics of both species. Two distinct glassy states involving different dynamical arrest transitions are consistently described, namely, a double glass with the simultaneous arrest of the self and collective dynamics of both species, and a single glass of large particles in which the self dynamics of the small species remains ergodic. In the single glass scenario, spatial modulations in the collective dynamics of both species occur due to the structure of the large spheres, a feature not observed in the double glass domain. The theoretical results, obtained within the self-consistent generalized Langevin equation formalism, are in agreement with both simulations and experimental data, thus providing the first stringent validation of this theoretical framework in the description of dynamical arrest in highly asymmetric mixtures. Our findings are summarized in a state diagram that classifies the various amorphous states of highly asymmetric mixtures by their dynamical arrest mechanisms.

Correlations of plasticity in sheared glasses

Physical review. E, Statistical, nonlinear, and soft matter physics, 2014

In a recent paper [Mandal et al., Phys. Rev. E 88, 022129 (2013)], the nature of spatial correlations of plasticity in hard-sphere glasses was addressed both via computer simulations and in experiments. It was found that the experimentally obtained correlations obey a power law, whereas the correlations from simulations are better fitted by an exponential decay. We here provide direct evidence-via simulations of a hard-sphere glass in two dimensions (2D)-that this discrepancy is a consequence of the finite system size in the 3D simulations. By extending the study to a 2D soft disk model at zero temperature [Durian, Phys. Rev. Lett. 75, 4780 (1995)], the robustness of the power-law decay in sheared amorphous solids is underlined. Deviations from a power law occur when either reducing the packing fraction towards the supercooled regime in the case of hard spheres or changing the dissipation mechanism from contact dissipation to a mean-field-type drag in the case of soft disks.

Microstructure and dynamics of a polymer glass subjected to instantaneous shear strain

Journal of Physics: Condensed Matter, 2008

The application of instantaneous shear deformations to a polymer glass modifies the energy landscape of the glass in non-trivial ways. Using molecular dynamics simulations on a freely jointed chain model, we investigate the effect of the strain on the dynamical heterogeneities in the glassy system. The resulting behaviour can be separated into two regimes: elastic for small shear deformations, and plastic for large deformations. Dynamic heterogeneity in the system tends to diminish with deformation. This increased homogenization can be seen, for instance, through changes in the distribution of particle mobilities, both in space and in time, as the glass relaxes following an affine deformation. We are able to directly correlate changes in the overall diffusion in the system with local configurations of mobile and immobile particles. The effect of the deformation on the ageing process is briefly addressed, as we present some new insight into the local dynamics of the polymer chains.