Spatially heterogenous dynamics in dense, driven granular flows

Interest in the dynamical arrest leading to a fluid --> solid transition in thermal and athermal systems has led to questions about the nature of these transitions. These jamming transitions may be dependent on the influence of extended structures on the dynamics of the system. Here we show results from a simple driven, dissipative, non-equilibrium system which exhibits dynamical heterogeneities similar to those observed in a supercooled liquid which is a system in thermal equilibrium. Observations of the time $\tau_R(r)$ required for a particular particle to move a distance $r$ reveal the existence of large-scale correlated dynamical regions with characteristic timescales chosen from a broad distribution. The mean squared displacement of ensembles of particles with varying characteristic $\tau_R(r)$ reveals an intriguing spatially heterogenous mobility. This suggests that a unified framework for jamming will have to be based on the connection between the nature of these heterogeneities and the effective dynamics.Comment: 6 pages, 5 figures, submitted to Europhys. Let

Entropy-vanishing transition and glassy dynamics in frustrated spins

In an effort to understand the glass transition, the dynamics of a non-randomly frustrated spin model has been analyzed. The phenomenology of the spin model is similar to that of a supercooled liquid undergoing the glass transition. The slow dynamics can be associated with the presence of extended string-like structures which demarcate regions of fast spin flips. An entropy-vanishing transition, with the string density as the order parameter, is related to the observed glass transition in the spin model.Comment: 4 pages,5 figures, accepted in PRL January 200

Shear induced rigidity in athermal materials: a unified statistical framework

Recent studies of athermal systems such as dry grains and dense, non-Brownian suspensions have shown that shear can lead to solidification through the process of shear jamming in grains and discontinuous shear thickening in suspensions. The similarities observed between these two distinct phenomena suggest that the physical processes leading to shear-induced rigidity in athermal materials are universal. We present a non-equilibrium statistical mechanics model, which exhibits the phenomenology of these shear-driven transitions: shear jamming and discontinuous shear thickening in different regions of the predicted phase diagram. Our analysis identifies the crucial physical processes underlying shear-driven rigidity transitions, and clarifies the distinct roles played by shearing forces and the density of grains.Comment: Comments welcom