Signatures of dynamical heterogeneities in dense granular flows

Recent interest in understanding the dynamical arrest in both thermal and athermal systems has led to questions about the nature of these jamming transitions (PRL {\bf 86}, 111 (2001), Nature {\bf 411}, 772 (2001)), as well as the role extended structures may play in determining the dynamics of the system (Science {\bf 287}, 627 (2000)). Simulations of steady-state gravity-driven flows of inelastically colliding hard disks show the formation of large-scale linear chains of particles with a high collision frequency even at flow velocities well above the jamming transition (EPL {\bf 66}, 277 (2004)). These chains can be shown to carry much of the collisional stress in the system due to a dynamical correlation that develops between the momentum transfer and time between collisions in these "frequently-colliding" particles. Several striking features develop which may be connected to the presence of the chains, including a strong anisotropy in the distribution of collision angles and an increase in collision frequency as the granular temperature is decreased. The granular temperature displays a different dependence on flow velocity than is predicted by current kinetic theory (PRE {\bf 65}, 011303 (2001)); the velocity fluctuations are observed to die out more rapidly than expected, as observed in experiments (Science {\bf 275}, 1920 (1997)). Understanding the effects that these long-lived dynamical stress chains have on dense, flowing granular materials can lead to further insight into the nature of these systems.