Granular materials, such as sand, grain, powders, and many pharmaceuticals, are large aggregates of macroscopic, individually solid particles. However, far from being simple materials with simple properties, granular materials display an astounding range of complex behavior that often defies their categorization as either solid, liquid or gas. In many ways, we may even think of them as presenting a new, different state of matter. In the engineering community, there has been a long-standing interest in describing and predicting the response of granular materials. This is obvious from their tremendous importance for industrial processes in areas like agriculture or civil engineering. Nevertheless, there is so far no detailed understanding as it exists, e.g., for ordinary fluids. At least in part this comes about because granular materials exist far from equilibrium and in very inhomogeneous configurations. Furthermore, because of friction, they respond in highly nonlinear ways to external forcing. Over the last 10+ years, precisely these aspects have made granular materials a model system in the physics community for the understanding of many aspects of driven, non-equilibrium behavior. This talk will review some of these unusual aspects of granular materials, and will discuss both our current understanding and some new directions for future research.

ANL Physics Division Colloquium Schedule