Science in the sandbox: Fluctuations, friction and instabilities

The study of granular materials is a novel and rapidly growing field. These materials are interest for a number of reasons, both practical and theoretical. They exhibit a rich of novel dyanamical states, and they exhibit 'phases'-solid, liquid, and gas-that resemble conventional thermodynamic phases. However, the presence of strong dissipation through friction and inelasticity places these systems well outside the usual class of systems that can be explained by equilibrium thermodynamics. Thus, there are important challenges to create new kinds of statistical physics and new analytical descriptions for the mean and fluctuating behavior of these materials. We explore recent work that focuses on several important issues. These include force propagation and fluctuations in static and driven systems. It is well known that forces propagate through granular structures along networks-force chains, whose structure is a function of history. It is much less clear how to describe this process, and even what kind of structures evolve in physical experiments. After a brief overview of the field, we consider models of force propagation and recent experiments to test these models. Among the latter are experiments that probe force profiles at the base of sandpiles and experiments that determine the Green's function response to point perturbations in granular systems. We also explore the nature of force fluctuations in slowly evolving systems, particulary sheared granular systems. These can be very strong-with rms fluctuations in the force that are as strong as the mean force. Finally, we pursue the analogy between conventional phases of matter, where we particularly focus on the transition between fluid and solid granular states in the presence of sustained horizontal shaking