Dynamics of grains in driven granular media

We use diffusing-wave spectroscopy (DWS) to study microscopic dynamics in the interior of 3-dimensional granular systems. We study two granular systems where particle motions are excited by different driving mechanisms - gravity-driven channel flow and a gas-fluidized bed. In both instances we obtain detailed information about short-time collisional dynamics such as rms velocity fluctuations, mean free paths, and collision frequencies. We also observe a slow crossover from short-time ballistic motions to long-time, grain-scale diffusive motions.