Experimental measurements of time dependent structure in shock waves and gravitational-collapse of a granular gas

We experimentally measure time-dependent velocity statistics in a quasi-2D granular gas under gravity. High resolution video particle tracking measurements show that the vibration induced shock waves have a serrated substructure on the scale of a particle diameter. We identify this substructure as an experimental visualization of collisional transport. Collisional transport is non-local in granular variables and is the mechanism for increased sound speed at higher densities. We also measure the time-dependent statistical properties of the gas after the energy input has been halted. The cooling occurs in two distinct stages separated by a high temperature settling shock. In the final stage, the temperature of a fluid packet decreases as a power law until the gas reaches a static state in agreement with recent theoretical work on this problem. However, the measured power law exponent is significantly larger than the theoretical value.