Experimental investigation of the freely cooling granular gas
  Using diamagnetically levitated particles we investigate the dynamics of the freely cooling granular gas. At early times we find good agreement with Haff's law, where the time scale for particle collisions can be determined from independent measurements. At late times, clustering of particles occurs. This can be included in a Haff-like description taking into account the decreasing number of free particles. With this a good description of the data is possible over the whole time range. 
C.C. Maa{\ss}, N. Isert, G. Maret, and C.M. Aegerter