Free Cooling of a Granular Gas in Microgravity*

Granular gases as dilute ensembles of particles in random motion are not only at the basis of elementary structure-forming processes in the universe and involved in many industrial and natural phenomena, but also excellent models to study fundamental statistical dynamics. A vast number of theoretical and numerical investigations have dealt with this seemingly simple non-equilibrium system. The essential difference to molecular gases is the energy dissipation in particle collisions, a subtle distinction with immense impact on their global dynamics. Its most striking manifestation is the so-called granular cooling, the gradual loss of mechanical energy in absence of external excitation. We report the first experimental study of homogeneous cooling of three-dimensional (3D) granular gases in microgravity. The asymptotic scaling E(t)∼ t^-2 obtained by Haff's minimal model [J. Fluid Mech. 134 401 (1983)] proves to be robust, despite the violation of several of its central assumptions.