Free Cooling of a Granular Gas in Three Dimensions

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 apparently 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 an experimental study of homogeneous cooling of three-dimensional (3D) granular gases in microgravity. Surprisingly, the asymptotic scaling $E(t)\propto t^{-2}$ 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. The shape anisotropy of the grains influences the characteristic time of energy loss quantitatively, but not qualitatively. We compare kinetic energies in the individual degrees of freedom, and find a slight predominance of the translational motions. In addition, we detect a certain preference of the grains to align with their long axis in flight direction, a feature known from active matter or animal flocks, and the onset of clustering.