Title: Free Cooling of a Granular Gas in Three Dimensions,
Abstract: 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.