Rotation and translation dynamics in a granular gas of rough spheres in microgravity*

Granular gases are dilute ensembles of grains, interacting only in rare collision events, where energy is dissipated. Their collective dynamics have been studied analytically and numerically in numerous publications. One striking scenario is granular cooling, the decay of kinetic energy from an initially agitated state, originally modeled by P. Haff in1983 [1]. Experimental proof of the scaling laws was achieved only recently in 3D ensembles in microgravity [2,3]. It revealed striking quantitative disagreement with theory and previous simulations. One question concerns the role of particle rotations. For rods, rotaions around the short axis can be easily excited without friction, and those have been evaluated [2]. The purely frictionally or contact asymmetry excited rotation of spheres has previously not been addressed, for technical reasons. This is the

We present microgravity experiments using soft frictional spheres in a 3D setting. We analyze the role of translations  and rotations  during their granular cooling.

[1] P. K. Haff, Grain flow as a fluid-mechanical phenomenon, J. Fluid Mech. 134, 401 (1983).

[2] K. Harth, T. Trittel, S. Wegner, R. Stannarius, Phys. Rev. Lett. 120, 214301 (2018).

[3] P. Yu, M. Schröter, M. Sperl, Phys. Rev. Lett. 124, 208007 (2020).