Experimental study of clusters in dense granular gas and implications for the particle stopping time in protoplanetary disks

In protoplanetary disks, zones of dense particle configuration promote planet formation. Solid particles in clouds alter their motion through collective effects and back reaction to the gas. The effect particle-gas feedback with ambient solid-to-gas ratios $\epsilon > 1$ on stopping time is investigated. experiments board International Space Station we studied evolution a granular gas while interacting air. We observed diffusion clusters released at onset an experiment but also formation new dynamical clusters. mass ratio outside cluster varied range about $\epsilon_{\rm avg} \sim 2.5 - 60$. find that concept drag viscous medium still holds, even if strongly dominated by solids. However, factor has be used, depending $, i.e. force reduced 18 highest ratios. Therefore, flocks grains disks move faster collide than constituents might suggest.