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