Laboratory Experiments on the Motion of Dense Dust Clouds

In laboratory experiments, we study the motion of levitated, sedimenting clouds of sub-mm grains at low ambient pressure and at high solid-to-gas ratios $\epsilon$. The experiments show a collective behavior of particles, i.e. grains in clouds settle faster than an isolated grain. In collective particle clouds, the sedimentation velocity linearly depends on $\epsilon$ and linearly depends on the particle closeness $C$. However, collective behavior only sets in at a critical value $\epsilon_{\rm crit}$ which linearly increases with the experiment Stokes number St. For $\rm St <0.003$ particles always behave collectively. For large Stokes numbers, large solid-to-gas ratios are needed to trigger collective behavior, e.g. $\epsilon_{\rm crit} = 0.04$ at $\rm St = 0.01$. Applied to protoplanetary disks, particles in dense environments will settle faster. In balance with upward gas motions (turbulent diffusion, convection) the thickness of the midplane particle layer will be smaller than calculated based on individual grains, especially for dust. For pebbles, large solid-to-gas ratios are needed to trigger instabilities based on back-reaction.