Laboratory Experiments on the Motion of Dense Dust Clouds

TL;DR

This study experimentally investigates how dense clouds of sub-millimeter dust grains settle faster collectively than individually, revealing dependencies on solid-to-gas ratio and Stokes number, with implications for protoplanetary disk dynamics.

Contribution

It provides the first detailed laboratory analysis of collective dust cloud behavior, quantifying how sedimentation velocity depends on particle density and flow parameters.

Findings

Particles in dense clouds settle faster than isolated grains.

Sedimentation velocity depends linearly on solid-to-gas ratio and particle closeness.

Collective behavior emerges above a critical solid-to-gas ratio that depends on the Stokes number.

Abstract

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.