# Dense Particle Clouds in Laboratory Experiments in Context of Drafting and Streaming Instability

## TL;DR

This study conducts laboratory experiments on particle clouds in a rotating system to investigate the transition from individual to collective behavior driven by dust-to-gas ratios, providing insights into streaming instabilities relevant to planetesimal formation.

## Contribution

It presents experimental evidence of particle behavior transition in a laboratory setting, aiding the validation of astrophysical models of streaming instability.

## Key findings

- Particles behave as test particles at low dust-to-gas ratios.
- Particles exhibit faster sedimentation in denser regions at higher ratios.
- Experimental results can test and improve astrophysical hydrocodes.

## Abstract

The streaming instability, as an example of instabilities driven by particle feedback on a gas flow, has been proven to have a major role in controlling the formation of planetesimals. Here, we present experiments to approach this situation in the laboratory for particles in the Knudsen flow regime. In these experiments, we observe a particle cloud trapped for about 30 s in a rotating system under Earth's gravity. For average dust-to-gas ratios up to 0.08, particles behave like individual test particles. Their sedimentation speed is identical to that of a single free-falling particle, even in locally denser regions. However, for higher dust-to-gas ratios, the motion of particles becomes sensitive to clumping. Particles in locally denser regions now sediment faster. Their sedimentation speed then depends linearly on the overall dust-to-gas ratio. This clearly shows a transition from tracerlike behavior to collective behavior. Beyond these findings, these types of experiments can now be used as a gauge to test the particle feedback models in astrophysical hydrocodes which are currently used for numerical simulations of streaming instabilities.