Experimental evidence for granular shear-flow instability in the Epstein regime

TL;DR

This study experimentally demonstrates a granular shear-flow instability in a dust-gas mixture under Epstein drag conditions in microgravity, resembling Kelvin-Helmholtz instability, informing planet formation theories.

Contribution

First experimental observation of granular shear-flow instability in Epstein regime using microgravity, providing a benchmark for two-fluid planet formation models.

Findings

Observed periodic velocity field indicative of shear instability

Resembles Kelvin-Helmholtz instability in behavior

Provides experimental benchmark for two-fluid theories

Abstract

Stability analysis of two-fluid protoplanetary disc models has enriched our understanding of how solids can grow into larger bodies called planetesimals. Dust particles entrained in a gas stream modify the flow, creating shear layers prone to instability. In such environments, drag occurs in the free-molecular (Epstein) regime. Recreating these two-phase flows on Earth is difficult due to gravity-driven buoyancy. Here, we use particle image velocimetry to study a low-pressure dust-gas mixture at Knudsen numbers up to 10 in microgravity. We observe a granular shear flow instability, characterized by a periodic velocity field, which can be modeled to first order as a Kelvin-Helmholtz (KH) instability. This behavior resembles a Kelvin-Helmholtz instability and provides a benchmark for two-fluid theories relevant to planet formation.