Dynamics of ultrasonically levitated granular rafts

Macroscopic particles in an acoustic trap can self-assemble into close-packed granular rafts consisting of hundreds of particles. These rafts are formed and stabilised due to a sonic depletion force mediated by scattering, which establishes short-range attractions between the constituent particles [1]. We show that this cohesive granular material displays fluid-like behaviour, forming circular “granular droplets” with an emergent surface tension and viscosity. These droplets interact with the acoustic field, inducing forces and torques that drive the droplets to merge, deform, and break-up. We focus on a rotational instability that provides a persistent torque to objects moving in the acoustic field. As the angular momentum of a granular droplet is increased, it deforms from a circle to an ellipse, eventually pinching off into two separate droplets. We use hydrodynamic models for rotating liquid drops to describe the granular dynamics and extract the droplet surface tension.
References:
[1] MX Lim, A Souslov, V Vitelli, and HM Jaeger, “Cluster Formation via Sonic Depletion Forces in Levitated Granular Matter”, arXiv:1808.03862 (2018)