Mechanics of ultrasonically levitated active granular membranes

We explore granular rafts in an acoustic trap consisting of hundreds of macroscopic particles. These close-packed rafts are self-assembled by a sonic depletion force mediated by scattering, which establishes short-range attractions between the constituent particles [1,2]. We show that droplets of this granular fluid display emergent surface tension and elasticity. These droplets interact with the acoustic field, inducing forces and torques that drive coalescence, deformations, and break-up. We use a persistent torque in the acoustic field to extract the droplet surface tension. At the same time, active fluctuations in the acoustic field act as an effective temperature, driving the droplet to explore its configurational space. Microstructural measurements, and the fluctuation spectra of the droplet perimeter, reveal the far-from-equilibrium dynamics of this granular active membrane.

References:
[1] MX Lim, A Souslov, V Vitelli, and HM Jaeger: “Cluster formation by acoustic forces and active fluctuations in levitated granular matter”. Nature Physics 15,460 (2019)
[2] MX Lim, KA Murphy, and HM Jaeger, “Edges control clustering in levitated granular matter”. Granular Matter 21, 77 (2019)