The deformation and flow of a levitated granular membrane

Many phenomena in nature can be modeled by condensed phases of hard spheres held together by cohesive forces. However, there has been a lack of experimental systems to systematically probe the self-assembly and mechanics of these systems on the mesoscale, particularly in the presence of nonequilibrium driving. Here, we investigate one such model system: sub-millimetre objects acoustically levitated in air. Driven by scattered sound, levitated grains self-assemble into a monolayer of particles, forming mesoscopic granular membranes, with a surface tension and bending rigidity that emerge from long-range interactions. Detuning the acoustic trap can give rise to stochastic active forces and torques that impart angular momentum to levitated objects. As the angular momentum of a quasi-two-dimensional granular membrane is increased, it deforms from a circle to an ellipse, eventually pinching off into multiple smaller clusters. We use hydrodynamic models for rotating liquid drops to describe the granular dynamics and extract the droplet surface tension, and show that long-range acoustic forces and the anisotropy of the droplet give rise to a surface tension that scales with the droplet size. We show how this surface tension is modified by active fluctuations.