Acoustically Levitated Granular Matter

An intense ultrasound field can be used not only to levitate particles and manipulate their positions, but sound scattered off individual solid objects gives rise to tunable attractive forces that acoustically bind the particles into larger aggregates. For levitation in air, the small viscosity then makes it possible to explore the regime of under damped far-from-equilibrium dynamics in a strongly coupled multi-particle system. I will discuss recent experiments that exploit acoustic levitation to self-assemble small particles into mono layer rafts freely floating in air, while tracking their dynamics with high-speed video imaging. By changing the inter particle spacing and controlling the acoustic energy density, the rafts can transform from close-packed solids into soft lattices that can ‘melt’ into 2D liquids and eventually expand into 3D particle swarms. Along the way, acoustically levitated granular matter provides an exciting new platform to study the nature of instabilities induced by many-body hydrodynamic interactions.