Using Sound to Levitate and Activate 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 forces that acoustically bind the particles into larger aggregates [1]. For levitation in air, furthermore, the small viscosity makes it possible to explore the regime of underdamped dynamics in a driven, strongly coupled multi-particle system. This talk will discuss recent experiments that exploit acoustic levitation to self-assemble small particles, from a few tens of micrometers to a few hundred micrometers in diameter, into monolayer rafts freely floating in air, while tracking individual particle movement with high-speed video imaging. By controlling the particle size and the sound energy density, rafts can be transformed from close-packed solids into soft lattices that ‘melt’ into 2D liquids. Mediated by the fluid (air) surrounding the particles, non-reciprocal multibody forces due to scattered sound as well as microstreaming give rise to emergent collective dynamics that activates the particles. In small particle clusters this can lead to self-propulsion or internal engine-like motion, and in larger rafts produces unusual lattice vibrations.