Self-Assembling Matter with Sound

Ultrasound can levitate particles and create tunable interactions through sound scattered by nearby objects. In air, low viscosity permits underdamped collective dynamics far from equilibrium. Heinrich Jaeger discusses experiments that assemble small particles into freely suspended clusters and monolayer rafts while tracking their motion with high-speed video. Changing particle separation and acoustic energy transforms close-packed solids into soft lattices and unusual two-dimensional liquids. These experiments provide a platform for investigating non-reciprocal many-body forces and the collective behaviour they produce.