Acoustically Levitated Granular Matter: From Meso-Scale Particle Assembly to Tabletop Asteroids*

Granular matter can serve as a prototype for exploring the rich physics of many-body systems driven far from equilibrium. This talk will outline a new frontier for granular physics with macroscopic particles, where acoustic levitation compensates the forces due to gravity and eliminates frictional interactions with supporting surfaces in order to focus on particle interactions. Levitating small particles by intense ultrasound fields in air makes it possible to manipulate and control their positions and assemble them into larger aggregates. Furthermore, sound scattered off individual, levitated solid particles gives rise to tunable attractive forces among neighboring particles. The small air viscosity implies that a regime of complex, underdamped dynamics can be explored where inertial effects are important, and at the same time hydrodynamic instabilities can induce active fluctuations. I will discuss recent work that exploits acoustic levitation to self-assemble small particles, 10s to 100s of microns in diameter, into freely floating rafts and track their interactions with high-speed video imaging. These rafts can be manipulated by external forcing into regimes of extreme plastic deformation or rotated rapidly to the point of breakup similar to rubble pile asteroids [1]. By changing the interparticle spacing and controlling the energy density in the acoustic cavity, the rafts can transform from close-packed solids into extremely soft lattices that exhibit intermittency and dynamic heterogeneity [2], and 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 hydrodynamic coupling in multi-particle systems.