Mechanical Properties of Acoustically Levitated Granular Rafts

We investigate a model system for the rotational dynamics of inertial many-particle clustering, in which submillimeter objects are acoustically levitated in air. Driven by scattered sound, levitated grains selfassemble into a monolayer of particles, forming mesoscopic granular rafts with both an acoustic binding energy and a bending rigidity. Detuning the acoustic trap can give rise to stochastic forces and torques that impart angular momentum to levitated objects. As the angular momentum of a quasi-two-dimensional granular raft is increased, the raft deforms from a disk to an ellipse, eventually pinching off into multiple separate rafts, in a mechanism that resembles the breakup of a liquid drop. We extract the raft effective surface tension and elastic modulus and show that nonpairwise acoustic forces give rise to effective elastic moduli that scale with the raft size. We also show that the raft size controls the microstructural basis of plastic deformation, resulting in a transition from fracture to ductile failure.

Automated summary

We investigate a model system for the rotational dynamics of inertial many-particle clustering, where submillimeter objects are acoustically levitated in air. Driven by scattered sound, these levitated grains selfassemble into a monolayer of particles, forming mesoscopic granular rafts. Detuning the acoustic trap imparts angular momentum to the levitated objects, causing the rafts to deform and eventually break into separate rafts, resembling the breakup of a liquid drop.