Mechanical properties of acoustically levitated granular rafts

TL;DR

This study explores the mechanical behavior of acoustically levitated granular rafts, revealing their elastic properties, deformation mechanisms, and failure modes, which resemble liquid drop dynamics and are influenced by acoustic forces.

Contribution

It introduces a model system for levitated granular rafts, quantifies their surface tension and elastic modulus, and demonstrates size-dependent deformation and failure behaviors.

Findings

Rafts exhibit measurable surface tension and elastic modulus.

Deformation transitions from fracture to ductile failure with size.

Non-pairwise acoustic forces influence elastic properties.

Abstract

We investigate a model system for the rotational dynamics of inertial many-particle clustering, in which sub-millimeter objects are acoustically levitated in air. Driven by scattered sound, levitated grains self-assemble 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 break-up of a liquid drop. We extract the raft effective surface tension and elastic modulus, and show that non-pairwise 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.