Probing many-body effects in the shear response of acoustically levitated granular rafts

Cohesive granular materials are found throughout nature and industry, such as very fine powders and agglomerations of dust in space. These materials typically have short-ranged, pairwise-additive interactions. When granular particles are levitated in an ultrasound field, they experience longer-ranged, anisotropic acoustic forces; this experimental system enables the study of a unique granular material with tunable cohesion strength. Acoustically levitated particles tend to self-assemble into two-dimensional raft structures. We utilize custom probes to perform mechanical shear tests on this soft solid, applying and measuring acoustic forces in situ with sub-μN precision. Our results indicate that the work required to shear a levitated raft appears to scale with raft size significantly more than we would expect for other granular materials. This allows for investigation into how acoustic interactions result in many-body effects in structures composed of granular particles.