Deformation of a soft, acoustically-bound granular solid under applied stress

Mesoscale particles can be levitated in an ultrasonic standing wave, in which acoustic scattering produces attractive interparticle forces that cause these particles to self-assemble into quasi-two-dimensional rafts. Previously, indirect methods have been used to investigate mechanical properties of this cohesive granular material by modeling a stochastically rotating raft as a liquid droplet.[1] This material, however, can also be viewed as a soft, semicrystalline solid. By using custom probes to carefully modify the acoustic field near a sample, we perform micromechanical tests on this material, such as tensile testing. We observe millimeter-sized rafts deform under applied stresses of just tens of Pa. We measure forces on the μN scale with a thin cantilever and can examine elastic and plastic deformation at the scale of individual particles. Prior work on this athermal system has found that the material's properties can depend significantly on sample size.[1] This implies that the binding potential exhibits many-body effects, a phenomenon which is investigated here through direct mechanical testing.