Complex Acoustic Fields for Driving Levitated Granular Media

Granular systems are inherently nonequilibrium due to strong dissipative effects. When granular systems are driven by external forces which compensate for this dissipation, the behavior of the grains can resemble equilibrium atomic or molecular systems. However, these driven granular materials also exhibit behaviors that are uniquely nonequilibrium. Here, we investigate a model system for driven granular media with attractive forces: sub-millimeter objects acoustically levitated in air. For unsteady acoustic fields with nontrivial mode shapes, levitated objects experience complex driving forces and torques. Furthermore, multiple levitated objects have secondary scattering interactions, driving aggregation into a membrane-like monolayer of close-packed grains. We apply the Lattice Boltzmann Method (LBM) to conduct direct numerical simulations of levitated objects, which permits investigation of fluid-structure interactions, including momentum transfer by acoustic wave scattering and viscous dissipation. Acoustic fields which carry angular momentum, as well as non-spherical levitated objects are investigated.