Granular Dynamics in Spectral Acoustic Holograms*

Acoustic traps use forces exerted by structured sound waves to transport granular materials along specified trajectories in three dimensions. The structure of the acoustic force landscape is governed by the amplitude and phase profiles of the sound's pressure wave. These profiles can be controlled through deliberate spatial modulation of monochromatic waves, by analogy to holographic optical trapping. Alternatively, spatial and temporal control can be achieved by interfering a small number of sound waves at multiple stationary frequencies to create acoustic holograms based on spectral content. We demonstrate spectral holographic trapping by projecting acoustic conveyor beams that move millimeter-scale objects along prescribed paths. Spectrally-rich sound waves projected by just two sources can implement a superposition of stationary and dynamic force landscapes whose Moire interaction drives trapped particles through dynamical states analogous to those in sliding charge density waves and the recently-discovered wave-driven oscillator.