Acoustic Levitation

Mechanically agitated granular matter often serves as a prototype for exploring the rich physics associated with hard sphere systems, with an effective temperature introduced by vibrating or shaking. While depletion interactions drive clustering and assembly in colloids, no equivalent short-range attractions exist between macroscopic grains. To overcome this limitation, we acoustically levitate grains in air.

Scattered sound establishes short-range attractions between small particles, while detuning the acoustic trap generates active fluctuations. 

We showed that in small clusters, the magnitude of active fluctuations controls not only the assembly rates but also their assembly pathways and ground state statistics. Intriguingly, when particles with facets are levitated, they preferentially attach by sharing a single edge, forming a granular hinge. 

As the levitating particles are made smaller, new physics enters, with short-range repulsive forces that arise from the oscillation of the viscous fluid medium surrounding the particles (even air has some viscosity).  The result is a rich set of complex dynamical behaviors.

The following paper is a review of work in the emerging field of acoustically levitated multi-body structures and dynamics:

Melody X. Lim, Bryan VanSaders, and Heinrich M. Jaeger, "Acoustic manipulation of multi-body structures and dynamics", Reports on Progress in Physics 87, 064601 (2024). link