Ultrasonic 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 introduce an experimental approach based on acoustic levitation and trapping of granular matter.

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

To illuminate the interplay between attractions and fluctuations, we investigate transitions among ground states of two-dimensional clusters composed of a few particles.

Experiments and modeling reveal that, in contrast to thermal colloids, in non-equilibrium granular ensembles the magnitude of active fluctuations controls not only the assembly rates but also their assembly pathways and ground-state statistics.

These results open up new possibilities for non-invasively manipulating macroscopic particles, tuning their interactions, and directing their assembly.

Publication: "Cluster formation by acoustic forces and active fluctuations in levitated granular matter", Nature Physics 15, 460–464 (2019) , link, associated News & Views, UChicago press release

Supplementary movies available here: (1), (2), (3), and (4)