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.
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.
We also showed that when particles with facets are levitated, they preferentially attach by sharing a single edge, forming a granular hinge.
"Edges control clustering in levitated granular matter", Granular Matter 21: 77 (2019), link
"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)