Physics

Aerospace engineering

Meteorology

Cluster (spacecraft)

Granular matter

Granular material

Thermal fluctuations

Mechanics

Biology

Statistical physics

Engineering

Magnet

Active matter

Programming language

Range (aeronautics)

Thermal

Condensed matter physics

Computer science

Quantum mechanics

Acoustic levitation

Levitation

Cell biology

Classical mechanics

Nature Physics

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 shaking1–6. While depletion interactions drive clustering and assembly in colloids7–10, no equivalent short-range attractions exist between macroscopic grains. Here we overcome this limitation and investigate granular cluster formation by using acoustic levitation and trapping11–13. Scattered sound establishes short-range attractions between small particles14, while detuning the acoustic trap generates active fluctuations15. To illuminate the interplay between attractions and fluctuations, we investigate transitions among ground states of two-dimensional clusters composed of a few particles. Our main results, obtained using experiments and modelling, 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. Acoustically levitated granular particles offer a platform on which to study self-assembly in a macroscopic system. Precise acoustic tuning reveals how cluster statistics and assembly pathways change as the system moves out of equilibrium.

Cluster formation by acoustic forces and active fluctuations in levitated granular matter