Sculpting Matter with Sound: From Tunable Interactions to Rotational Dynamics in Acoustically Levitated Systems

Acoustic levitation provides a unique platform for studying matter far from equilibrium without the constraints of gravity or container walls. In typical standing-wave levitators, attractive acoustic scattering forces drive particles to merge into dense aggregates. We expand the accessible assembly space by introducing controlled electrostatic charging, superimposing Coulombic repulsion on acoustic attraction. This approach yields diverse assemblies ranging from compact clusters to fully separated configurations and hybrid states that combine both components. We observe a wide spectrum of collective dynamics, most notably synchronized particle oscillations induced by the spontaneous rotation of selected compact clusters. To uncover the mechanism of this selective rotation, we fabricate nanoprinted chiral spinner particles with precisely defined geometries. We find that chirality sets the rotation direction, while the number of spinner arms controls the rotation speed. Beyond single-particle effects, we demonstrate that nonreciprocal forces can also induce rotation in many-particle systems: a symmetric, electrostatically expanded triangular assembly remains stationary, but introducing asymmetry in a single particle spins up the entire structure. Our results show how microscopic asymmetry and nonreciprocal acoustic interactions generate torque and drive macroscopic collective motion---pointing toward programmable, reconfigurable, acoustically driven micromachines.