Nonreciprocal interactions between acoustically levitated granular particles lead to liquidlike droplets

Granular particles can be levitated against gravity in an acoustic resonance cavity, where they self-assemble into two-dimensional rafts as a result of anisotropic, attractive interparticle forces. If particles are near in size to the viscous boundary layer that forms around particle surfaces, attractive forces balance with repulsive forces that arise from microstreaming flows. When considered as interparticle forces, these repulsive interactions can be nonconservative, non-pairwise-additive, and nonreciprocal even for collections of identical particles. The nonreciprocal forces in this underdamped system lead to continuous hydrodynamic fluctuations that act as an effective temperature which we control via the sound volume. Rafts of these particles become very active and increasingly disordered at higher effective temperatures. This particle motion is fundamentally athermal, yet the bulk displays behavior that qualitatively resembles a molecular liquid. We perturb the system with microscale probes to study its behavior, observing breakup and deformation at the scale of individual particles, as well as analyzing how "thermal" this acoustically-driven liquidlike phase is.