Hydrodynamic Coupling Melts Acoustically Levitated Crystalline Rafts

Particles interacting hydrodynamically often exhibit chaotic trajectories, even for as few as three particles. Such interactions can agitate the degrees of freedom of the particles, producing configuration dependent athermal fluctuations. Acoustic levitation, where granular particles are levitated with intense ultrasound, is an ideal platform to study such systems. Here, we acoustically levitate and assemble a granular raft, where particles form an open lattice with tunable spacing. Oscillating air flows between neighboring particles in the levitated raft, establishing a hydrodynamic coupling even when particles are at rest. The hydrodynamic coupling gives rise to spontaneous excitations in the lattice, in turn driving intermittent particle rearrangements. Under the action of these fluctuations, the raft transitions from a predominantly quiescent, crystalline structure, to a two-dimensional liquid-like state. We show that this transition is characterized by dynamic heterogeneity and intermittency, as well as cooperative particle movements, that produce an effectively `cageless' crystal. These findings shed light on fluid-coupling driven excitations that are difficult to isolate and control in many other hydrodynamic systems.