Freezing an acoustically levitated granular liquid

Phase transitions far from equilibrium are behind many diverse and intriguing phenomena in both nature and the laboratory. Here we present a study of an out-of-equilibrium phase transition for a membrane of acoustically levitated particles, 30 microns in diameter. This quasi-2D membrane is a dissipative but driven granular system; the energy density of the acoustic cavity plays the role of an effective temperature and determines the fluctuations in the system. Furthermore, the scattered sound between such small particles establishes interactions that are repulsive at a close approach but become attractive upon contact. Underdamped, collisional dynamics in this complex potential landscape give rise to two kinetically distinct states, which we call “liquid” and “solid”. In the “liquid” state, particles freely diffuse throughout the membrane. In the “solid” state, particles interconnect to form a kinetically arrested chain-like structure. The structures formed after the transition can be tuned by adjusting the rate of change of the effective temperature: as the effective temperature is decreased more slowly, particles assemble into a lacey structure consisting of many open loops.