Experimental Investigation of Freely Interacting Walking Droplets

Droplets may walk along the surface of a vertically vibrating fluid bath through resonant interaction with their self-excited wavefield. This macroscopic system, which exhibits dual wave-particle behaviors, has given rise to a range of Hydrodynamic Quantum Analogs that push the boundaries of classical mechanics. However, the experimental exploration of quantum many-body analogs has been hindered by droplet coalescence upon collision. Here, we demonstrate that freely interacting walking droplets can be sustained experimentally by increasing ambient pressure, which widens the range of Weber numbers over which coalescence is prevented. Moreover, because walking droplets bounce subharmonically relative to the bath oscillations, they may chaotically switch out of phase with one another, leading to fundamentally distinct collective phenomena due to wave interference. For experiments requiring vertical phase synchronization, we thus draw from previous 'superwalker' studies to show that a second driving frequency may synchronize all droplets' vertical phase. More broadly, our experimental platform opens new avenues for exploring hydrodynamic analogs of quantum many-body systems, as well as collective phenomena in wave-dressed active matter.