Solid particles walking on a vibrating interface*

Since the discovery of walking droplets in 2005, an outstanding question has been whether solid particles can similarly break symmetry and spontaneously walk along a vibrating fluid interface, self-propelled by their own wave fields. Despite significant efforts motivated by fundamental and practical interests, the generalization of this walking symmetry breaking to solid spheres has remained elusive for over twenty years. Factors such as the particle's surface roughness and wettability affect the lubrication layer between the particle and the bath, while fluid properties alter the relative magnitudes of the bouncing and Faraday thresholds. Inadequate combinations of these properties may result in vertical bouncing dynamics that are drastically different from those of walking droplets, typically disrupting the period-doubling cascade essential for the particle to resonate with the underlying Faraday waves and thus acquire motility. We finally provide an answer to this long-standing problem by demonstrating that millimetric glass beads can indeed self-propel at the interface between two immiscible fluids subject to vertical oscillations. We characterize the particle self-propulsion over a range of particle sizes, forcing amplitudes, and frequencies. Unlike walking droplets, where the droplet and the bath are made of the same liquid and the surrounding air has a weak influence, in our two-liquid system, the particle's added mass, viscous dissipation during flight, and the density ratio between phases play a prominent role, which we discuss. Special attention is given to the rationalization of the experimental observations through a minimal theoretical model of particle-fluid interaction. We conclude by demonstrating a range of collective phenomena that suggest new directions in active granular matter with wave-mediated interactions.