Capillary surfers: wave-driven particles at a fluid interface.

Active systems have recently attracted considerable interest for the possibility of extending statistical mechanics to incorporate non-equilibrium phenomena. Among active systems, self-propelled particles are natural or artificial objects that convert energy from the environment into directed motion. Vibrating platforms are suitable sources of diffuse energy to observe macroscopic self-propelled particles and their collective behaviour with table-top experiments. Here we introduce capillary surfers: solid particles propelled by their self-generated wave field on the surface of a vibrating liquid bath. The surfer speed and its interaction with the environment can be tuned through the particle shape, mass, fluid properties and the vibration parameters. The wave nature of the interactions among surfers allows for multistability of states and promises a number of novel collective behaviours. Capillary surfers benefit from the interplay between dissipation and inertia manifesting as a field of unsteady surface waves, and thus have the potential to fill the gap between dissipation- and inertia-dominated active systems.