Walking and orbiting droplets

Small drops can bounce indefinitely on a bath of the same liquid if the container is oscillated vertically at a sufficiently high acceleration1. Here we show that bouncing droplets can be made to ‘walk’ at constant horizontal velocity on the liquid surface by increasing this acceleration. This transition yields a new type of localized state2,3,4,5 with particle–wave duality: surface capillary waves emanate from a bouncing drop, which self-propels by interaction with its own wave and becomes a walker. When two walkers come close, they interact through their waves and this ‘collision’ may cause the two walkers to orbit around each other6,7,8.