A remarkable series of experiments has recently been conducted by Couder and
co-workers. First, they demonstrated that a droplet is able to bounce indefinitely
without coalescing on the surface of a vertically vibrated liquid bath (Couder et al.
2005a). In certain regimes, a bouncing droplet moves laterally through its interaction
with its own wave field (Couder et al. 2005b; Proti`ere et al. 2005; Proti`ere, Boudaoud
& Couder 2006). As the droplet thus walks across the surface, its wave field probes
the surroundings, detecting solid obstacles that may be used to guide the droplets.
When many such walkers are present, they may interact to form stable orbits or
lattice structures (Lieber et al. 2007). More surprising yet, when a droplet passes