A hydrodynamic counterpart to the Kapitza-Dirac effect.

An electron beam can undergo diffraction when interacting with a standing wave of light, a phenomenon commonly known as the Kapitza-Dirac effect. We explore an analog of this effect using pilot-wave hydrodynamics. A drop bouncing on a periodically-forced fluid bath can self-propel through a resonant interaction with its own wave field. We here report the results of an experimental study in which we sent such walking drops towards a standing Faraday wave, and recovered a distribution of the deflection angles reminiscent of the Kapitza-Dirac diffraction pattern. Our accompanying theoretical modeling illustrates that successfully replicating key aspects of the observed diffraction pattern required that we model explicitly the droplet's vertical dynamics, and so relax the often-made assumption of resonance between droplet and wave.