Diffraction of walking drops by a standing Faraday wave

TL;DR

This study demonstrates a diffraction-like phenomenon in pilot-wave hydrodynamics where droplets are deflected by a standing Faraday wave, revealing complex interactions and phase-dependent behaviors similar to quantum effects.

Contribution

It introduces the observation of diffraction patterns in hydrodynamic droplets influenced by standing waves, drawing parallels to the Kapitza-Dirac effect in quantum physics.

Findings

Droplets exhibit diffraction patterns in deflection angles.

Droplet speed and sorting depend on impact phase.

Differences and similarities with quantum Kapitza-Dirac effect are discussed.

Abstract

The Kapitza-Dirac effect is the diffraction of quantum particles by a standing wave of light. We here report an analogous phenomenon in pilot-wave hydrodynamics, wherein droplets walking across the surface of a vibrating liquid bath are deflected by a standing Faraday wave. We show that, in certain parameter regimes, the statistical distribution of the droplet deflection angles reveals a diffraction pattern reminiscent of that observed in the Kapitza-Dirac effect. Through experiments and simulations, we show that the diffraction pattern results from the complex interactions of the droplets with the standing wave. Our study highlights non-resonant effects associated with the detuning of the droplet bouncing and the bath vibration, which are shown to lead to drop speed variations and droplet sorting according to the droplet's phase of impact. We discuss the similarities and differences between our hydrodynamic system and the discrete and continuum interpretations of the Kapitza-Dirac effect, and introduce the notion of ponderomotive effects in pilot-wave hydrodynamics.