Phase-controlled lateral forcing of droplets on a vibrating bath

Abstract

We introduce a method for controlling the lateral motion of droplets bouncing on a vibrated liquid bath by superimposing a small periodic rocking motion onto the conventional vertical forcing. We show experimentally that the lateral velocity varies linearly with the rocking amplitude $\alpha$ and sinusoidally with its phase $\varphi$ relative to the vertical forcing, while the response depends strongly on the droplet's vertical bouncing mode. A time-averaged model of a harmonic bouncer yields a time-averaged lateral force ${\overline{F}}_x=mg\alpha \sin ({\Phi }_b+\varphi )$, where ${\Phi }_b$ is the droplet impact phase. Periodic rocking thus provides a simple, programmable means of exerting lateral forces on bouncing droplets, enabling controlled manipulation of passive and active droplets in pilot-wave hydrodynamics.