Behavior of self-propelled acetone droplets in a Leidenfrost state on liquid substrates

TL;DR

This paper investigates the behavior of self-propelled acetone droplets in a Leidenfrost state on liquid substrates, revealing mechanisms of non-coalescence, oscillation, bouncing, and self-propulsion, with implications for understanding droplet dynamics on liquids.

Contribution

It introduces experimental observations and a theoretical model for acetone droplets on liquid substrates, including a new strategy to estimate vapor layer thickness and insights into droplet self-propulsion and immersion.

Findings

Droplets can hover and exhibit damped oscillations or bounce depending on impact velocity.

Self-propelled droplets follow straight-line trajectories due to Marangoni effects.

Droplets can become immersed beneath the water surface, affecting drag and dynamics.

Abstract

It is demonstrated that non-coalescent droplets of acetone can be formed on liquid substrates. The fluid flows around and in an acetone droplet hovering on water are recorded to shed light on the mechanisms which might lead to non-coalescence. For sufficiently low impact velocities, droplets undergo a damped oscillation on the surface of the liquid substrate but at higher velocities clean bounce-off occurs. Comparisons of experimentally observed static configurations of floating droplets to predictions from a theoretical model for a small non-wetting rigid sphere resting on a liquid substrate are made and a tentative strategy for determining the thickness of the vapor layer under a small droplet on a liquid is proposed. This strategy is based on the notion of effective surface tension. The droplets show self-propulsion in straight line trajectories in a manner which can be ascribed to a Marangoni effect. Surprisingly, self-propelled droplets can become immersed beneath the undisturbed water surface. This phenomenon is reasoned to be drag-inducing and might provide a basis for refining observations in previous work.