Self-excited motions of volatile drops on swellable sheets

TL;DR

This paper investigates the complex self-excited motions of volatile droplets on swellable sheets, revealing how swelling, buckling, and evaporation lead to pulsatile, oscillatory droplet movements through a combined experimental and theoretical approach.

Contribution

It introduces a novel model of droplet motion driven by nonequilibrium swelling and evaporation, providing a phase diagram and scaling laws for droplet excitability.

Findings

Droplet motion exhibits pulsatile spinning and sliding behaviors.

A phase diagram maps droplet excitability based on size and film thickness.

Scaling laws describe the droplet's motion dynamics.

Abstract

When a volatile droplet is deposited on a floating swellable sheet, it becomes asymmetric, lobed and mobile. We describe and quantify this phenomena that involves nonequilibrium swelling, evaporation and motion, working together to realize a self-excitable spatially extended oscillator. Solvent penetration causes the film to swell locally and eventually buckle, changing its shape and the drop responds by moving. Simultaneously, solvent evaporation from the swollen film causes it to regain its shape once the droplet has moved away. The process repeats and leads to complex pulsatile spinning and/or sliding movements. We use a one-dimensional experiment to highlight the slow swelling of and evaporation from the film and the fast motion of the drop, a characteristic of excitable systems. Finally, we provide a phase diagram for droplet excitability as a function of drop size and film thickness and scaling laws for the motion of the droplet.