Active volatile drops on liquid baths

TL;DR

This paper demonstrates how a volatile drop can self-propel on a liquid surface due to thermocapillary stresses caused by evaporation, with a theoretical model matching experimental observations.

Contribution

It introduces a new mechanism of active drop propulsion driven by thermal effects, expanding understanding beyond solutal-driven active drops.

Findings

Drop speed depends on evaporation flux.

Thermocapillary stresses induce self-propulsion.

Theoretical model aligns with experimental data.

Abstract

In most experimental studies, active drops propel in a liquid bulk due to self-generated interfacial stresses of solutal origin. Here, we demonstrate the self-propulsion of a volatile drop on the surface of a liquid bath due to stresses of thermal origin. Evaporative heat pumping is converted into directed motion driven by thermocapillary stresses, which emerge on the drop surface as a result of a symmetry breaking of the drop temperature field. The dependence of the drop speed on the activity source, i.e. the evaporation flux, is derived with scaling arguments and captures the experimental data.