Spontaneous motion of a volatile droplet on a warm substrate

When a volatile droplet is deposited on a uniformly heated wettable substrate, one expects to see it spread into a thin film and evaporate at an enhanced rate. However, we demonstrate that at higher substrate temperatures, still below the boiling temperature of the liquid, the deposited droplet starts to contract. Even to that, above a threshold temperature, the droplet surprisingly starts to spontaneously move. We describe and quantify the contraction and the self-propulsion, both arising due to thermocapillary convection induced by a gradient in temperature along the droplet interface. We show that the relative thermal conductivity of the substrate and liquid plays a crucial role in determining the directionality of the themo-capillary flow. Finally, we provide a scaling law that captures the dependence of the moving speed of the droplet on the dynamic contact angle that changes with substrate temperature.