Snap evaporation of droplets on smooth topographies

The evaporation of droplets on solid surfaces is important for a broad range of applications, including ink-jet printing and surface cooling. Despite its apparent simplicity, the precise configuration of an evaporating droplet on a solid surface has proven notoriously difficult to predict and control. This is because droplet evaporation typically proceeds as a `stick-slip' sequence (a combination of pinning and de-pinning events) caused by microscopic structure of the solid surface. Here we show how smooth, pinning-free, solid surfaces of non-planar topography give rise to a different process called snap evaporation. During snap evaporation the morphology of an evaporating droplet follows a reproducible sequence of steps, where the liquid-gas interface is quasi-statically reduced by mass diffusion until it undergoes an out-of-equilibrium snap. Experimentally, we demonstrate this process using a water droplet evaporating on a wavy ultra-smooth lubricant-infused surface. Mathematically, we use full hydrodynamics lattice-Boltzmann simulations, and a model based on bifurcation theory that reveals the points where snap events are triggered, which obey a strict hierarchy dictated by the underlying surface topography.