Non-wetting droplets on hot superhydrophilic surfaces.

Controlling wettability by varying surface chemistry and roughness or by applying external stimuli is of interest for a wide range of applications including microfluidics, drag reduction, self-cleaning, water harvesting, anti-corrosion, anti-fogging, anti-icing and thermal management. It has been well known that droplets on textured hydrophilic, that is superhydrophilic, surfaces form thin films with near-zero contact angles. Here we report an unexpected behaviour where non-wetting droplets are formed by slightly heating superhydrophilic microstructured surfaces beyond the saturation temperature (>5 °C). Although such behaviour is generally not expected on superhydrophilic surfaces, an evaporation-induced pressure in the structured region prevents wetting. In particular, the increased thermal conductivity and decreased vapour permeability of the structured region allows this behaviour to be observed at such low temperatures. This phenomenon is distinct from the widely researched Leidenfrost and offers an expanded parametric space for fabricating surfaces with desired temperature-dependent wettability.