Droplet dynamics on heated superhydrophobic substrates: Cassie-Wenzel transition to lift-off

Abstract

A water droplet on a textured superhydrophobic substrate can either rest on top of the pillars (Cassie state) or impale the surface textures (Wenzel state). Here, we report on the Cassie to Wenzel transition and associated dynamics of a water droplet on heated superhydrophobic substrates. Below the saturation temperature of the liquid, the Cassie to Wenzel transition of the droplet occurs at a specific droplet volume which is dependent on the surface morphology and temperature. Near the Leidenfrost temperature of the superhydrophobic substrate (140–170 °C), partial impalement into the textures and accompanying increased vapor pressure leads to an explosive out-of-plane lift-off behavior of the droplet. The substrate morphology affects the lubrication pressure due to vapor flow underneath the droplet which dictates the lift-off volume. In addition, the detachment of the droplet from the substrate is also observed to be caused by local bubble nucleation and resulting capillary wave along the liquid–vapor interface. We use the pressure-based analytical transition criteria to predict the volume of drop corresponding to Cassie-Wenzel transition for temperatures lower than the saturation temperature and that for the out-of-plane lift-off at higher temperatures. The predictions agree reasonably well with the experimental observation over the entire range of substrate temperatures and for different surface morphology.