Superhydrophobic surfaces that can selectively trap a drop based on temperature

A water drop will bounce on a surface if the surface is sufficiently superhydrophobic. The degree of superhydrophobicity can be tuned by modulating the chemistry and microstructure of the surface, thus enabling external control of whether a particular drop bounces or sticks. A challenge in these approaches is that they require separate sensing, processing, and actuating steps. Here we explore how one might design a smart superhydrophobic surface in which the surface can sense a property of the drop, here its temperature, and, if above a critical threshold, passively adjust its functionality so that it will capture the drop in the absence of external control. Specifically we model two potential mechanisms in which a superhydrophobic surface could trap a sufficiently hot drop within milliseconds: melting of microtextured wax and condensation of the vapor within the superhydrophobic texture. We then test these mechanisms through systematic drop impact experiments in which we independently vary the substrate and drop temperatures on a waxy superhydrophobic Nasturtium leaf. In this regime a critical temperature threshold for bouncing can be controlled by considering the relative timescales between condensation growth and drop residence time.