Characterization of Coalescence-Induced Droplet Jumping Height on Hierarchical Superhydrophobic Surfaces

Coalescence-induced condensate droplet jumping from superhydrophobic surfaces can be exploited in condensation heat-transfer enhancement, imparting self-cleaning behavior to surfaces, anti-icing coatings, and other industrial uses. An intriguing application would exploit this phenomenon to achieve thermal rectification using a sealed vapor chamber with opposing superhydrophilic and superhydrophobic surfaces. During forward operation, continuous evaporation occurs from the heated superhydrophilic evaporator surface; self-propelled jumping returns the condensate droplets from the cooler superhydrophobic surface to the evaporator, allowing passive recirculation of the working fluid without a reliance on gravity or capillary wicking for fluid return. In reverse operation, when the superhydrophobic surface is heated, the absence of a mechanism for fluid return from the cooler superhydrophilic side to the evaporator restricts heat transport across the (low conductivity) vapor gap. The effectiveness of this jumping-droplet thermal diode can be enhanced by maximizing the distance between the opposing superhydrophobic and superhydrophilic surfaces. It is, therefore, important to investigate the height to which the condensate droplets jump from the superhydrophobic surfaces such that replenishment of liquid to the superhydrophilic surface is maintained. In this study, we systematically investigate the height to which the condensate droplets jump from the hierarchical superhydrophobic surfaces having truncated microcones coated with nanostructures. The condensate droplet jumping height increases with a decrease in microcone pitch. A general theoretical model that accounts for surface adhesion, line tension, and initial wetting states of multiple coalescing droplets of different radii is used to predict and explain the experimentally observed trend of droplet jumping height with surface roughness. The insights provided regarding the effect of surface topography on droplet jumping height are critical to the design of high-performance thermal diode devices.