Coalescence-induced droplet jumping on atmospheric-mediated superhydrophobic surfaces

Coalescence-induced droplet jumping has received much attention over the past decade due to its ability to passively remove microscale droplets thereby enhancing condensation heat transfer, anti-icing, self-cleaning, and energy harvesting performance. However, droplet-jumping relies on surface superhydrophobicity, which results from the joint contributions of surface roughness and low-surface-energy conformal coatings such as alkyl and perfluorinated molecules. In spite of fantastic laboratory scale demonstrations, jumping-droplet surfaces fail to gain traction in real-life applications due to poor durability of the low surface energy coatings required to achieve superhydrophobicity. Here, we demonstrate that by exposing rationally designed intrinsically hydrophilic copper-based hierarchically structured CuO surfaces to ambient air, robust superhydrophobicity enabling coalescence-induced droplet jumping can be achieved. The as-prepared CuO surfaces experienced a transition from superhydrophilic to superhydrophobic with final apparent advancing contact angle and roll-off angle of >160° and <10°, respectively. X-ray photoelectron spectroscopy (XPS) confirmed that the wettability transition from wetting to non-wetting arises due to adsorption of airborne volatile organic compounds (VOCs) on the high-aspect-ratio and high-surface-area nanostructures. Due to the permanent and reliable source of VOCs in ambient air, the superhydrophobicity was shown to be retrievable after organic solvent and plasma cleaning. Most importantly, high-speed optical microscopy revealed the presence of stable coalescence-induced droplet jumping during atmospheric water vapor condensation. Our work not only promises an economic and facile way of fabricating superhydrophobic surfaces without the need for application of low-surface-energy chemistries, it also develops a platform for the development of next-generation durable superhydrophobic surfaces that can self-heal in the presence of ambient air.