Effect of repeated immersions and contamination on plastron stability in superhydrophobic surfaces

Development of superhydrophobic surfaces is of great interest for drag-reducing applications as air layers retained underwater greatly reduce fluidic drag. However, liquid flow over these surfaces can result in the collapse of the lubricating air layer. Here, we investigate the dynamic stability of retained air layers on three different superhydrophobic surfaces against repeated immersion and motion through various viscous liquids. The three surfaces investigated are a highly ordered polytetrafluoroethylene micropillar array, a two-level hierarchical random polycarbonate nanofur, and a double-scale hierarchical Teflon AF wrinkled surface. Both repeated immersions and contamination by viscous liquids accelerated the rate of plastron decay on the pillar array and the nanofur, while the Teflon wrinkles remained dry. Five topographical features were identified as correlated to a dynamically stable retained air layer, and a relation between these stability-enhancing parameters and the drag-reducing capabilities is found. Furthermore, resistance of superhydrophobic surfaces against contamination is studied and the directionality of the Cassie-to-Wenzel wetting transition on air-retaining surfaces is demonstrated. Together, an understanding of these properties allows for the rational design of new superhydrophobic surfaces fit for application.