Hierarchically structured re-entrant microstructures for superhydrophobic surfaces with extremely low hysteresis

This paper reports a new type of hierarchically structured surface consisting of re-entrant silicon micropillars with silicon nanowires atop for superhydrophobic surface with extremely low hysteresis. Re-entrant microstructures were fabricated on a silicon substrate through a customized one-mask microfabrication process while silicon nanopillars were created on the entire surface of microstructures, including sidewalls, by a metal-assisted-chemical etching process. The strategy of constructing hierarchical surfaces aims to reduce the actual contact area between liquid and top part of solid surface, thereby increasing the contact angle and reducing the sliding angle. The strategy of using re-entrant profile of the microstructure aims to prevent a liquid droplet from falling into cavities of roughened structures and decrease the actual contact area between the liquid droplet and sidewalls of solid structures, therefore reducing adhesion forces acting on the liquid droplet. Our measurement shows that the surface incorporating both hierarchical and re-entrant strategies exhibits a sliding angle as low as 0.5 degrees, much lower than sliding angles of surfaces only incorporating either one of the strategies.