Mechanisms of superhydrophobicity on hydrophilic substrates

Abstract

Surface microstructures of solids play a significant role in producing superhydrophobicsurfaces. In the present paper, the Cassie–Baxter and Wenzel models on a rough substrateare examined from the viewpoints of geometry and energy. The result shows that if theair beneath a droplet on a sinusoidal substrate is open to the atmosphere, thesuperhydrophobic state can exist only when the substrate is hydrophobic, and that thegeometric parameters of the microstructure have a great influence on the wetting behavior.Two mechanisms that may lead to a superhydrophobic property from a hydrophilicsubstrate are addressed. Firstly, for closed or airproof microstructures (e.g. honeycombstructures), a negative Laplace pressure difference caused by the trapped air under thedrop can keep the balance of the liquid/vapor interface. Secondly, some specialtopologies of surface structures satisfying a certain geometric condition may alsolead to the formation of a Cassie–Baxter state even if the microstructures areopen to the air. Therefore, some surface morphologies may be designed to obtainsuperhydrophobic properties on hydrophilic surfaces. The present study is also helpful tounderstand some superhydrophobic phenomena observed in experiments and in nature.