Mechanisms of superhydrophobicity on hydrophilic substrates

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