A 3-D model for thermodynamic analysis of hierarchical structured superhydrophobic surfaces

Abstract

Lotus leaves and rose petals are the two typical natural examples with superhydrophobicity, also known as the lotus effect and petal effect, respectively. It is generally recognized that the hierarchical structures on their surfaces are responsible for the lotus effect and petal effect. However, the relations between surface geometries and the corresponding wetting states have not been understood completely. In this paper, a 3-D model is built to thermodynamically analyze the wetting behavior of hierarchical structured surfaces. The free energy (FE) and free energy barrier (FEB) of different wetting states are analyzed in detail. Furthermore, equilibrium contact angle (CA), receding and advancing CAs, as well as contact angle hysteresis (CAH) can be predicted by this approach. More importantly, the critical Nano pillar height used for characterizing the transitions between the two wetting states is clearly defined in terms of geometric parameters of solid surfaces, and the final stable wetting state dominated by surface roughness and chemical composition can be determined based on the obtained results. The approach presented in this study can also be used as a guidance for fabricating superhydrophobic surfaces.