Synergistic effect of micro- and nano-structure for superhydrophobic surfaces

Superhydrophobic surfaces have special surface effects and have broad application prospects in many fields. Hierarchical structures by combining microstructures with nanostructures are the key factor for achieving robust superhydrophobicity on solid surfaces. However, the synergistic effect of micro- and nano-structure has rarely been studied. In particular, it is still unclear how the existence of nanostructures affects the design of microstructures. The main focus of this paper is as follows: Firstly, the force balance model was established according to the relationship between hydrostatic pressure (Δph), Laplace pressure (ΔpL), and capillary pressure (Δpc) in the two-stage structure, and the formation conditions of the four wetting states are determined. Secondly, the theory predicted that the nanostructure has a significant effect on improving the stability of the Cassie wetting state compared with the micron structure. Finally, the influence of structure parameters on wettability was discussed. The results show that when the solid-liquid contact area fraction fn of the nanostructure is 0.25, 0.4, 0.6, 0.8, and 1, respectively, the smaller the fn is, the larger the value range of the superhydrophobic first-order micron structure is. At the same time, the apparent contact angle θ is larger. Under the same nanostructure, the smaller the solid-liquid contact area fraction fm of the micron structure, the larger the θ. These findings strengthen our fundamental understanding of superhydrophobic surfaces and are helpful for their practical application. [Display omitted]