Intermediate wetting state at nano/microstructured surfaces.

A general partial wetting model to describe an intermediate wetting state is proposed in this study to explain the deviations between the experimental results and classical theoretical wetting models for hydrophobic surfaces. We derived a theoretical partial wetting model for the static intermediate wetting state based on the thermodynamic energy minimization method. The contact angle based on the partial wetting model is a function of structural parameters and effective wetting ratio f, which agrees with the classical Wenzel and Cassie-Baxter models at f = 1 and 0, respectively. Si samples including porous surfaces, patterned surfaces and hierarchical nano/microstructured surfaces were prepared experimentally, having the same chemical composition but different physical morphology. We found that the experimental water contact angles deviate significantly from the classical Wenzel and Cassie-Baxter models but show good agreement with the proposed partial wetting model.