Wetting Behavior of a Surface with Dual-Scale Structures.

Theoretical and numerical studies were conducted to investigate the transitional interpillar spacing for dual-scale structures, where wetting transition between the Wenzel and Cassie-Baxter states occurs in the primary and secondary pillars. A theoretical formula was derived for the transitional interpillar spacing based on the continuum picture of water. Molecular dynamics (MD) simulations were carried out by varying the interpillar spacing for the primary pillars for single- and dual-scale structures with various pillar heights. The results obtained from the theoretical formula agreed reasonably well with the results obtained from MD simulations, especially when the primary pillar height was relatively high. The transitional interpillar spacing increases as the pillar height and the number of secondary pillars increase. The effect of the secondary pillars on the transitional interpillar spacing was also evaluated using the difference in the grand potentials between the Wenzel and Cassie-Baxter states. These results show that the dual-scale structures increase the transitional interpillar spacing with an increase in the surface hydrophobicity.