Importance of moderate size of pillars and dual-scale structures for stable superhydrophobic surfaces: A molecular dynamics simulation study

Abstract

In this work, we first create three-dimensional molecular-dynamics (MD) model to study the effect of structure parameters including pillar size, pillar height, pillar groove, and intrinsic contact angle (θe, or Young’s angle) on wetting behavior of drops on the one-scale nanopillared surface. We find that drops tend to exhibit the Cassie state on the nanopillared surface with small pillar size and large θe. Considering comprehensively the stability of the Cassie state, excellent hydrophobicity, and mechanical durability of microstructures, the pillar height and the pillar groove should be moderate. Then, only considering the wettability, we could see the dual-scale structure as the one-scale structure with large θe* that results from the small-scale structure for increasing the θe and cannot be achieved on the smooth surface in experiments. Importantly, we observe that the dual-scale structure, corresponding to large θe* resulting from the small-scale structure for increasing the θe, are important for the formation and the stability of the Cassie state. Moreover, the dual-scale structure can prevent the transition of drops from the Cassie state to the Wenzel state. Interestingly, fabricating the small-scale structure on top of the large-scale structure (corresponding to large θe* of the one-scale structure) can facilitate the transformation of the Wenzel state of drops on the large-scale structure to the Cassie state, and the Cassie state is stable. This study provides an inspiration for the fabrication of stable artificial superhydrophobic surfaces.