Spontaneous transition of a water droplet from the Wenzel state to the Cassie state: a molecular dynamics simulation study

Abstract

It is widely accepted that the superhydrophobic state is attributed to the formation of the Cassie state. The Cassie state is mostly metastable, which can be turned into the Wenzel state. Therefore, the superhydrophobic state is generally considered to be unstable. In this study, the wetting behaviors of a water droplet on different pillar surfaces are simulated. The spontaneous transition from the Wenzel state to the Cassie state is achieved, which is significant for the stable existence of superhydrophobicity. The transition process is analyzed in detail and can be chronologically divided into two stages: the contact area decreases and the water droplet rises. Moreover, the transition mechanism is studied, which is due to the combined effect of the surrounding pillars and the central pillar. The surrounding pillars form a no-wetting gap under the droplet, and the central pillar forces the droplet to move upward. Furthermore, three parameters that may influence the transition are studied: the pillar height, the droplet size and the hollow size.