Wetting Transition from Wenzel to Cassie States: Thermodynamic Analysis

Superhydrophobicity is closely linked to the chemical composition and geometric characteristics of surface roughness. Building on our structural studies on water and air-water interfaces, this work aims to elucidate the mechanism underlying the wetting transition from the Wenzel to the Cassie state on a hydrophobic surface. In the Wenzel state, the grooves are filled with water, meaning that the surface roughness becomes embedded in the liquid. To evaluate the effects of surface roughness on water structure, a wetting parameter (W_Roughness) is proposed, which is closely related to the geometric characteristics of roughness, such as pillar size, width, and height. During the wetting transition from Wenzel to Cassie states, the critical wetting parameter (W_Roughness,c) may be expected, which corresponds to the critical pillar size (a_c), width (w_c), and height (h_c). The Cassie state is expected when the W_Roughness is less than W_Roughness,c (<W_Roughness,c), which can be achieved by altering the geometric characteristics of the roughness, such as increasing pillar size (>a_c), decreasing width (<w_c), or increasing height (>h_c). Additionally, molecular dynamic (MD) simulations are conducted to demonstrate the effects of surface roughness on superhydrophobicity.