Abstract:

It is well known that surface roughness has a very important effect on superhydrophobicity. The Wenzel and Cassie-Baxter models, which correspond to the homogeneous and heterogeneous wetting respectively, are currently primary instructions for designing superhydrophobic surfaces. However, the particular drop shape that a drop exhibits might depend on how it is formed. A water drop can occupy multiple equilibrium states, which relate to different local minimal energy. In some cases, both equilibrium states can even co-exist on a same substrate. Thus the apparent contact angles may vary and have different values. We discuss how the Wenzel and Cassie-Baxter equations determine the homogeneous and heterogeneous wetting theoretically. Contact angle analysis on hierarchical surface structure and contact angle hysteresis has been put specific attention. In particular, we study the energy barrier of transition from Cassie-Baxter state to Wenzel state, based on existing achievement by previous researchers, to determine the possibility of the transition and how it can be interpreted. It has been demonstrated that surface roughness and geometry will influence the energy required for a drop to get into equilibrium, no matter it is homogeneous or heterogeneous wetting.

Key words: superhydrophobic surface, contact angle, wetting transition, energy balance, biomimetics