What Happened when a Superhydrophobic Surface was Immersed in Water? A Study by Optical Transmission Microscopy

Here we present a simple and fast optical transmission microscopy study on the wetting transitions on hierarchical polymer surfaces immersed in water. We analyze the influence of immersion time and the liquid pressure on wetting states of individual micro-cavities on these surfaces, as well as the lifespan of their superhydrophobicity. We show that transitions between the three wetting states (Cassie, Cassieimpregnating, and Wenzel) occur at a certain pressure threshold. Below this threshold, the transitions between the Cassie and the Cassie-impregnating states are reversible, while above the threshold, irreversible transitions to the Wenzel state start to occur. The transitions between the different wetting states can be explained by taking into account both the Young-Laplace equation for the water menisci in the cavities and the diffusion of dissolved gas molecules in the water. In addition, the wetting transitions had a stochastic nature, which may result from the diffusion of dissolved gas molecules in the water between neighboring cavities. Further, we compared the contact angle properties of two polymeric materials (COC and PP) with moderate hydrophobicity. We attributed the different water repellent properties of the two materials to a difference in the wetting of their nanostructures. The experimental observations indicate that both the diffusion of gas molecules in water, and the geometry of nanostructures influence the sustainability of superhydrophobicity of surfaces under water, understanding these factors can help improve the structural design of superhydrophobic surfaces.