Dewetting Transitions on Superhydrophobic Surfaces: When Are Wenzel Drops Reversible?

On superhydrophobic surfaces, drops in the Wenzel state can be switched to the suspended Cassie state in some cases but in other cases are irreversibly impaled in the surface roughness. To date, the question of when dewetting transitions are possible for Wenzel drops has not been resolved. Here, we show that pinned Wenzel drops being stretched out-of-plane cannot reduce their contact angle below a critical value where unstable pinch-off occurs, preventing dewetting for Wenzel drops that exhibit receding contact angles beneath this critical pinch-off angle. Dewetting transitions are therefore only possible for Wenzel drops with moderately large receding contact angles (theta approximate to 90 degrees), which requires low surface roughness for one-tier surfaces or a Partial Wenzel wetting state for two-tier surfaces.