Fig. 1.

Wetting states and transitions between them on surfaces with different topographies. (A) With single-level topography, positive Laplace pressure causes a transition from the Cassie to the Wenzel state. The transition involves wetting of the whole solid surface and is irreversible in most cases. (B) Hierarchical topography. Nanofilaments suppress the transition to the Wenzel state and positive Laplace pressure will cause a transition to the nano-Cassie state instead, where wetted solid area remains small due to the plastron in the nanofilament layer. This transition can be reversed by negative Laplace pressure. Constant pressure in the plastron is maintained by a gas reservoir such as a macroscopic bubble on the edge of the surface. Scale bar in the scanning electron micrographs is 10 μm (in the inset, 500 nm). The post height in the micrographs is 5 μm.