Nano to micro structural hierarchy is crucial for stable superhydrophobic and water-repellent surfaces

Water-repellent biological systems such as lotus leaves and water strider's legs exhibit two-level hierarchical surface structures with the smallest characteristic size on the order of a few hundreds nanometers. Here we show that such nano to micro structural hierarchy is crucial for a superhydrophobic and water-repellent surface. The first level structure at the scale of a few hundred nanometers allows the surface to sustain the highest pressure found in the natural environment of plants and insects in order to maintain a stable Cassie state. The second level structure leads to dramatic reduction in contact area, hence minimizing adhesion between water and the solid surface. The two level hierarchy further stabilizes the superhydrophobic state by enlarging the energy difference between the Cassie and the Wenzel states. The stability of Cassie state at the nanostructural scale also allows the higher level structures to restore superhydrophobicity easily after the impact of a rainfall.