Water droplets on the bounce

Self-cleaning, anti-icing and condensation-controlling surfaces are important in nature and a broad range of technologies. Thomas Schutzius and colleagues now show that water droplets resting on suitably textured superhydrophobic surfaces in a low-pressure environment can 'self remove' through spontaneous levitation and trampoline-like bouncing, even though the surface is completely rigid. Such behaviour seemingly violates the second law of thermodynamics, but is explained by an overpressure beneath the droplet that builds up due to fast droplet vaporization while substrate adhesion and the pillar texture of the surface restrict vapor flow. The effect, which can even remove icy droplets as they freeze, illustrates how insight into droplet-surface interactions can guide the rational design of surfaces to put such interactions to unexpected use.