Bouncing water drops

Abstract

When a liquid drop impacts a solid, it spreads (with possibly beautiful fingering patterns) up to the point when kinetic energy is dissipated by viscosity. Then, it can retract (if the solid is partially wetted by the liquid), or not. A very different behaviour can be observed on highly hydrophobous solids. On such solids, the contact angle is close to 180°, so that the kinetic energy of the impinging drop can be transferred to surface energy, without spreading. Thus, the drop can fully bounce. However, the liquid nature of this kind of spring imposes a limit for the restitution coefficient, which is due to the fact that the drop, after the lift-off, oscillates.