Bouncing water drops

When a liquid drop impacts a solid, it spreads (with possibly beautiful, fi ngering patterns) up to the point when kinetic energy is dissipated by visc osity. Then, it can retract (if the solid is partially wetted by the liquid ), or not. A very different behaviour can be observed on highly hydrophobou s solids. On such solids, the contact angle is close to 180 degrees, so tha t the kinetic energy of the impinging drop can be transferred to surface en ergy, without spreading. Thus, the drop can fully bounce. However, the liqu id nature of this kind of spring imposes a limit for the restitution coeffi cient, which is due to the fact that the drop, after the lift-off, oscillat es.