Faraday instability in floating drops
In 1831, Michael Faraday discovered that a liquid undergoing vertical vibration is unstable to surface waves. We show the Faraday instability in floating liquid drops, as an example of hydrodynamic instability that develops in a domain with flexible boundaries. A surprising mutual adaptation between the waves and the drop's boundary occurs, as a result of the competition between the wave radiation pressure and the capillary response of the drop's border. Two types of behaviour are observed. In the first, stable drop shapes are obtained experimentally and they result from the equilibrium between wave radiation pressure and capillary pressure. In the second, the radiation pressure exceeds the capillary response of the drop's border and leads to non-equilibrium behaviours, with breaking into smaller drops that have a complex dynamics including spontaneous propagation.