Spontaneous symmetry-breaking of geometrically confined Faraday waves*

When a fluid bath is vibrated vertically, there exists a critical driving acceleration beyond which the flat hydrostatic surface becomes unstable and transitions to a field of subharmonic standing waves, the so-called Faraday waves. A further increase in forcing will see the system undergo a secondary instability in which the standing Faraday pattern spontaneously becomes chaotic. When the bath is large with respect to the characteristic wavelength, the waves form elongated patterns that appear, drift and disappear randomly on the free surface. We demonstrate that, by geometrically confining these waves, their chaotic motion can be harnessed to produce coherent directed motion. In particular, in annular geometries, these chaotic waves can spontaneously develop into fast-moving travelling waves in either clockwise or anti-clockwise directions. We rationalize the mechanism responsible for this new instability in terms of the streaming flows generated at the boundary layers on the vertical walls, which are critically enhanced by the presence of a meniscus. Potential applications of these out-of-equilibrium waves will be discussed.