Spontaneous symmetry-breaking meniscus streaming*

When a fluid bath is vibrated vertically beyond a critical driving acceleration, the free surface destabilizes into a field of subharmonic standing waves, the so-called Faraday waves. A further increase in forcing gives rise to a secondary instability in which the standing Faraday pattern spontaneously becomes chaotic. When the bath is large relative to the characteristic wavelength, the waves form elongated patterns that appear, drift and disappear randomly on the free surface. Drawing inspiration from active-matter systems, we demonstrate that these out-of-equilibrium waves can spontaneously lead to coherent directed motion under confinement. In particular, chaotic Faraday waves may spontaneously develop into fast-moving travelling waves in either clockwise or anti-clockwise directions in annular geometries. Combining experiments and simulations, we rationalize the mechanism responsible for this instability in terms of the streaming flows generated near the vertical walls, which are critically enhanced by the meniscus. Moreover, we demonstrate how this instability can be harnessed for a range of applications in flow transport, mixing and particle sorting, and discuss the potential of these out-of-equilibrium waves as a platform to investigate new types of active meta-materials.