Spontaneous symmetry-breaking Faraday instabilities

When a fluid bath is vibrated vertically beyond a critical driving acceleration, the free surface destabilizes into a field of standing waves, the so-called Faraday waves. A further increase in forcing will see the emergence of a secondary instability in which the subharmonic 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. Taking cues from active-matter systems, we demonstrate that these chaotic waves can spontaneously lead to coherent motion under confinement. Specifically, we present a spontaneous symmetry-breaking instability that transforms standing Faraday waves into rapidly traveling waves, rotating either clockwise or anti-clockwise, in annular geometries. Combining experiments and simulations, we show that this traveling instability is driven and significantly enhanced by capillary effects, including wettability and contact-line dynamics. Moreover, we demonstrate how this instability may be leveraged for applications in flow transport, mixing and particle sorting, and explore the potential of these non-equilibrium waves as a versatile platform for investigating novel classes of active meta-materials.