Parametrically driven standing waves — stripes, hexagons, squares
Faraday waves (Michael Faraday, 1831) appear when a fluid container is vibrated vertically. The surface responds at half the drive frequency (subharmonic resonance). Above a critical amplitude, the flat surface destabilizes via a parametric instability. The geometry depends on the drive frequency and fluid properties, selecting stripes, hexagons, or squares through nonlinear competition.