Faraday wave

If a layer of liquid is placed on top of a vertically oscillating piston, a pattern of standing waves appears which oscillates at half the driving frequency. These Faraday waves can take the form of stripes, close-packed hexagons, or even squares or quasicrystalline patterns. Faraday Waves are commonly observed as fine stripes on the surface of wine in a wineglass which is ringing like a bell. Faraday waves also explain the 'fountain' phenomenon in the Chinese Singing Bowl.