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If the interface between a gas and a liquid oscillates back and forth, if the amplitude of oscillation is low, then the interface motion is synchronous with that of the driver and the body of the liquid. However if the oscillatory acceleration of the interface exceeds a certain value, then surface waves are set up on the interface. This was first demonstrated by Faraday in the 19th Century, when a plate of water was oscillated up and down on a plank of wood 18 feet long (click here for a reconstruction). The first surface mode to be excited has a frequency half that of the driver, and is known as the Faraday wave. As the amplitude of the driver increases further, more modes are excited (each with their own threshold for onset). As these various modes superimpose, the interface looks increasingly chaotic. This is seen in the above video. A gas bubble is suspended beneath a glass surface, and pulsates as it is subjected to a sound field. The amplitude of the pulsation is too small to see, but a Faraday wave can be seen around the bubble equator. As the film progresses, the amplitude of the driving acoustic field is increased, and more modes are excited
(Video: PR Birkin, YE Watson, TG Leighton).
Click here to see more movies of Faraday waves on bubble walls (warning: large file size).
This page was last updated by TG Leighton, 27 September 2004