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The dance of soap films

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No need for brushes to paint with music, it is possible with soap! Researchers from the Matter and Complex Systems Laboratories and the Interdisciplinary Physics Laboratory have presented a device for observing the effect of sound on soap films: several physical phenomena draw on them spectacular patterns. This work is published in the journal European Journal of Physics.

If our breath makes it possible to form a bubble, how does a soap film react when exposed to music? While studying the propagation of sound in soap foams, researchers from the Matter and Complex Systems Laboratory (MSC, CNRS / Paris Diderot University) and the Interdisciplinary Laboratory of Physics (LIPhy, CNRS / Université Grenoble Alpes) have designed a set-up to observe these mechanisms. They have installed a loudspeaker at the end of a tube at the end of which a film of soap is formed. The liquid membrane starts to vibrate like the skin of a drum, which causes several phenomena. When a sound track that is capable of exciting the resonant frequencies of the tube is played, dynamic structures appear on the film and fascinate both by their beauty and by the richness of the underlying physics.

The phenomenon works with all types of music, but the dimensions of the tube allow to choose the resonant frequencies according to the instruments. Thus, the fundamental frequency of the tube used is 460 Hertz, which corresponds to the tone of the flute. Depending on the frequency and volume of the music, capillary waves appear as well as a rich surface dynamics. The sound waves cause the formation of several vortices, which varies the thickness of the film and delays the bursting of the film. Since the thickness of the film is comparable to the wavelength of the incident white light, interferences appear and decompose this light into different colors. Thus changes in the thickness of the film are manifested by variations in color. The device makes it possible to illustrate the flows in the soap films subjected to an acoustic wave in a particularly pedagogical and artistic way.

The article has been published in the European Journal of Physics and the open version is in HAL.

Note: This news first appeared on the CNRS site and has been translated here with their permission.