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Shedding light on soap-film physics

10 Aug 2009

High-speed photography reveals how punctures expand in soap films.

Physicists in France have taken high-speed photographic images showing that a punctured soap film ripples for a moment before bursting into a fine mist of water droplets. The researchers attribute this strange behaviour to the same physics which makes a flag edge flap in the wind.

The fact that the rim of a punctured film does not expand in a smooth manner was first noted by the physicist Lord Rayleigh back in 1891. But it was not until the late 1960s that physicists first identified that punctures in soap films enlarge via distinctive "fingers", which are responsible for the observed ejection of water droplets. Until now, this phenomenon has only been studied from a frontal view, which has led physicists to link the observed instabilities with the internal kinetics of the liquid soap.

In this latest research, however, Henri Lhuissier and Emmanuel Villermaux at the Aix-Marseille University have taken a new approach by observing a soap film from an angle and studying how it behaves when it bursts. To create the film the researchers pulled a rigid frame out of a solution of the commercial detergent "Dreft". The bursting of the soap film is then performed in a vapour-saturated atmosphere.

Using a high-speed camera with a framing rate up to 25,000/s, the researchers captured a soap film receding in air with each full sequence typically lasting less than 0.01 s. They find that, in addition to the puncture growing parallel to the film, there also exists a secondary motion perpendicular to the plane, which causes the region of film close to the perforation to oscillate in a flapping-like manner.

Publishing their findings in Physical Review Letters, the French physicists attribute this flapping to the velocity difference between the fast-receding film and the slower-moving atmosphere. As the soap molecules begin their transit, the air flows around them leading to instability – just as a flag begins to flap when a rapid air current (the wind) flows around it. Once a film has receded far enough for the flapping undulations to pass a given threshold, this lateral motion can trigger the second instability which gives rise to the indentations or ligaments.

To provide further evidence that this flapping is related to the external medium the researchers also captured images of a flapping soap film in sulphur hexafluoride (SF6), which is five times denser than air. In this case, the destabilization of the puncture rim developed significantly faster, leading the researchers to conclude that the speed of soap film bursting increases with the density of the supporting medium.

Lhussier told our sister website physicsworld.com that he is now adapting his high-speed photography technique to study a more common phenomenon in nature – bubbles. "The application of such theories to bubble caps or shells is very important since the tiny spray they generate while bursting is responsible for a large part of the natural aerosols formation and thus plays a major role in the climate through the ocean-atmosphere exchanges," he says.

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