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The rupture of a vertical soap film is a deterministic phenomenon


Understanding the stability of bubbles remains a challenge for scientists. Currently, it is impossible to predict the maximum size of a bubble and only empirical know-how gives a recipe for making large bubbles. To better understand the rupture phenomenon, physicists have tried to simplify it by studying not bubbles, but vertical films. The work to date on horizontal films has shown that their bursting is ultimately ruled by chance. But what happens when a vertical film ruptures during its generation ?

This question was answered by physicists from the LPS. By measuring and modeling the vertical lifetime of films, they showed that the rupture is in this case perfectly deterministic and that it depends on the rate of formation, the nature and concentration of the soap molecules used and the viscosity of the solution. They have shown that it is the concentration gradient of the soap molecules between high and low film which has a stabilizing effect. This work, published in the journal Soft Matter opens the way for new models of the foamability of surfactant solutions widely used in industry.


 

To accomplish this work, the researchers built an experimental device for their systematic analysis of the production and breakdown of vertical soap film. A metal frame of 2 inches wide and 4 inches high is pulled out of a soapy solution at velocities between 0.01 and 10 mm/s. The researchers then measured the lifetime and the maximum length of the films produced and observed that unlike the random nature of the rupture of horizontal films, the rupture of vertical films during their generation is deterministic. Furthermore, a surprising dependence of the life time with the thickness of the films was observed : the thicker films have shorter lifetimes but they are longer at the time of rupture. Typically, a film drawn at 6 mm/s can reach 3 cm whereas a film drawn at 0.6 mm/s will only reach 6 mm. This led the researchers to propose a scenario for the rupture of soap films : soap molecules are not distributed evenly at the surface of the film, but are less concentrated in the area freshly created which causes a force that holds the weight of the film. This is known as the Marangoni effect. At a given time, the film reaches a maximum length at which its weight becomes so important that the Marangoni force is too weak to support the film and it breaks. This work also permits, for the first time, to measure the surface tension difference between the top surface and the bottom film and shows that this difference in surface tension increases linearly with the pulling velocity.

 

Contact :
Emmanuelle Rio

Reference :
A study of generation and rupture of soap films
L. Saulnier, L. Champougny, G. Bastien, F. Restagno, D. Langevin et E. Rio
Soft Matter, 10, 2899-2906, (2014).

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