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Generation and bursting of vertical soap films
If a frame is pulled out of a soapy solution, a colored ephemeral free standing film will be created. How thick is this film and how long will it last ? These are the two questions we will address. We first show that the film is composed of two zones : a zone of well-defined thickness at the bottom and a thinning zone at the top. The thickness of the bottom zone is usually described by Frankel’s model. We show that this model breaks down if the film is generated at too high velocity and that the fitting of the data by a model including surface elasticity allows using film generation as a surface rheometer. The film rupture during its generation is then explored. We demonstrate that the rupture is tuned by the drainage of the film and that the maximum length of the film is given by a balance between the weight of the film and the gradient of surface tension at the liquid/air interfaces. Our work makes it possible to get a prediction for the surface tension difference across the whole film. This difference is found to be surprisingly small, of the order of 1 - 2 mN/m.