”Young” soap films

If we pull out rapidly a metallic frame out of a surfactant solution, we arrive at a ”young” soap film with relatively simple features, as noticed first by Lucassen. The weight of the film is equilibrated by a vertical gradient of surface tension. At each level, the local solution concentration c(z) equilibrates with the local monolayers, of surface concentration Γ(z). A detailed analysis of the young films was started by us in 1987. We present here an approach which is more illuminating a) the concentration profiles decay exponentially at large heights, with a characteristic length λ ∼ meters b) the surface is protected up to a thickness hm larger than λ c) we also review the dynamic requirements. The surfactant must reach the surface in a time shorter than the free fall time of a pure water film. This discussion explains (to some extent) the compromise which is achieved in practice by good foaming agents.