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 T(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 lambda similar to meters; (b) the surface is protected up to a thickness h, larger than lambda; (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.