"Young" soap films

If we pull out rapidly a metallic frame out of a surfactant solution, we ar rive at a "young" soap film with relatively simple features, as noticed fir st by Lucassen. The weight of the film is equilibrated by a vertical gradie nt of surface tension. At each level, the local solution concentration c(z) equilibrates with the local monolayers, of surface concentration T(z). A d etailed analysis of the young films was started by us in 1987. We present h ere an approach which is more illuminating: (a) the concentration profiles decay exponentially at large heights, with a characteristic length lambda s imilar to meters; (b) the surface is protected up to a thickness h, larger than lambda; (c) we also review the dynamic requirements. The surfactant mu st reach the surface in a time shorter than the free fall time of a pure wa ter film. This discussion explains (to some extent) the compromise which is achieved in practice by good foaming agents.