The
evolution of a foam driven by the transfer of two gases of
different solubilities across the soap films is studied. A bamboo
foam, or a train of films, is used as a model system; it is made of
a poorly soluble gas and put into contact with a reservoir of a soluble
gas at an initial time. The measurement of the time evolution of the
volume of each bubble shows that the foam swells as it progressively
incorporates the soluble gas. The dynamics is modeled from the gas
fluxes across each film. The continuous limit of this model at a large
number of bubbles is studied in detail: it gives an effective nonlinear
diffusion equation, which fits the data very well. The corresponding
diffusion constant, given by the product of the permeability of the
soluble gas and the initial size of the bubbles, is shown to be the
key parameter governing the coarsening dynamics of the foam.