Nobody knows why a soap bubble collapses. When the liquid film forming the bubble, stabilised by surfactants, becomes too thin, it collapses. This seemingly simple problem, ruled by the classical laws of
fluid mechanics and of statistical physics, is still a challenge for the physicist. The rupture criteria based on a stability analysis in the vicinity of the film equilibrium state fail to reproduce the observations. However the
film ruptures in a foam obey some simple phenomenological laws, which suggest that underlying fundamental laws exist and wait to be determined. The state-of-the-art conjecture is that ruptures are related
to hydrodynamical processes in the films. Similarly, the apparent viscosity of a liquid foam sample is not understood yet, and determines the way a foam flows.
Foams are widely used in industry and most of the stability and flows issues have been solved. Nevertheless, most of the industrial formulations must
currently be modified in order to use green surfactants. This adaptation will be extremely more efficient and possible if the flows in the liquid part of the foam are understood.
With this device, we quantified for the first time the transfer of surfactant from one film to another, across their common meniscus. On this basis, we built a theoretical model to predict the dissipation induced by the elementary foam deformation. An upscaling of these local observations and predictions leads to the effective viscosity of a foam sample.
We also discovered unexpected new instabilities playing an important role in the soap film aging, and we evidenced the existence of a line tension in soap film, at the boundary between film domains of different thicknesses. These processes control the film thinning dynamics, and thus its life time.
