Stability of foam films: marginal regeneration and drainage – DRAINFILM

Liquid foams are materials whose mechanical and insulating properties are widely used in industry. However, their stability remains largely an open question. The properties of liquid foams are closely related to those of the foam films they contain. The films can thicken, disappear or appear when the foam moves. The rearrangement of the liquid inside these films under the effect of an external stress such as gravity then alters the thickness of the films, which affects their stability but also their gas permeability. It is therefore crucial to understand the flows in the films to explain the stability of the foams.

However, the appearance of thickness inhomogeneities seems to be linked to the appearance of complex flows such as marginal regeneration, which is the subject of this project. Indeed, a pinch appears at the edge of the films and can become destabilized, generating thinner portions. The sliding puzzle between the thick and thin portions of the film then contributes to the thinning of the film. This phenomenon of marginal regeneration has been little explored since the historical work of Mysels. Pinching has been predicted by Aradian et al. but its dynamics has never been observed experimentally. Moreover, the mechanism and conditions of destabilization of this thin zone are largely unexplored. However, it is certain that it is necessary to describe this phenomenon precisely in order to give a global description of the thickness distributions in the films and thus their fragility with respect to rupture.

Understanding this phenomenon of marginal regeneration and its implication on the stability of foams is a difficult problem which requires skills in the fields of fluid mechanics and free interfaces, the physico-chemistry of surfactants and nonlinear physics. In this project we bring together three partners with complementary expertises and innovative solutions, both experimental and theoretical.

In particular, the originality of our project is to combine the expertise developed in recent years in the foam community (measurement of the velocity field in films, their thickness and tensions, control of evaporation and physico-chemistry) with a new experiment that allows to control gravity by centrifuging the films. We will approach the phenomenon experimentally by developing experiments to answer the questions (i) of the pinch dynamics, (ii) of its destabilization and (iii) of the contribution of marginal regeneration to film drainage and film rupture. These experimental tasks will be complemented by original theoretical descriptions, based on the extension of classical lubrication equations, allowing us to describe our results and extrapolate them to foams.

Thanks to recent developments, the subject is ripe to be finally understood and we believe that the results of this project may lead to numerous publications in high-impact journals. Furthermore, although fundamental, the question raised has obvious implications in terms of applications and we intend to use our contacts with industry to enable a rapid transfer of the results. To this end, we will set up a workshop at the end of the project to share our results with industrial contacts and organize future collaborations. Finally, the consortium is very involved in popularizing science and we will take advantage of the aesthetic character of marginal regeneration to build an exhibition around the foams and develop our science popularization activities.