Foam Ripening
Résumé
The diffusion of gas between bubbles is an important phenomenon by which liquid foam evolves towards thermodynamic equilibrium. By this process, bubbles smaller than the average size shrink, whereas larger bubbles grow, resulting in a growing of the average size of the bubbles over time, and a spontaneous evolution of the radii distribution towards a statically invariant one. This coarsening, or disproportionation, is analogous to the ripening or aging of emulsions and crystals. This process is strongly coupled with drainage, rheological and optical foam properties. This is mainly due to the increase of the average bubble size, but note also that ripening induces internal dynamics that are central in the complex rheological behavior of foams. We will present fondamentals for describing ripening in dilute suspensions of spherical bubbles, in dry and wet foams. In each case, kinetics for bubble growth will be given as a function of the relevant parameters. Finally, several strategies for controlling bubble ripening will be suggested.