On the generation of a foam film during a topological rearrangement

T1 topological rearrangement, i.e. switching of neighbouring bubbles in a liquid foam, is the elementary process of foam dynamics, and it involves film disappearance and generation. It has been studied extensively as it is crucial in foam rheology or foam collapse. T1 dynamics depends mainly on the surfactants used to generate the foam, and several models taking into account surface viscosity and/or elasticity have been proposed. By performing experiments in a cubic assembly of films, we go a step forward in this global analysis and investigate experimentally the mechanism of formation of the new film. In particular, the flow velocity field is probed by particle tracking and the film thickness is measured by light absorption and interferometric measurements. Two limit behaviours for the film are reported: it may (i) undergo an homogeneous extension, or (ii) resist elongation and remain at rest, new film being created from liquid exchange with connecting meniscus. Both T1 dynamics and film thickness are shown to depend on the competition between these two behaviours. Interestingly, their balance is set by the surfactant solution used, but it is also shown to vary during a single T1 relaxation process.