Abstract

In many consumer goods and industrial processes, liquid foams are notably used for their viscoelastic behaviour. Given that their liquid matrices are Newtonian fluids with viscosities lower by several orders of magnitude, their strong dissipative properties are surprising, and recent advances have shown that they may originate from a very localised zone near the junctions between the foam films. To further investigate the different mechanisms involved there, we designed a special frame allowing us to create and observe three soap films connected together with a meniscus. We can control the lengths of the films using three synchronized motors to explore different geometries of deformation. Using a fluorescent dye along with a photobleaching setup, hyperspectral cameras and tracking the position of the meniscus, we can have access to the thickness and velocity fields in the fluid, as well as the difference of surface tension between the films.

We thus quantify the balance of surfactants going in and out of the meniscus during the imposed deformation. This allows us to conclude that the soap films exchange surfactants between them without exchanging with the meniscus as they are deformed, which is a key information to add to our model describing the rheology of this elementary foam.