Stress transmission between soap films: the role of the Plateau border*

The origin of the high effective viscosity of a liquid foam remains unclear due to the missing description of the local flow in the liquid matrix. In particular, the transmission of a stress from a thin film to another through a meniscus is still not well understood. Here, we experimentally investigate this elementary brick of foam mechanics using a horizontal soap film connected, through 2 meniscus, to 2 peripheral films on each sides to mimic the foam structure. Simultaneous stretching/compression of the peripheral films is imposed and the thickness and velocity field of the horizontal film are measured during the deformation. The central film exhibits a global motion toward the stretched side where some fluid flows into the meniscus, while a much thicker film is extracted from the meniscus of the compressed side. This indicates that the deformation is not screened by the meniscus as commonly assumed in local models. This study reveals the flows and their associated dissipation at the root of the foam effective viscosity.