Rayleigh Taylor instability in a soap film*

Measuring the local flows in the liquid phase of a foam during thin film deformations is a crucial step to understand the origin of the effective viscosity of the foam. In this aim, we built a deformable frame to produce three liquid films connected by a horizontal meniscus. When the vertical film, suspended below the meniscus, is suddenly stretched, it first get larger and thinner. However, at longer time, a new film, much thicker, is extracted from the meniscus and the initial film almost recovers its initial shape. Finally, this new film destabilises and mushrooms of thick film fall down in the thinner film, with a well defined wave length. The linear stability analysis of this phenomenon allows to identify this instability as a Rayleigh-Taylor instability, the film thickness difference playing the role of the density difference. We compare the measured wave lengths to the fastest mode of this instability in the linear regime.

These results open new perspectives on the processes governing the average film thickness in a sheared foam and provide a nice example of a perfectly 2D Rayleigh Taylor instability.