Surfactant Exchanges between Deformed Soap Films

The effective viscosity of liquid foams is controlled by Marangoni forces and therefore by the transport of surfactants. Direct tracking of the latter during foam deformation is out of reach. Besides, the competition between diffusion and convection on the interfaces and in the bulk of complex assemblies of thin films and menisci is still an open problem. These shortcomings severely limit our understanding of foam rheology and stability. In this Letter, we use a comprehensive characterization of in-plane flows in an elementary foam during imposed deformation to determine indirectly the flux of surfactants from a film to its first neighbor. We show that the meniscus connecting the films, despite its potential role as a surfactant reservoir, is not involved in surfactant transport. This information has been identified as a key missing piece in local foam viscosity models of the literature.