Transport of convected soluble surfactants on and within the foam film in the context of a foam fractionation process

This study models convective transport of soluble surfactant in a foam fractionation system with reflux. Owing to reflux, initial surfactant concentration in films is lower than in Plateau borders. Marangoni flows and film drainage flows arise convecting surfactant both on the film surface and in the bulk. An interface is set up within the film bulk called a separatrix: this divides two regions of uniform surfactant concentration, one with concentration equal to that of the initial film and one with concentration equal to that in the Plateau border. The evolution of the separatrix is tracked to determine surfactant recovery and enrichment for the film. Surfactant lean films benefit most from reflux. However for films that are initially comparatively surfactant rich, recovery might actually decrease at long times owing to film drainage. Nonetheless surfactant lean films and those containing surfactant with only moderate solubility benefit from reflux even despite film drainage.