Liquid coating from a surfactant solution

A solid drawn out of a pure wetting liquid comes out coated with a layer of liquid, of thickness given by the Landau equation (for small coating velocities). If the solid is drawn from a solution containing surfactants, we show that the film is thicker than predicted by Landau: when drawing the solid, a gradient of surface concentration appears, provoking a Marangoni flow and thus a thickening of the film. The thickening factor we measure is found to reflect a balance between convection (responsible for the gradient) and absorption from the volume (which tends to erase the gradient).