Recently, we have observed that surfactants deposited on liquid films can cause complex flows in confined environments. Surfactants are molecules typically found in soaps, but also in other substances. They can alter the surface tension between two phases, such as a liquid phase (water) and a gas phase (air). When the concentration of surfactants at an air-water interface is nonuniform, this creates a Marangoni tension, which leads to the spreading of the surfactants to even
out their concentration. In most liquids, there is a trace of endogenous surfactants at very low concentrations, due to natural contamination. These endogenous surfactants can interact with exogenous surfactants locally deposited on the interface and alter the Marangoni-driven spreading.