Surface rheology is a description of the rheological properties of a free surface. When perfectly pure, the interface between fluids usually displays only surface tension. But when surfactants are adsorbed on the interface, because they lower the surface tension, the stress within the interface is affected by the flow for several reasons. * Change in the surface concentration of surfactants when the in-plane flow tends to alter the surface area of the interface (Gibbs' elasticity). * Adsorption/desorption of the surfactants to/from the interface.