Hydrodynamic coupling of surfactant-influenced gas/liquid interfaces

Gas/liquid interfaces are rarely free of surfactants. In many fluid dynamic systems that involve a gas/liquid interface, the transport of mass, momentum, and energy is strongly influenced by the presence of even a minute amount of surfactant, due to the resultant viscoelasticity of the interface. In this paper we present results from a joint theoretical and experimental study of the interfacial viscoelastic properties and their coupling to subsurface fluid flow. A vorticity formulation for the stress boundary conditions, including non-constant viscoelastic properties, is derived and utilized in a numerical simulation of flow in a cylinder driven by the constant rotation of the bottom endwall with the free (top) surface covered by an insoluble surfactant. The relative effects of elastic properties, surface dilatational viscosity K^s and surface shear viscosity μ^s on the dynamics of the flow are discussed. The required interfacial properties which appear in the stress boundary conditions are studied through experiments, including free-surface boundary layer measurements made during the interaction of a vortex pair with a surfactant-influenced interface for the purpose of directly evaluating the surface excess viscosities (k^s + μ^s).