Spreading of a new surfactant in the presence of a pre-existing surfactant distribution is investigated both experimentally and theoretically for a thin viscous substrate. The experiments are designed to provide a better understanding of the fundamental interfacial and fluid dynamics for spreading of surfactants instilled into the lung. Quantitative measurements of spreading rates were conducted using a fluorescent new surfactant that was excited by argon laser light as it spread on an air–glycerin interface in a petri dish. It is found that pre-existing surfactant impedes surfactant spreading. However, fluorescent microspheres used as surface markers show that pre-existing surfactant facilitates the propagation of a surface-compression disturbance, which travels faster than the leading edge of the new surfactant. The experimental results compare well with the theory developed using lubrication approximations. An effective diffusivity of the thin film system is found to be Deff = (E*Γ¯)/(μ/H¯), which indicates that the surface-compression disturbance propagates faster for larger background surfactant concentration, Γ¯, larger constant slope of the σ*−Γ* relation, −E*, and smaller viscous resistance, μ/H¯. Note that σ* and Γ* are the dimensional surface tension and concentration, respectively, μ, is fluid viscosity, and H¯ is the unperturbed film thickness.

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