Surfactant and liquid transport in a thin viscous film with pre-existing surfactant and periodic wall stretch

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. Film thickness measurements are made using a constructed light technique. The experimental results compare well with the theory which is developed using lubrication approximations. An effective diffusivity of the thin film system is found and indicates that the surface-compression disturbance propagates faster for larger background surfactant concentration, larger slope of the surfactant isotherm, and smaller viscous resistance.;Soluble surfactant and airway surface liquid transport are examined using a mathematical model of Marangoni flows which accounts for airway branching and for cyclic radial and longitudinal airway stretching. Both uniform and linear longitudinal wall strain are considered. The model allows for variation of the amplitude and frequency of the motion. The soluble surfactant dynamics of the thin fluid film are modeled by either linear sorption or squeeze-out phenomena. The model is general, and can handle either delivery of surfactants into the lung, by setting the proximal boundary condition to a higher concentration compared to the distal boundary condition, or removal from it by switching these end conditions. Starting with a steady-state, non-cycled, non-uniform, surfactant distribution we find that transport of surfactant into the lung is enhanced for larger strain amplitudes and frequency, though the latter is less important. For surfactant clearance from the lung, larger strain amplitude enhances transport. However, cycling frequency has the opposite effect with larger frequencies leading to reduced transport. For larger strain amplitudes and slower frequencies, liquid clearance is enhanced.