Surface viscosity of surfactant films from human lungs

The surface shear viscosity of films of human lung surfactant was measured using a rotary traction viscometer. At low shear rates dilute surfactant films (surface coverage 2.5--10 cm2/microgram lung extract) exhibited a low viscosity. Increasing the film concentration initially produced only a small increase in viscosity but, after a certain concentration, the film viscosity increased very markedly. The viscosity range for the most disperse (coverage 10 cm2/microgram extract) to the most concentrated film (0.6 cm2/microgram extract) was about 2.0 X 10(-4) g . sec-1 to 2.2 X 10(-2) g . sec-1. Surfactant film viscosity was also found to depend on the temperature of the subphase, higher viscosities being achieved when the subphase temperature was lowered. At low surface concentrations, surfactant films behaved in a strictly Newtonian manner. In contrast, at concentrations above 5.5 cm2/microgram extract they showed mildly non-Newtonian behaviour in that the magnitude of their surface viscosity was dependent on the rate of shear. The results suggest that films of human lung surfactant probably consist of both lipids and protein, and that the protein fraction plays a major part in determining the rheological properties of the films.