Effect of viscosity and compression rate on the collapse phase transition of pulmonary surfactant at an air-water interface

Pulmonary surfactant adsorbs at the air-water interface in the lungs, reducing its surface tension, particularly during exhalation, when the decreasing surface area compresses the interfacial film. In vitro, however, a collapse phase transition, in which the two-dimensional film transforms to three-dimensional structures, limits access to low surface tensions. When compressed on the surface of a captive bubble, pulmonary surfactant films partially avoid collapse and reach low surface tensions if compressed faster than a threshold rate. At progressively lower surface tensions, rates of collapse initially increase but then pass through a maximum and slow. In this paper, we model the collapse phase transition using nucleation and growth theory, and simulate the behavior of a spherical bubble with surfactant at its interface. If interfacial shear viscosity varies according to free area theory within the film, and free area vanishes with vanishing surface tension, our model correctly predicts the experimental behavior.