Surfactant Spreading on a Thin Film Is Sensitive to Film Thickness: Implications for In Vivo Pulmonary Systems Versus In Vitro Scenarios

The role of pulmonary surfactant is critical to the mechanics of the lung. Pulmonary surfactant forms a monolayer film at the air-liquid interface of a thin film of fluid lining the alveoli. Without surfactant, the air-liquid surface tension would be too high and the lungs would collapse during exhalation, as is the case with premature babies who develop respiratory distress syndrome (RDS) because they lack a sufficient amount of pulmonary surfactant [1]. Because an increase in surfactant surface concentration correlates with a decrease in surface tension, the surface tension gradients resulting when a monolayer of surfactant is deposited on a thin fluid film cause the surfactant to spread. This self-spreading phenomenon is of interest for applications such as surfactant replacement therapy in infants suffering from RDS and for drug delivery [1,2].