Probing Collapse Mechanisms of Two-Dimensional Lipid Monolayers

Lung surfactant is a lipid monoalyer and protein mixture that lines the alveolar air-liquid interface to lower surface tension. When lung surfactant is subjected to a large compression, such two-dimensional thin film collapses. To date, several collapse mechanisms are known for lipid monolayers: fracture, solubilization, and folds. We present a novel method that enables us to distinguish “folds” from other mechanisms and, furthermore, quantify folds. We simply measure fluctuations of the surface tension when the collapse occurs, and compute the mean squared fluctuation, which provides various properties of folds (e.g. a size and time that folds remain). We validate this technique using mixed lipid monolayers of DPPC and POPG, a good model system of lung surfactant, which exhibits the different mechanisms of collapses by varying POPG fractions. Moreover, visualization of such monolayers with the measurements reveals that formation of folds occurs randomly, and depends strongly on compression rates and a composition of POPG.