Measuring The Interfacial Rheological Properties Of Lung Surfactant Monolayers

Lung surfactant (LS) is essential to respiration because it reduces surface tension at the interface between water and air in the alveoli. In addition, LS stabilizes the alveoli against collapse during exhalation. LS is composed of lipids, mainly dipalmitoylphosphatidylcholine (DPPC), and proteins. We are interested in the rheological properties of a pure DPPC film as a first step towards understanding complex LS rheology. During exhalation, we hypothesize that a large surface tension gradient occurs between the alveolar sacs and the bronchioles which should cause LS to flow away from the alveoli. Because LS does not readily leave the alveoli through the trachea, there must be the drag effect with an opposing direction to the flow induced form the surface tension gradient. Previous work has shown that the surface shear viscosity of LS increases exponentially at low surface tensions, which may be sufficient to prevent the flow of surfactant out of the lungs. Additionally, a yield stress in the interfacial LS film would also help to prevent the flow. Here, we define the yield stress as a minimum stress below which no flow occurs. As a result, we have shown the existence of the yield stress in pure DPPC monolayer, and how to increase or decrease the yield stress based on the addition of other lipid molecules.