Structure and Rheology of DPPC-Cholesterol Monolayers

Lipids are key components of biological interfaces and are constantly subjected to external stress and shear flow. In this work, we focus on DPPC-cholesterol monolayers at water-air interfaces and study their response to compression and shear. The choice of lipids is motivated by their crucial role in regulating the surface activity and mechanical properties of pulmonary surfactants at the interface of alveoli. Using a Langmuir-Blodgett setup coupled to an interfacial shear rheometer and a fluorescence microscope, we observe that pure DPPC monolayers exhibit a first-order phase transition (liquid-expanded to liquid-condensed) upon compression and a weak viscoelastic response under oscillatory shear. We then report a significant shift in both the structural and mechanical properties with the addition of cholesterol. In particular, the collapse pressure is reduced, the surface viscosity drops by more than one magnitude, and the monolayers exhibit a percolated microstructure at high cholesterol concentrations.