Solutal Marangoni spreading in the presence of pre-deposited insoluble surfactant monolayers

Marangoni spreading can be used for pulmonary drug delivery. However, endogenous surfactant in the lung may inhibit that spreading. Here we investigate how the density of pre-existing phospholipid monolayers impacts Marangoni flows induced by exogenous surfactants on a thin fluid subphase. A dipalmitoylphosphatidylcholine (DPPC) monolayer is pre-deposited at a predetermined lateral density on an aqueous subphase before depositing a drop of oleic acid, an insoluble surfactant. Talc tracer particles monitor the extent of spreading, and a trans-illumination method is used to measure the temporal evolution of the subphase surface deformation. This reveals the outwardly moving “Marangoni ridge”. As long as the surface pressure of the DPPC monolayer is less than that of an oleic acid monolayer, spreading proceeds until the surface pressures of the monolayers are equal. The final area per molecule of DPPC in the compressed monolayer is thus the same in each case. The trajectory and degree of interface deformation caused by Marangoni flow are altered by increasing initial DPPC concentration. Unlike Marangoni flow on bare subphases, there is surface flow ahead of the Marangoni ridge as the DPPC monolayer is compressed.