Fluorinated Langmuir monolayers are more viscous than non-fluorinated monolayers

The surface shear viscosity of the liquid expanded phase in Langmuir monolayers of fluorinated alcohols differs by orders of magnitude from the corresponding surface shear viscosity of Langmuir monolayers of their non-fluorinated counterparts. The line tension between the liquid expanded and the gaseous phase on the other hand is of similar magnitude for both fluorinated and non-fluorinated surfactants. The difference of fluorinated versus non-fluorinated monolayers is measured by active microrheology and by observing the shape relaxation of distorted liquid expanded domains toward a circular shape. Our microrheology measurements are supported by measurements of the relaxation rates of distortions, which in fluorinated liquid expanded phases are proportional to the deviation of the curvature from the mean curvature, whereas they are proportional to the square of the deviation of the curvature from the mean curvature in non-fluorinated monolayers.