HYDRODYNAMIC COUPLING OF MONOLAYERS WITH SUBPHASE

The surface canal viscometry technique was used to determine the in-plane steady shear viscosity of monolayer assemblies of several amphiphilic molecules at the air/water interface. By varying the subphase viscosity by a factor of about 2.5 without affecting its surface activity, we were able to deduce the surface viscosity, which is independent of the subphase viscosity, only after correcting for the hydrodynamic coupling as prescribed by the theory of Harkins and Kirkwood. Alternatively, the subphase shear viscosity has been correctly deduced from the observed area flow rate through a canal in the limit of a vanishingly small surface viscosity where the flow rate now depends solely on the subphase viscosity through hydrodynamic coupling. By testing in these two limits, we evaluated and confirmed for the first time the validity of the hydrodynamic coupling between the monolayers and the subphase, as proposed by the theory of surface viscous flow of Harkins and Kirkwood.