Coupling of the interfacial and bulk flow in a knife-edge surface viscometer

After more than 50 years of investigating how to measure surface (excess) shear viscosity, it remains a controversial issue. The complications stem from the fact that to measure a viscous response the system needs to be flowing, and for a surface film on a liquid substrate this means that the liquid in the bulk will also be flowing. Macroscale measurements, which generally provide greater accuracy, are often made at Reynolds numbers that are large enough for inertia to be non-negligible. However the theoretical models against which the measurements are compared have so far failed to properly account for the coupling. Results will be presented from a numerical study on the coupling between the interfacial and bulk flow. Also, experimental results will be presented for the lung surfactant component DPPC.