Inferring Boundary Viscosity Values from Shear Deformation of Molecularly Thin Films

Derjaguin and co-workers in 1946 introduced a shear technique for inferring the boundary viscosity of ultrathin liquid films. Air blown through a slender horizontal slit is used to apply a constant shear stress to the free surface of an initially flat and uniform film. Newtonian films tend to distort streamwise into a wedge shape whose slope decreases in time. The viscosity of the film can then be extracted from the wedge slope. High resolution measurements of the deformed film shape are normally obtained by interferometry for microscale films or ellipsometry for molecular scale films. Over the years, it has become evident that liquid nanofilms can exhibit a shear response that deviates significantly from this ideal linear behavior. Various physical mechanisms have been proposed to help resolve discrepancies between theory and experiment. Here we present finite element simulations of the liquid deformation process to evaluate help assess the influences of different mechanisms. Based on quantitative comparison to experimental data, we describe which candidate mechanisms best fit the trends observed.