Response of Molecularly Thin Films to an Applied Shear Stress: Theory vs Experiment

Derjaguin and co-workers in 1946 introduced the "blow-off technique" for inferring the boundary viscosity of ultrathin liquid films supported on a solid substrate. Air blown through a slender horizontal slit is used to create a constant wall shear stress that ideally deforms an initially flat free surface film into a streamwise wedge shape whose slope decreases in time. The boundary viscosity can then be inferred from the slope value. High resolution measurements of the film shape are normally obtained by interferometry for microscale films or ellipsometry for molecular scale films. Over the years, it has become evident that many molecular scale films exhibit shear response that deviates significantly from ideal linear behavior. Various physical mechanisms have been proposed to help resolve discrepancies between theory and experiment. Here we present results of computational studies of the 1D thin film equation which describes the liquid deformation process in time. We evaluate the influence of different proposed mechanisms based on quantitative comparison to experimental data and discuss which candidate mechanisms best fit the trends observed.