Predicting shear rheology of soft interfaces*

Predicting non-Newtonian shear rheology of soft matter systems at fluid-fluid interfaces has been compromised by using linear (Newtonian) constitutive equations to determine rheological properties. Predicting this nonlinear behavior is integral to the development of engineered products and explaining many biophysical processes. Here, we model the interfacial viscosity as a generalized function of the imposed shear rate and present a non-Newtonian constitutive equation for interfaces under steady shear. We also introduce non-Newtonian material properties that control nonlinear and linear shear responses of an interfacial system. Combining flow field predictions from the new equation and experiments in a knife-edge flow geometry, we demonstrate that monolayers of DPPC – the primary constituent of mammalian cell walls and pulmonary surfactant – are shear-thinning at near-physiological surface packing over six decades of shear-rate.