Predicting and measuring shear rheology of soft interfaces

Measuring the non-Newtonian shear responses of monomolecular films or macromolecules like proteins at fluid-fluid interfaces has been limited by using linear (Newtonian) constitutive equations to determine rheological properties. Predicting this nonlinear behavior is integral to comprehending many biophysical processes such as breathing and in the pathology of diseases such as Alzheimer’s, which are marked by protein denaturation in vivo. We present a generalized 2-D constitutive equation for interfaces under steady shear with the interfacial shear viscosity generalized to be a function the imposed shear-rate. We 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 monomolecular films 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. Also, the role of interfacial rheology of an adsorbed protein film of insulin in the process of denaturation and subsequent amyloid fibril formation at the air/water interface is delineated.