Shear rheology of viscous interfaces coupled to bulk flow

Abstract

We present a novel interfacial constitutive equation for the material at inter- faces that takes a Newtonian functional form but allows shear viscosity to be a function of the local shear rate. We show excellent agreement between measured interfacial non-Newtonian responses and flows predicted at interface. We highlight that DPPC behaves as a shear-thinning material across physiologically relevant shear rates and surface packings, introducing material properties that include consistency and shear-thinning indices. The results and theory introduced in this thesis make significant strides towards unifying the disparate rheological behavior of DPPC films reported in the literature.

This thesis describes the Newtonian (linear) and non-Newtonian (nonlinear) interfacial shear rheology of phospholipid dipalmitoylphosphotidylcholine (DPPC) deformed in a direct-shear contact knife-edge ow geometry. DPPC plays a crucial role in human health and physiology. Monolayers of DPPC regulate the surface tension during breathing and control the ow of the liquid lining the alveolus in human lungs. DPPC also assembles into bilayers that constitute the cell membrane, imparting fluidity that facilitates nutrient and gas exchange in the living cell.

This study focuses on modeling and measuring the viscous response of DPPC and impact of the inherently coupled ows in the bulk and interfacial phases on rheology. Depending on the surface packing and corresponding reduction in surface tension, monolayers of DPPC exhibit a wide range of phase morphologies. Upon shearing the monolayer, its viscous response varies from that of an essentially inviscid film at low surface packing, to that of a highly viscous non-Newtonain (shear thinning) film when the packing is dense. The thesis highlights that significant primary and secondary fl ows are produced in the bulk phase when a finitely viscous film is sheared at the interface. The more viscous the film, the stronger the driven bulk ow. This nonlinear coupling is measured across hydrodynamic regimes straddling the Stokes ow limit to regimes where the inertia of the bulk phase is significant.

At low and intermediate surface packing, it was found that DPPC films behave Newtonian (single shear viscosity across shear rates for a given surface packing) and the traditional Boussinesq-Scriven interfacial stress model could capture the experimentally measured viscous interfacial responses and the driven bulk ows. However, at higher surface packings, the nonlinear coupling could not account for the departures from the predicted Newtonian response at the interface. This motivated the development of a a generalized two dimensional (2-D) theoretical framework that could predict observed shear-rate dependent yield stress-like behavior, shear thinning, and Newtonian responses of DPPC films.