Molecular Orientation in Large-Amplitude Oscillatory Shear (LAOS) of Complex Fluids

Abstract

Polymer rheological properties are important to modern manufacturing, and though polymers have been studied for decades, we still do not fully understand how polymers flow. Subjecting a polymeric liquid to oscillatory shear flow is a common method for probing polymer rheology. Oscillatory shear flow provides rheologists with necessary information on the dynamic behavior of polymers. This thesis aims to deepen our understanding of polymer flow by generating a comprehensive modernization of molecular theories that predict polymer behavior during oscillatory shear. Additionally, this work provides a detailed investigation of rigid dumbbell macromolecular theory by dissecting the orientation distribution function to find the pieces that contribute to important rheological properties like the shear stress response and the normal stress differences. Orientation is the source of all rheological property predictions for rigid dumbbell theory. Thus, this thesis also provides a comprehensive experimental analysis of macromolecular orientation. Specifically, light scattering measurements are conducted on a wormlike micelle solution that is simultaneously subjected to large-amplitude oscillatory shear. These light scattering measurements provide information on micelle orientation throughout an oscillatory shear cycle. We compare these experimental orientation results with the orientation predictions of rigid dumbbell theory. We find that rigid dumbbell theory cannot capture all aspects of wormlike micelle rheology during large-amplitude oscillatory shear (LAOS) flow. Rigid dumbbell theory is especially incapable of predicting scattering caused by concentration fluctuations arising from micelle breakup, when the flow changes direction. Additionally, rigid dumbbell theory predicts orientations that are entirely out-of-phase with the measured orientations, such that there are no times during an oscillatory shear cycle when rigid dumbbell theory and orientation measurements match. We complete this study by blending different light scattering theories to formulate a descriptive model for our results.