Rheology, Kinematics, and Structure of Shear Banding Wormlike Micelle

Wormlike micelles (WLMs) are ubiquitously involved in different industrial processes and consumer products. The successful applications of WLMs are subject to various viscoelastic instabilities in flow, which are critically related to the flow-induced rearrangement of microstructure configuration. The fundamental mechanisms of these instabilities at the microscopic scale are still the subject of current research. This dissertation is focused on a specific type of viscoelastic instability, namely shear banding, which involves spontaneous development of distinct regions with widely differing shear rates in geometries approximating simple shear. The overall objective is to advance the understanding of the macroscopic rheological and flow properties of WLMs that display shear banding in both rheometric and complex flows, and to relate them to the underlying flow-microstructure interactions.;To fulfill the objectives of this dissertation, a combination of rheological, kinematic, and structural characterizations are made on several linear WLMs systems that have previously been reported to display shear banding. To avoid misidentifying shear thinning WLMs as shear banding, a rigorous, rheological model-free approach is developed and applied to high-resolution velocimetry data in Taylor-Couette flow, which allows for shear banding velocity profiles to be distinguished from shear thinning profiles in cases where this difference is indistinguishable by visual observation. As an initial step to explore the signatures of shear banding in more realistic processing flows, flow profiles of shear banding WLMs are characterized in both large-gap Taylor-Couette flow and for a two-dimensional extensional flow. At a microscopic scale, shear banding in WLMs has been hypothesized to arise from the influence of shear on the dynamics of micelle scission and reformation, which shifts the length distribution of micelle population containing both entangled and unentangled micelle strands. To test this hypothesis, a combination of rheology, velocimetry, and small angle neutron scattering (SANS) measurements was made with a series of shear banding WLMs with symmetrically varying average length at equilibrium. Experimental data show that a shift in the equilibrium micelle length distribution toward entangled strands leads to a narrowing of the instability window between critical shear rates due to an increase in the high shear viscosity. The flow-SANS measurement not only shows drastically different degrees of micelle alignment within the high and the low shear bands, but also implies flow-enhanced micelle scission and a decrease in average micelle length above the upper critical shear rate for shear banding. The flow and structural measurements are quantitatively compared to predictions from the Vasquez-Cook-McKinley (VCM) constitutive equation, which enables explicit predictions of the relative populations of unentangled micelles and entangled strands both at equilibrium and in flow. Significant discrepancies between the experimental data and the model predictions are found, which is believed to arise primarily from the model's oversimplification of the constitutive behavior and length distribution of WLMs.