Rheo-NMR of Wormlike Micelles

Abstract

In this work, experimental evidence that reveals the phenomena of shear banding, shear-induced nematic phase transition, velocity fluctuations and wall slip is presented. This evidence is based on Rheo-NMR imaging and spectroscopy in a micelle system of CPyCl /NaSal at 10 % w/v in 0.5 M NaCl, this concentration being far from the isotropic-to-nematic phase transition. Time averaged experiments provide evidence for shear banding, velocity fluctuations and wall slip. In these measurements, however the time scale of the velocity fluctuations was not provided. Moreover each experiment takes around 5 hours to perform. In contrast, real time fast velocimetry experiments were carried out, these measurements provide information about shear banding, velocity fluctuation, wall slip and the time scales of velocity fluctuations, and have the added advantage that measurements were around four orders of magnitude quicker than measurements using the time averaged method. However, while the real time experiment provided more insight on fluctuations, the spatial resolution was exchanged for speed, giving lower resolved velocity profiles. Using conventional Rheology, start-up experiments showed shear stress oscillations, and in this thesis these shear stress oscillations were associated with velocity fluctuations obtained using the time averaged measurements. A model is proposed which is based on an "experimental" flow curve with a Newtonian branch at low shear rates described by the Jonhson-Segalman model up to the onset of the shear stress plateau, and followed by two nearly "Newtonian" regions separated by unstable regions. Using both 1H and 2H NMR spectroscopy, molecular properties were associated with the shear banding effect. The NMR spectra showed that the aliphatic chain mobility of the surfactant molecules in the wormlike micelle system was shear-dependent and using spatially resolved spectroscopy the change of the aliphatic chain mobility follows the velocity band pattern and suggests the lever rule. One of the principal findings using 2H NMR spectroscopy was evidence of a shear-induced nematic phase transition at shear rates above γc. The shear banding phenomenon is very sensitive to variations in the surfactant and salt concentration. Shear banding dynamics have been found to change drastically in spatial scale and time scale on changing the batch of surfactant and salt used to prepare the samples.