Transient and steady state Rheo-NMR of shear banding wormlike micelles

Abstract

Over many years, the combination of nuclear magnetic resonance (NMR) techniques with rheometry, referred to as Rheo-NMR has been used to study materials under shear noninvasively. Rheo-NMR methods can provide valuable information on the rheological responses of materials or their behavior by temporally and spatially resolved mapping of the flow field. In this thesis, 1D velocity profiles across the fluid gap of a Couette shear cell are recorded using Rheo-NMR velocimetry to investigate the wormlike micelles (WLMs) surfactant system under transient and steady state flow conditions. The WLM system was a solution of 6 wt. % cetylpyridinium chloride (CPCl) and sodium salicylate (NaSal) in 0.5 M NaCl brine which is well-known for its ability to exhibit a mechanical response during flow known as shear banding. The shear banding phenomena is simply defined as the splitting of the flow into two macroscopic layers, a high and low shear band bearing different viscosities and local shear rates. Elastic instabilities are well known to develop in the unstable high shear band and manifest as fluctuations in the 1D measurements. Recently, it has been suggested that 1D velocimetry alone cannot reveal information about those observed fluctuations in terms of a sequence of elastic instabilities and 2D or 3D measurements are required. In this thesis, new Rheo-NMR equipment and quantitative analysis are used to characterize those fluctuations and show that 1D velocity measurements still have the potential to provide valuable information about 3D flows. Transient and steady state shear banding was observed for a range of shear rates across the stress plateau and the impact of several flow protocols were studied. The evolution of the high, low, and true shear rates, as well as interface position with time after shear startup was used to evaluate changes in the kinetics of shear band formation as a function of applied shear rate and flow protocol. Ultimately, these results will help in understanding the correlation between the macroscopic flow field and the microscopic structure and dynamics of WLMs and can also be a way to gain information about the presence and the dynamic of secondary flow without the need of a 3D measurement.