Experimental and numerical investigations of the non-linear rheological properties of viscoelastic surfactant solutions: application and failing of the one-mode Giesekus model

Abstract

In a series of experiments, we investigated the non-linear rheological properties of aqueous solutions of entangled wormlike micelles (WLMs) in steady-state shear flow and in large amplitude oscillating shear (LAOS) experiments. On grounds of their monoexponential stress relaxation properties, we studied semi-dilute solutions of the cationic surfactants cetylpyridinium chloride (CPyCl) or cetyltrimethylammonium bromide (CTAB) after addition of different amounts of sodium salicylate. The rheological data of these networks of WLMs were systematically compared with the numerically calculated results of the one-mode Giesekus constitutive equation. It turned out that the viscous resistance and the first normal stress difference, measured in steady-state shear flow, start-up, and relaxation experiments, were accurately predicted by the one-mode Giesekus model. In rheological tests, where we applied large oscillating shear amplitudes (LAOS), the transient shear stress could also approximately be described by means of the Giesekus model. The non-linear oscillating first normal stress difference, however, showed large deviations in respect to the theoretical predictions. These discrepancies between different rheological experiments, which we observed in oscillating and stationary flow, pointed to the existence of flow instabilities, which occurred in the LAOS regime. These, more complicated rheological processes, were induced by shear-banding and/or the presence of flow-induced phase transitions, which can occur in oscillatory and stationary shear. The non-linear phenomena, discussed in this article, are of general importance, and they can be equally observed in entangled solutions of flexible macromolecules.