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.

The Method of Harmonic Balance for the Giesekus Model under Oscillatory Shear

“…comprises contributions from the polymer (σ p ), and Newtonian solvent, σ s = η s γ, where γ is the deformation tensor and η s is solvent viscosity. Subsequently, it has been applied to wormlike micellar surfactant solutions [62,63,64,65,15], protein dispersions [66,67], concentrated dispersions [68], etc.…”

First-Order and Third-Order Nonlinearities from Medium-Amplitude Oscillatory Shearing of Hydrogen-Bonded Polymers and Other Viscoelastic Materials

“…28 A different kind of separation leads to the Giesekus model, 34,35 which may be viewed as a dumbbell model with anisotropic drag. 36 Here, the material's total stress response is thought to be composed of the stress contribution of the polymer on the one hand and that of the solvent on the other, where the coupling of orientation and flow is an adjustable parameter within this model. Other polymer-related models start by considering orientational distribution functions of rigid dumbbells or chains thereof.…”

“…Other approaches relying on time-deformation separability , exploit the damping function or the molecular stress function (MSF). , Such a separability is widely but by no means universally applicable . A different kind of separation leads to the Giesekus model, , which may be viewed as a dumbbell model with anisotropic drag . Here, the material’s total stress response is thought to be composed of the stress contribution of the polymer on the one hand and that of the solvent on the other, where the coupling of orientation and flow is an adjustable parameter within this model.…”

Synthesis of Carboxymethyl Flaxseed Gum and Study of Nonlinear Rheological Properties of Its Solutions

“…The LAOS represents the real-life processing and application conditions more realistically [15]. The most common method of quantifying LAOS tests and analyzing the higher harmonics is Fourier transform (FT) rheology [16][17][18]. In addition to FT rheology, the framework of Chebyshev stress decomposition was proposed by Ewoldt [19] to analyze the nonlinear oscillatory response and obtain meaningful physical parameters.…”