Rheological behavior of flexible elongated micelles: temperature effect in an isotropic phase

Abstract

We present experimental results on the linear and non-linear rheology of elongated flexible micelles with salt. Steady shear experiments were carried out on aqueous surfactant solutions made of cetylpyridinium chloride and sodium salicylate in 0.5M brine. The isotropic phase of entangled micelles was investigated for total surfactant mass fraction φ between 4 and 14% and for temperatures ranging from 25 to 39°C. The viscosity curves show clearly four distinct domains: a Newtonian region (I) followed by a shear thinning part which starts above a critical shear rate γ̇c then an unstable zone (III) and finally a second shear thinning behavior (IV) at very high shear rates. The viscosity decrease in domain II can be related to the disentanglement of the micelles and the apparition of large scale anisotropic structures which grow and become more pronounced when the shear rate is increased. This phenomenon is characterized by a constant shear stress value σc in flow curves. A temperature dynamic phase diagram has been derived from the σc values.Measurements of steady shear viscosity by temperature sweeps at different shear rates indicate that the viscosity remains constant until a critical temperature is reached then falls abruptly. This behavior is only noticed for high enough shear rates corresponding to region IV.Domains I and II have been fitted using both Carreau model and a power law. The Newtonian viscosity η0 is described by an Arrhenius relationship. The critical shear rate γ̇c required to orient the micelles in the flow direction is found to increase with temperature. However, the exponent of the power law is independent of temperature. For each mass fraction, viscosity curves at various temperatures are well superposed on a master curve when appropriate reduced variables are used. Interpretation of unstable and shear thinning behaviors observed in the third and fourth parts, respectively, is in progress.