Rheological behavior of flexible elongated micelles: temperature effect inan isotropic phase

We present experimental results on the linear and non-linear rheology of el ongated flexible micelles with salt. Steady shear experiments were carried out on aqueous surfactant solutions made of cetylpyridinium chloride and so dium salicylate in 0.5 M brine. The isotropic phase of entangled micelles w as investigated for total surfactant mass fraction phi between 4 and 14% an d for temperatures ranging from 25 to 39 degrees C. The viscosity curves sh ow clearly four distinct domains: a Newtonian region (I) followed by a shea r thinning part which starts above a critical shear rate (gamma) over dot(c ) 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 r elated to the disentanglement of the micelles and the apparition of large s cale anisotropic structures which grow and become more pronounced when the shear rare is increased. This phenomenon is characterized by a constant she ar stress value sigma(c) in flow curves. A temperature dynamic phase diagra m has been derived from the sigma(c) values. Measurements of steady shear viscosity by temperature sweeps at different s hear rates indicate that the viscosity remains constant until a critical te mperature is reached then falls abruptly. This behavior is only noticed for high enough shear fates corresponding to region IV. Domains I and II have been fitted using both Carreau model and a power law. The Newtonian viscosity eta(0) is described by an Arrhenius relationship. The critical shear rate (gamma) over dot(c) 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 frac tion, viscosity curves at various temperatures are well superposed on a mas ter curve when appropriate reduced variables are used. Interpretation of un stable and shear thinning behaviors observed in the third and fourth parts, respectively, is in progress. (C) 1998 Elsevier Science B.V. All rights re served.