The rheological behavior of micellar solutions is analyzed under nonhomogeneous velocity and stress flow conditions. The framework is based on the extended irreversible thermodynamics and the transient network formulation coupled to the underlying kinetics embodying two relevant processes: formation of wormlike chains from a free micellar solution through a thermally activated process and their flow induced degradation. The second kinetic process consists in the formation of entanglements from the free wormlike chains and their flow-induced breakage. These processes are modeled in a coupled kinetic scheme constituted by a set of reversible kinetic equations describing the evolution in average of the three microstates (free short rod-like micelles, free wormlike chains, and entangled wormlike chains) that reflect the complexity of macromolecular interactions. The predictions of the shear stress and first normal stress difference as a function of shear-rate under banded flow are in good agreement with experimental data.

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In this contribution a methodology to compute and classify shear-induced structural and phase transitions in surfactant/water mixtures from rheological measurements is presented. Non-linear rheological experiments, considering variations in surfactant concentration and temperature, are analyzed. In particular, the parameters of the BMP (Bautista-Manero-Puig) model, obtained from the fitting of the shear stress versus shear rate data, which are functions of surfactant concentration and temperature, allow classifying structural and phase transition boundaries. To test this methodology, we consider the analysis of the shear-induced structural and phase transitions of two micellar systems, cetyltrimethylammonium tosylate (CTAT)/water as a function of CTAT concentrations and Pluronics P103/water as a function of temperature. We found that the CTAT/water system presents a first-order phase transition at 30 C, and around 31 to 32 wt.% from isotropic to nematic phases, whereas a 20 wt.% Pluronics P103 aqueous micellar solution has two second-order (structural) phase transitions, one from spherical to cylindrical micelles at 33.1 C, and another one from cylindrical micelles to a nematic phase at 35.8 C and one first-order phase transition around 37.9 C at high shear rates near to the cloud point previously reported. The proposed methodology is also able to identify the instability regions where the wormlike micelles are broken, producing the typical shear banding behavior.

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