Isotropic-to-nematic transition in wormlike micelles under shear

We report on the linear and nonlinear rheology of surfactant solutions of elongated wormlike micelles. The surfactant solutions placed under scrutiny are made of cetylpyridinium chloride (CP+, Cl-j and sodium sahcylate (Na+, Sal-) diluted in 0.5M Nacl~bfine. Both semidilute and concentrated regimes of entangled micelles were investigated. Rheological experiments were performed at ambient temperature (T = 25 °C ) for surfactant concentrations 4 %-30 %. When submitted to a steady shear high enough (for shear rate j typically higher than I-lo s-') the solutions of wormlike micelles exhibit a first~order isotropic-to-nematic transition for all suriactant concentrations ~b m 6 %. The transition is characterized by a true plateau in the shear rate dependence of the shear stress al #). For # above the transition rate #jj~, « remains constant at «jj~. In the concentrated regime, the transition is clearly first-order. However, the first-order character weakens upon increasing dilution, suggesting that at some critical concentration 4~ it becomes second-order. Below ~b~, the transition ceases to occur the «(#)-behavior rather indicates a progressive and homogeneous orientation of the micelles throughout the sample. Moreover, in the two-phase domain (where both isotropic and nematic phases coexist) a characteristic transient behavior of the shear stress «(t) measured at constant # ~ #j~ as a function of time has been observed and investigated in detail. In agreement with the picture of the first-order phase transition (in the domain of metastability), the transient behavior could be interpreted quantitatively in terms of nucleation and one-dimensional growth