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-) and sodium s alicylate (Na+, Sal-) diluted in 0.5 M NaCl-brine. Both semidilute and concentrated regimes of entangled micelles were investigated. Rheolog ical experiments were performed at ambient temperature (T = 25-degrees -C) for surfactant concentrations phi = 1 %-30 %. When submitted to a steady shear high enough (for shear rate gamma typically higher than 1 -10 s-1) the solutions of wormlike micelles exhibit a first-order isot ropic-to-nematic transition for all surfactant concentrations phi grea ter-than-or-equal-to 6 %. The transition is characterized by a true pl ateau in the shear rate dependence of the shear stress sigma (gamma). For gamma above the transition rate gamma(I/N), sigma remains constant at sigma(I/N). In the concentrated regime, the transition is clearly first-order. However, the first-order character weakens upon increasin g dilution, suggesting that at some critical concentration phi(c) it b ecomes second-order. Below phi(c), the transition ceases to occur : th e sigma(gamma)-behavior rather indicates a progressive and homogeneous orientation of the micelles throughout the sample. Moreover, in the t wo-phase domain (where both isotropic and nematic phases coexist) a ch aracteristic transient behavior of the shear stress sigma(t) measured at constant gamma > gamma(I/N) as a function of time has been observed and investigated in detail. In agreement with the picture of the firs t-order phase transition (in the domain of metastability), the transie nt behavior could be interpreted quantitatively in terms of nucleation and one-dimensional growth process. These results are finally compare d to recent predictions by Spenley, Cates and MacLeish (Ref. [7]) who described the nonlinear rheology of wormlike micelles in terms of mech anical instability of shear-banding type.