Linear viscoelasticity of salt-free wormlike micellar solutions

We review recent results on the viscoelasticity of salt-free wormlike micellar solutions of a series of surfactants with same cation entity but with counterions of different lipophilicity. One observes a sharp crossover between a dilute regime of weak micellar growth and an onset of rapid micellar growth at a well-defined surfactant concentration. This concentration strongly depends on the micellar ionisation degree, which can be determined by light scattering experiments. The regime of rapid micellar growth is characterized by a fast increase of the viscoelastic relaxation time with surfactant concentration, which is then followed by a maximum assumed to be caused by intermicellar branching. Such a behaviour was first discovered by Hoffmann et al. back in the 1980s, then was explained by a model proposed by MacKintosh et al. We present a quantitative comparison between the experimental findings for the crossover transition of the zero-shear viscosity and the predictions of this model.