Self-similar shear-thickening behavior in CTAB/NaSal surfactant solutions

The e ect of salt concentration Cs on the critical shear rate γ̇c required for the onset of shear thickening and apparent relaxation time λ of the shear-thickened phase, has been investigated systematically for dilute CTAB/NaSal solutions. Experimental data suggest a self-similar behavior of γ̇c and λ as functions of Cs. Speci cally, γ̇c ∼ Cs whereas λ∼ Cs such that an e ective Weissenberg numberWe ≡ λγ̇ for the onset of the shear thickened phase is only weakly dependent on Cs. A procedure has been developed to collapse the apparent shear viscosity versus shear rate data obtained for various values of Cs into a single master curve. The e ect of Cs on the elastic modulus and mesh size of the shear-induced gel phase for di erent surfactant concentrations is discussed. 1 Electronic mail: [email protected] 1 ar X iv :0 70 6. 30 63 v2 [ ph ys ic s. fl udy n] 1 7 D ec 2 00 7 Experiments performed using di erent ow cells (Couette and cone-and-plate) show that γ̇c, λ and the maximum viscosity attained are geometry-independent. The elastic modulus of the gel phase inferred indirectly by employing simpli ed hydrodynamic instability analysis of a sheared geluid interface is in qualitative agreement with that predicted for an entangled phase of living polymers. A qualitative mechanism that combines the e ect of Cs on average micelle length and Debye parameter with shearinduced con gurational changes of rod-like micelles is proposed to rationalize the selfsimilarity of SIS formation.