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

The effect of salt concentration Cs on the critical shear rate γ c required for the onset of shear thickening and apparent relaxation time λ of the shear-thickened phase has been investigated systematically for dilute Cetyl-trimethylammonium bromide/sodium salicylate solutions. Experimental data suggest a self-similar behavior of γ c and λ as functions of Cs. Specifically, γ c ∼ Cs -6 whereas λ∼ Cs6 such that an effective Weissenberg number Weλ γ for 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 effect of Cs on the elastic modulus and mesh size of the shear-induced gel phase for different surfactant concentrations is discussed. Experiments performed using different flow cells (Couette and cone-and-plate) show that γ c, λ and the maximum viscosity attained are geometry independent. The elastic modulus of the gel phase inferred indirectly by employing simplified hydrodynamic instability analysis of a sheared gel-fluid interface is in qualitative agreement with that predicted for an entangled phase of living polymers. A qualitative mechanism that combines the effect of Cs on average micelle length and Debye parameter with shear-induced configurational changes of rod-like micelles is proposed to rationalize the self-similarity of shear induced structure formation.