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

TL;DR

This study investigates how salt concentration influences shear-thickening behavior in CTAB/NaSal solutions, revealing self-similar scaling laws and developing a master curve for viscosity data, with implications for understanding gel phase properties.

Contribution

The paper uncovers self-similar scaling laws for shear-thickening onset and relaxation time in surfactant solutions and introduces a method to collapse viscosity data into a universal master curve.

Findings

Critical shear rate scales as Cs^(-6).

Relaxation time scales as Cs^(6).

Shear-thickening behavior is geometry-independent.

Abstract

The effect of salt concentration Cs on the critical shear rate required for the onset of shear thickening and apparent relaxation time of the shear-thickened phase, has been investigated systematically for dilute CTAB/NaSal solutions. Experimental data suggest a self-similar behavior of the critical shear rate and relaxation time as functions of Cs. Specifically, the former ~ Cs^(-6) whereas the latter ~ Cs^(6) such that an effective Weissenberg number 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 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 the critical shear rate, relaxation time 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 SIS formation.