The effects of salts on the rheological characteristics of a drag-reducing cationic surfactant solution with shear-induced micellar structures

Abstract

The effect of the counterion salt sodium salicylate (Nasal) on the transient rheological properties of a drag-reducing surfactant system tris (2-hydroxyethyl) tallowalkyl ammonium acetate (TTAA) has been studied with both rheometric and rheo-optical methods. Three types of transient behavior for N1 and viscosity were identified in 5 mM TTAA solutions depending on the counterion concentration: induction and growth (below equimolar concentration); overshoot and growth (above equimolar concentration); and overshoot then plateau (at high concentrations of Nasal). The transient flow birefringence and orientation angle show trends similar to those of the viscosity and N1. The second type of transient behavior suggests a two-stage alignment and shear thickening process. The SIS buildup time from the quiescent state, the rebuilding time after a strong preshear, and the relaxation time were also obtained from N1 measurements, and show a maximum around equimolar conditions. The initial N1 and viscosity immediately after the flow startup, on the other hand, show a maximum around a ratio of 2.5 to 3 Nasal/TTAA. For solutions with a Nasal concentration in the ratio 1.5 to 3, the steady state values of N1 and viscosity do not show much variation with Nasal concentration over the shear rate range covered, however. The effect of an addition of sodium chloride (NaCI) to an equimolar Nasal/TTAA solution on the characteristic times and steady state values was also quantified. These rheological results provide us with tools to determine the optimal concentration ratio for practical drag reduction applications.