Cone angle effects, radial pressure profile, and second normal stress difference for shear-thickening wormlike micelles

Abstract

Many dilute surfactant solutions containing long, thin, wormlike micelles undergo a shear-thickening transition at which the apparent viscosity jumps to a much higher value due to the formation of shear-induced structures. However, the behavior of the normal-stress differences N1 and N2 has not been studied at the low-shear rates and shear stresses (<0.1 Pa) at which shear thickening occurs in most of the commonly studied systems, such as aqueous solutions of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSS). For dilute equimolar solutions of CTAB/NaSS, the validity of measurements of the shear flow properties η, N1 and N2 via cone-and-plate rheometry is assessed by systematically varying the cone angle, and by measuring the pressure profile during cone-and-plate flows. The dependence on the cone angle of the apparent viscosity value is consistent with a stratified fluid structure, as shown to occur in Couette rheometers by Hu and co-workers [Hu, Y. T. et al., J. Rheol. 42, 1209–1226 (199...