Rheological Properties of Shear-Induced Structure in CTAB/NaSal Aqueous Solution-Viscosity and Elasticity Change under Start-up Flows-

Abstract

Start-up shear flow was applied to CTAB/NaSal aqueous solutions which form wormlike micelles, and transient behaviors of viscosity and first normal stress difference were observed. In the transient shear flow, the shear-induced structure, SIS, was generated and strongly affected the flow properties. The maximum value of viscosity in the transient flow suddenly increased once the SIS appeared up to 40 times as high as the complex viscosity measured under oscillatory shear flow. The coefficient of first normal stress difference also increased by the appearance of SIS. Thus, SIS showed not only high viscosity but also strong elasticity. At low shear rates, the strain value at the viscosity maximum was smaller than that at the first normal stress difference maximum. With increasing shear rates, this difference in the two strains became smaller and dimmish when the flow instability was observed. Weissenberg number, a ratio of the first normal stress difference to the shear stress, was evaluated as a function of strain, which showed a sudden increase just before the flow-instability. These findings are influenced by the geometry used to generate shear flows. In the concentric cylindrical geometries, SIS was observed at shear rates lower than in the cone-plate and the parallel plate geometries.