Structure and viscoelasticity of wormlike micellar solutions under steady shear flows

Structure and viscoelastic properties of aqueous wormlike micellar solutions of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) were investigated by rheological measurements, flow visualization using small dispersed particles and small-angle neutron scattering under steady shear flows. The concentrations of CTAB and NaSal were respectively fixed at 0.15 mol L ^-1, so as to show Maxwell type relaxation behavior in the linear mechanical response region. According to the stress response of steady shear flow measurements, shear rate regions were classified into three regions, i.e. (A) a Newtonian region (for γ̇ < γ̇_l), (B) a stress plateau region (for γ_l̇< γ̇ < γ_ḣ), which had a characteristic behavior of the shear-banding, and (C) a stress turnup region (for γ̇> γ̇_h) where shear stress increased with γ̇ again. It was revealed that the fraction of the nematic phase flowing at γ̇_h was proportional to the shear rate in the region B. The first normal stress difference, N_l, was approximately proportional to the shear rate in the region B, while it gradually decreased with increasing shear rates in the region C.