Shear-Induced Structures in Concentrated Surfactant Micellar Phases

Abstract

Constant and oscillatory Couette shear flow have been used in combination with small-angle neutron scattering to observe the shear-induced ordering in concentrated surfactant micellar phases. For the lamellar phase of hexaethylene glycol monohexadecyl ether, C16 E 6, two distinct lamellae orientations have been identified. At low shear gradients the lamellae are ordered parallel to the flow–vorticity plane, whereas at higher shear gradients the lamellae order parallel to the flow-shear gradient plane, corresponding to a rotation through 90° of the axis of orientation. At intermediate values of constant shear and for oscillatory shear, both lamellae orientations are simultaneously observed for the first time in a surfactant lamellar phase. For the lamellar phase, a dispersion of the binary surfactant mixtures of dioleyl cationic and 2-ethyl hexaglycerol monoether surfactants, a high degree of alignment, in the direction parallel to the flow–vorticity plane, is observed at zero and low shear. With time, during the application of a shear gradient of 25 s−1, the lamellar phase transforms to a highly ordered solution of monodisperse multilamellar vesicles.