Shear-induced structural transition in a lyotropic lamellar phase studied using small angle neutron and light scattering

We study the effects of shear flow on structures of the lamellar phase formed in a nonionic surfactant C₁₆E₇ (hepta(oxyethylene glycol)-n-hexadecylether)/D₂O system by using small angle neutron scattering (SANS) and small angle light scattering (SALS) in the range of shear rate 10^-3-30 s^-1. As the shear rate increases from 0.3 to 1 s^-1, the lamellar repeat distance for the 48 wt% sample is decreased significantly and discontinuously. With further increase in the shear rate, d increases through a sharp minimum at 1 s^-1. The minimum value does not depend on the surfactant concentration very much and is nearly equal to the thickness of the bilayers. At this shear rate (1 s^-1), the intensity of the polarized SALS is strongly enhanced in the small angle region in the vorticity direction. These results suggest that water layers are excluded by shear flow and that large concentration fluctuations on the μm scale are induced, which corresponds to local segregation into two regions; one of these has concentrated lamellar structures and the other is a water-rich region. As the shear rate increases further (to 3 s^-1), a broad diffraction peak appears in the vorticity direction in the polarized SALS whereas a four-lobe pattern is observed in the depolarized SALS. There results, together with the two-dimensional SANS pattern, suggest the formation of close-packed onions elongated along the flow direction at 3 s^-1. Comparison is made with the study of Nettesheim et al (2003 Langmuir 19 3618) who report the formation of multilamellar cylinders as intermediate structures, between lamellae and spherical onions.