Effects of shear flow on structures of lamellar phase in a nonionic surfactant/water system

Effects of shear flow on the structure of a lamellar phase in C16E7 (hepta(oxyethylene glycol)-n-hexadecylether)/water system (4055 wt% Of C16E7) at 70degreesC are studied by using small-angle neutron scattering (SANS), small-angle light scattering (SALS), and shear stress-shear rate relationships. The repeat distance takes a deep minimum (referred to as d*) at the shear rate 0.1-1 s(-1). As the concentration of C16E7 decreases, the repeat distance at rest increases whereas d* remains almost constant and nearly equal to the thickness of bilayers obtained from the line shape analysis of small angle X-ray scattering at rest. These results suggest that the water layer is excluded by shear flow and that the lamellar phase segregates into surfactant-rich and water-rich regions. Although macroscopic phase separation does not occur, SALS intensity takes a maximum at the shear rate giving d*, which is consistent with the SANS results. Mechanism for the decrease in the repeat distance is discussed in relation to the change in the size of the lamellar domains.