Shear-induced ordering of lamellar and gyroid structures observed in a nonionic surfactant/water system

The shear-induced ordering of lamellar and gyroid structures of a nonionic surfactant C₁₆E₇/D₂O system in a Couette shear cell (0.001 < γ̇ < 10 s^-1, γ̇: shear rate) has been investigated by using a small angle neutron scattering technique. In the lamellar phase, the steady shear flow having γ̇ > 0.01 s^-1 suppresses undulation fluctuations of lamellae (Maxwell effect). This suppression of fluctuations brings two effects; 1) shear-induced lamellae ordering toward a parallel orientation and 2) obstruction of a lamellar→gyroid transition. It is quite interesting to note that there is a characteristic shear rate range (0.01 < γ̇ < 0.3 s^-1), where both effects take place. We have also investigated the shear effects on the gyroid phase. Below the characteristic shear rate range, the gyroid structure keeps three-dimensional network lattice, while above the characteristic shear rate range, the gyroid structure transforms to the parallel orientation lamellae (shear-induced gyroid-lamellar transition). Thus the shear flow having the characteristic shear rate plays very important roles in shear ordering phenomena.