Shear-induced structural transition in the lamellar phase of the C16E7/D2O system. Time evolution of small-angle neutron scattering at a constant shear rate

The time evolution of small-angle neutron scattering is measured for the lamellar phase of a nonionic surfactant C16H33(OC2H4)7OH (C16E7) in D2O at 48 wt% at 343 K under shear flow. At the shear rates of 0.3, 1 and 3 s−1, a new diffraction peak appears at higher q [where q = (4π/λ)sin θ, and λ and 2θ are the wavelength of the neutron beam and the scattering angle, respectively] about 1–2 h after applying shear flow and coexists with the initial diffraction peak. The coexistence of two peaks continues even after 5 h at 0.3 s−1 whereas at 1 and 3 s−1 the peak at lower q disappears after about 3 h. These results indicate that the repeat distance decreases discontinuously and so suggest some sort of transition. A plot of the repeat distance after 5 h versus shear rate shows a minimum at 1 s−1, which is in good agreement with our previous results obtained by increasing the shear rate stepwise.