Effects of Oscillatory Shear on the Orientation of the Inverse Bicontinuous Cubic Phase in a Nonionic Surfactant/Water System

The bicontinuous inverse cubic phase (V₂ phase) formed in amphiphilic systems consists of bilayer networks with a long-range order. We have investigated effects of oscillatory shear on the orientation of the V₂ phase with space group <i>Ia</i>3<i>d</i> formed in a nonionic surfactant (C₁₂E₂)/water system by using simultaneous measurements of rheology/small-angle X-ray scattering. It is shown that grain refining occurs by applying the large amplitude oscillatory shear (LAOS) with a strain amplitude (γ₀) of ∼20, which gives the ratio of the loss modulus (<i>G</i>″) to the storage modulus (<i>G</i>′) (<i>G</i>″/<i>G</i>′ = tan δ) of ∼100. On the other hand, orientation of the cubic lattice occurs when the small amplitude (γ₀ ≈ 0.0004) oscillatory shear (SAOS) in the linear regime is applied to the sample just after the LAOS. Interestingly, the orientation is strongly enhanced by the “medium amplitude” (γ₀ ≈ 0.05) oscillatory shear (“MAOS”) after the SAOS. When the MAOS is applied before applying the LAOS, orientation to a particular direction is not observed, indicating that the grain refining process by the LAOS is necessary for the orientation during the MAOS. The results of additional experiments show that the shear sequence “LAOS–MAOS” is effective for the orientation of the cubic lattice. When the LAOS and MAOS are applied to the sample alternatively, grain refining and orientation occur during the LAOS and MAOS, respectively, indicating reversibility of the orientation. It is shown that (i) the degree of the orientation is dependent on γ₀ and the frequency (ω) of the MAOS and (ii) relatively higher orientation can be obtained for the combination of γ₀ and ω, which gives tan δ = 2–3. The lattice constant does not change throughout all the shearing processes and is equal to that before shearing within the experimental errors, indicating that the shear melting does not occur. These results suggest a possibility to control the orientation of the cubic lattice only by changing the conditions of oscillatory shear without using the epitaxial transition from other anisotropic phases, such as the hexagonal and lamellar phases