In situ X-ray diffraction studies of a multilayered membrane fluid under confinement and shear

The structure of a fluid membrane system composed of surfactant-co-surfactant-oil-water mixtures has been investigated under confinement and shear conditions. Small angle x-ray scattering (SAXS) was employed with a second generation x-ray surface forces apparatus (XSFA-II) to study the time evolution of the orientational structure of the lamellar fluid under oscillatory shear. In a regime of relatively big confinement gap (∼800 μm) and small shear amplitude (∼40 μm), direct evidence of an "orientational phase separation" behavior, where a surface boundary layer adopts different orientation and separates from the bulk region, was observed for the first time. Under continuous shearing, the surface boundary layer grows in thickness and aligns towards a shear-favored (low friction) state while the bulk orientation remains unchanged. To further investigate the effects of surface confinement, we spatially mapped, in ∼1 μm sections, the orientation structure of the lamellar fluid sample confined between two glass surfaces using a micro-focused x-ray beam produced by a linear Bragg-Fresnel lens at the Advanced Photon Source. The data confirmed the expected trend that the smectic domains align progressively better with respect to the surface as they approach the surface.