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-surfacta nt-oil-water mixtures has been investigated under confinement and shear con ditions. Small angle x-ray scattering (SAXS) was employed with a second gen eration x-ray surface forces apparatus (XSFA-II) to study the time evolutio n of the orientational structure of the lamellar fluid tinder oscillatory s hear. In a regime of relatively big confinement gap (similar to 800 mum) an d small shear amplitude (similar to 40 mum), direct evidence of an "orienta tional phase separation" behavior, where a surface boundary layer adopts di fferent orientation and separates from the bulk region, was observed for th e first time. Under continuous shearing, the surface boundary layer grows i n thickness and aligns towards a shear-favored (low friction) state while t he bulk orientation remains unchanged. To further investigate the effects o f surface confinement, we spatially mapped, in similar to1 mum sections, th e orientation structure of the lamellar fluid sample confined between two g lass surfaces using a micro-focused x-ray beam produced by a linear Brag-Fr esnel lens at the Advanced Photon Source. The data confirmed the expected t rend that the smectic domains align progressively better with respect to th e surface as they approach the surface.