Structure under confinement in a smectic-A and lyotropic surfactant hexagonal phase

Simple and complex fluids undergo significant changes in their structural and rheological behaviour as they are progressively confined between narrowing walls. The understanding of these new fluid properties is of fundamental interest in applications ranging from thin film lubrication in micromachines to catalysis. The X-Ray surface forces apparatus (XSFA) is capable of non-destructive imaging of the structure of confined complex fluid systems, on length scales ranging from nanometers to several tens of microns. In this article, recent work is presented on studies of confined complex fluids with the XSFA. Confinement can significantly align the liquid crystalline smectic phase. The degree of orientation depends critically on the compliance of the confining surface: "soft surfaces" exhibit a critical gap for alignment of 3.4 ~tm, while "hard surfaces" do not exhibit gap dependent alignment. Shear-induced orientation has been shown to dominate over confinement induced alignment with the systems studied. In the final part of the paper, we discuss an important new development where it is demonstrated that the XSFA may be used as an alignment tool for lyotropic liquid crystals.