ABSTRACT
In the examples of the previous chapter we assumed that the surfaces impose uniform director alignment in the entire sample. Uniform alignment results in uniform pictures when observing the sample between crossed polarizers. For example, uniaxial samples with uniform homeotropic alignment would appear black between crossed polarizers, whereas those with uniform planar alignment appear bright when the directions of the crossed polarizers do not coincide with the optical axis. This is not the case for chiral materials, where the director has a tendency of forming a helical structure, which competes with the uniform surface alignment and results in defects. In addition, equivalent conditions for different director structures or impurities make uniform alignment exceptional and the director field is inhomogeneous unless special treatment is applied. Because the director field determines the local optical properties, we usually observe a wide variety of visual patterns. These patterns are due almost entirely to the defect structures that occur in the long-range molecular order of the anisotropic fluids. Indeed, historically the underlying structures of the nematic and smectic-A liquid crystal phases were proposed based on the stable structures of the defects that characterize these phases.
