Textures and Defects
The unique optical properties of liquid crystals enable them to be used in a variety of applications. This section explains how these characteristics arise.
Liquid Crystal Textures
The term texture refers to the orientation of liquid crystal molecules in the vicinity of a surface. Each liquid crystal mesophase can form its own characteristic textures, which are useful in identification. We consider the nematic textures here, postponing discussion of cholesteric textures until the section on polymer stabilized cholesteric liquid crystals.
If mesogenic materials are confined between closely spaced plates with rubbed surfaces (as described above) and oriented with rubbing directions parallel, the entire liquid crystal sample can be oriented in a planar texture, as shown in the following diagram. Mesogens can also be oriented normal to a surface with the use of appropriate polymer films, or in the presence of an electric field applied normal to the surface, giving rise to the homeotropic texture, as illustrated below.
Defects Under the Microscope
The abrupt changes in brightness seen in the pictures from the previous section on birefringence signal a rapid change in director orientation in the vicinity of a line or point singularity known as a disclination. A disclination is a region where the director is undefined. The following is a diagram that shows the orientation of the director around a disclination.
Focal conic defects are responsible for many of the structures in smectic liquid crystals. The following is a TEM micrograph of a focal conic.
The diagram and micrographs in this section were provided by Professor Steven Hudson of the Macromolecular Science Department, CWRU.
|