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| Schlieren Texture of a Nematic Film |
Schlieren texture of a nematic film with surface point defects (boojums). This picture was taken under a polarization microscope with polarizer and analyzer crossed. From every point defect emerge four dark brushes. For these directions the director is parallel either to the polarizer or to the analyzer. The colors are newton colors of thin films and depend on the thickness of the sample. Point defects can only exist in pairs. One can see two types of boojuns with "opposite sign of topological charge," one type with yellow and red brushes, the other less colorful. The difference in appearance is due to different core structures for these defects of different "charge."
About Liquid Crystals:
ReplyDeleteNormally, we consider matter to have three distinct states: solid, liquid and gas. However, there are states of matter which do not meet the necessary requirements of any of these three categories. For example, a substances such as mayonnaise is somewhere between a liquid and a solid.
Liquid crystals are not quite liquid and not quite solid. Physically, they are observed to flow like liquids, but they have some properties of crystalline solids. Liquid crystals can be considered crystals that have lost some or all of their positional order, while maintaining full orientational order.
Nematics are polarizable rod-like organic molecules on the order of 20 Angstroms in length. Because of their tendency to organize themselves in a parallel fashion, they demonstrate interesting and useful optical properties. Digital watches for example, functioned using nematic liquid crystals. Today, many more useful and interesting properties of nematics are known and exploited.
Smectic liquid crystals are different from nematics in that they have one more degree of orientational order than do the nematics. Smectics generally form layers within which there is a loss of positional order, while orientational order is still preserved. There are several different categories to describe smectics. The two best known of these are Smectic A, in which the molecules align perpendicular to the layer planes, and Smectic C, where the alignment of the molecules is at some arbitrary angle to the normal.