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Polarizing microscope texture of a thin, liquid crystalline film. This highly nonuniform structure reflects spatial distortions in molecular orientation that occur in a nematic fluid, the simplest form of a liquid crystal. The thin nematic film (1- to 2-micrometers thick) is spread over the surface of an isotropic fluid (glycerine).
The upper surface of the film is free (in contact with air). In the nematic, the rod-like elongated molecules are free to move around but tend to be parallel to each other. The average direction of orientation is called the director. By placing the nematic film between glycerine and air, one creates director distortions in the vertical plane, as the nematic-air interface favors normal (perpendicular) orientation of the director and the glycerine-nematic interface favors tangential (parallel) orientation. Since the direction of alignment in the plane of the film is not fixed, the film exhibits numerous distortions, with the director changing from point to point. The interference colors of the film result from different director tilt and some variation in the thickness of the film. Very often, the director distortions collapse into "strings" of practically constant width, seen in the texture as parallel dark extinction bands. The dark bands mark the regions where the orientation of liquid crystal molecules is parallel to either the polarizer or analyzer.Image credit: Oleg Lavrentovich, Liquid Crystal Institute, Kent State University
