Polarization Microscope Image of Liquid Crystals
Shown here is the polarizing microscope texture of a thin, liquid crystalline film. Two centers with emerging dark brushes represent "boojum," point defects in the molecular orientation of the liquid crystal. The defects are formed at the surface of a thin film of a nematic fluid, the simplest form of a liquid crystal. The thin 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. Since the direction of alignment in the plane of the film is not fixed, the film exhibits distortions, with the director changing from point to point. The changing interference colors of the film result from different director tilt near the "cores" of the defects. The dark bands mark the regions where the orientation of liquid crystal molecules is parallel to either the polarizer or analyzer of the optical microscope. Since both defects have a radial director orientation near the core, each defect emanates four dark brushes. The term "boojum" entered the world of science from Lewis Carroll's "The Hunting of the Snark," thanks to Professor David Mermin, who was the first to introduce the word in his Physical Review Letters article in 1977. At the time, Mermin was studying topological defects similar to those presented here, in a superfluid helium 3 that, amazingly, is also a liquid crystal in many respects.
Image credit: Oleg Lavrentovich, Liquid Crystal Institute, Kent State University