Liquid Crystals

03 Oct 1994 12:01

In normal crystalline solids, molecules or atoms sit in fixed locations, arranged in a pattern which repeats itself over space. (Actually, the locations aren't quite fixed, because of thermal motion, but that's a bit of a side issue.) In normal liquids, the molecules are free to move about, and their distribution over space is not point-like but smeared and more or less uniform. Liquid crystals are states of matter intermediate between normal liquids and normal crystals. The simplest to envision are the nematic liquid crystals, where the molecules are long and rod-like; in their fluid phase, the direction of the rods is random; in the nematic phase, nearby rods are oriented along a common axis, the ``director'', but their positions are still random. (In chiral nematics, the director rotates as you move through the material, arranging the molecules in a helical pattern.) In cholesteric liquid crystals, the molecules are oriented along a common axis, but also lie in regular, parallel planes, each one slightly twisted with respect to its neighbors --- imagine a heap of lengths of picket-fence, piled one atop the other. (Cholesterol, surprisingly enough, has a cholesteric phase.) In smectics, the molecules again lie in parallel layers, though they are free to move about within each layer, but now are oriented perpendicular, or nearly so, to the layers. (There are many sub-varieties of smectics.) These are the principal sorts of liquid crystal, but literally dozens of distinct phases have been identified.