These are nonisotropic materials — neither crystalline nor liquid — that are composed of long molecules parallel to each other in large clusters and that have properties intermediate between those of crystalline solids and liquids. It is estimated that 1 in every 200 organic compounds has the capability of being produced in the liquid crystal form.
There are three principal types of liquid crystals, based on the arrangement of the molecules. In the smectic type, the molecules are parallel with their ends in line, forming layers that are usually curved or distorted, but are still capable of movement over one another. In the nematic type, the molecules are essentially parallel, but there is no regular alignment of their ends. The cholesteric type is formed by optically active compounds that have the capability for molecular organizations of the nematic type.
Liquid crystals have some of the properties of liquids, such as fluidity, and some of the properties of crystals, such as optical aniso-tropy. A major use of liquid crystals is for digital displays, which consist of two sheets of glass separated by a sealed-in transparent liquid crystal material. The outer surface of the glass sheet is coated with a transparent conductive coating, with the viewing side coating etched into character-forming segments. A voltage applied between the two glass sheets disrupts the orderly arrangement of the molecules, thus darkening the liquid to form visible characters. Other typical applications of cholesteric liquid crystals are in skin thermography for tumor detection, in electronics for temperature mapping of circuits, and in nondestructive testing of laminates.
