Liquid crystals are sometimes referred to as the "fourth state" of matter following gases, liquids, and solids. The individual molecules flow past each other as they do in a liquid but tend to be oriented in the same manner, like the molecular arrangement in a solid crystal. However, the molecular forces maintaining the liquid crystal state are sufficiently weak that it can be maintained only over a limited range of temperatures and pressures. If the liquid crystal substance becomes too cold, it reverts to an ordinary solid. If it becomes too hot, it reverts to an ordinary liquid. It is also easily affected by changes in mechanical stress, electromagnetic fields, and chemical environment. Although it displays characteristics of both a crystal and a liquid, the liquid crystal state also has properties that are unique.
Not just an artificial contrivance, this phase occurs naturally and is found in biological systems, organizing and condensing processes such as the formation of chromosome and cellular membranes.
Liquid crystal behavior was first observed between 1850 and 1888 by researchers in chemistry, biology, medicine and physics. They observed that the optical properties of certain materials changed discontinuously with increasing temperatures. During 1922 in Paris, France, Georges Freidel suggested the classification scheme, which is used today with different phases of liquid crystals called smectic, nematic, and cholesteric.
Since the 1960s, a variety of applications incorporating liquid crystals have been developed, ranging from "mood rings" to simple thermometers and complex display screens for computers. The future promises to replace cathode ray tube TVs and computer monitors with liquid crystal displays. Another technology under development involves "switchable" windows that can be changed from clear to opaque with the flip of a switch.