LIQUID CRYSTAL

a substance that flows as a liquid but maintains some of the ordered structure characteristic of a crystal. Certain organic substances when heated will not melt directly but will turn from a crystalline solid to a liquid-crystal (mesomorphic) state. Upon further heating, a temperature is reached at which a true (isotropic) liquid is formed. The liquid-crystal state has some characteristics of a crystal and some of a liquid but generally has properties that are unique. Because the molecular forces producing liquid crystalline states are very weak, the structures are easily affected by changes in mechanical stress, electromagnetic fields, temperature, and chemical environment. Three main categories have been recognized: smectic, nematic, and cholesteric. Smectic liquid crystals consist of flat layers of cigar-shaped molecules with their long axes oriented perpendicularly to the plane of the layer. Each layer is one or two molecules thick, and the positions of the molecules within each layer can be ordered or random, depending upon the substance. The sheets flow freely over each other; the molecules within each layer, however, remain oriented and do not move between layers. Nematic liquid crystals are also oriented with their long axes parallel; but they are not separated into layers, and they behave like toothpicks in a box, maintaining their orientation but free to move in any direction. Nematic substances can be aligned by electric and magnetic fields, resulting in a number of characteristics such as the ability in some cases to be electrically switched from clear to opaque. This peculiarity gives rise to many technical applications such as in image display systems. Cholesteric liquid crystals form in thin layers, each one molecule thick; and within each layer the molecules are arranged with their long axes in the plane of the layer and parallel to each other, as a two-dimensional nematic structure. One of their unusual optical properties is circular dichroism, a phenomenon in which a beam of light is split, one wavelength becoming circularly polarized while the other wavelengths are reflected. Thus, when white light falls on cholesteric liquid crystals, the reflected light is an iridescent colour characteristic of the angle of the incident beam as well as of the temperature. The ability to react to minute variations in temperature by changing colour has many applications, such as the determination of temperature variations over surfaces such as the skin. substance that blends the structures and properties of the normally disparate liquid and crystalline solid states. Liquids can flow, for example, while solids cannot, and crystalline solids possess special symmetry properties that liquids lack. Ordinary solids melt into ordinary liquids as the temperature increasese.g., ice melts into liquid water. Some solids actually melt twice or more as temperature rises. Between the crystalline solid at low temperatures and the ordinary liquid state at high temperatures lies an intermediate state, the liquid crystal. Liquid crystals share with liquids the ability to flow but also display symmetries inherited from crystalline solids. The resulting combination of liquid and solid properties allows important applications of liquid crystals in the displays of such devices as wristwatches, calculators, portable computers, and flat-screen televisions. Additional reading Works on solids in general include Lawrence H. Van Vlack, Elements of Materials Science and Engineering, 6th ed. (1989), an elementary textbook; Charles A. Wert and Robb M. Thomson, Physics of Solids, 2nd ed. (1970), an intermediate-level text; Charles Kittel, Introduction to Solid State Physics, 6th ed. (1986), the standard college textbook; Neil W. Ashcroft and N. David Mermin, Solid State Physics (1976), an advanced textbook; George E. Bacon, The Architecture of Solids (1981), an introduction to bonding and structure; and Linus Pauling, The Nature of the Chemical Bond and the Structure of Molecules and Crystals, 3rd ed. (1960, reissued 1989), the classic reference work on chemical bonding. Gerald D. Mahan The history of liquid crystals in particular is surveyed by H. Kelker, History of Liquid Crystals, Molecular Crystals and Liquid Crystals, 21(1 and 2):148 (May 1973). The Nobel Prize acceptance lecture by P.G. de Gennes, Soft Matter, Reviews of Modern Physics, 64(3):645648 (July 1992), sets liquid crystals in a broader scientific context. Discussions of special topics in liquid crystals, frequently at a level close to this article, may be found in the periodical Condensed Matter News (bimonthly). More technical presentations are given in P.G. de Gennes, The Physics of Liquid Crystals (1974); S. Chandrasekhar, Liquid Crystals, 2nd ed. (1992); and P.S. Pershan, Structure of Liquid Crystal Phases (1988). Applications of liquid crystals are described in E. Kaneko, Liquid Crystal TV Displays (1987); and J. Funfschilling, Liquid Crystals and Liquid Crystal Displays, Condensed Matter News, 1:1216 (1991). Michael Widom