词条 | liquid crystal |
释义 | liquid crystal physics Introduction substance that blends the structures and properties of the normally disparate liquid and crystalline (crystal) 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 increases—e.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. Structure and symmetry Symmetries of solids and liquids ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() As a general rule, molecules solidify into crystal lattices with low symmetry at low temperatures. Both translational and rotational symmetries are discrete. At high temperatures, after melting, liquids have high symmetry. Translational and rotational symmetries are continuous. High temperatures provide molecules with the energy needed for motion. The mobility disorders the crystal and raises its symmetry. Low temperatures limit motion and the possible molecular arrangements. As a result, molecules remain relatively immobile in low-energy, low-symmetry configurations. Symmetries of liquid crystals Liquid crystals, sometimes called mesophases, occupy the middle ground between crystalline solids and ordinary liquids with regard to symmetry, energy, and properties. Not all molecules have liquid crystal phases. Water molecules, for example, melt directly from solid crystalline ice into liquid water. The most widely studied liquid-crystal-forming molecules are elongated, rodlike molecules, rather like grains of rice in shape (but far smaller in size). A popular example is the molecule p-azoxyanisole (PAA): ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() It was noted above that, as temperature decreases, matter tends to evolve from highly disordered states with continuous symmetries toward ordered states with discrete symmetries. This can occur through a sequence of symmetry-breaking phase transitions. As a substance in the liquid state is reduced in temperature, rotational symmetry breaking creates the nematic liquid crystal state in which molecules are aligned along a common axis. Their directors are all nearly parallel. At lower temperatures continuous translational symmetries break into discrete symmetries. There are three independent directions for translational symmetry. When continuous translational symmetry is broken along only one direction, the smectic liquid crystal is obtained. At temperatures sufficiently low to break continuous translational symmetry in all directions, the ordinary crystal is formed. ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() Selected phases characteristic of liquid-crystal-forming moleculesThere is a great variety of liquid crystalline structures known in addition to those described so far. The Table (Selected phases characteristic of liquid-crystal-forming molecules) relates some of the chief structures according to their degree and type of order. The smectic-C phase and those listed below it have molecules tilted with respect to the layers. Continuous in-plane rotational symmetry, present within smectic-A layers, is broken in the hexatic-B phase, but a proliferation of dislocations maintains continuous translational symmetry within its layers. A similar relationship holds between smectic-C and smectic-F. Crystal-B and crystal-G have molecular positions on regular crystal lattice sites, with long axes of molecules (directors) aligned, but allow rotation of molecules about their directors. These are the so-called plastic crystals. Many interesting liquid crystal phases are not listed in this table, including the discotic phase, consisting of disk-shaped molecules, and the columnar phases, in which translational symmetry is broken in not one but two spatial directions, leaving liquidlike order only along columns. The degree of order increases from the top to the bottom of the table. In general, phases from the top of the table are expected at high temperatures, and phases from the bottom at low temperatures. Liquid crystal compounds Liquid-crystal-forming compounds are widespread and quite diverse. Soap (soap and detergent) can form a type of smectic known as a lamellar phase, also called neat soap. In this case it is important to recognize that soap molecules have a dual chemical nature. One end of the molecule (the hydrocarbon tail) is attracted to oil, while the other end (the polar head) attaches itself to water. When soap is placed in water, the hydrocarbon tails cluster together, while the polar heads adjoin the water. Small numbers of soap molecules form spherical or rodlike micelles (micelle), which float freely in the water, while concentrated solutions create bilayers, which stack along some direction just like smectic layers. Indeed, the name smectic is derived from the Greek word for soap. The slippery feeling caused by soap reflects the ease with which the layers slide across one another. Many biological materials form liquid crystals. Myelin, a fatty material extracted from nerve cells, was the first intensively studied liquid crystal. The tobacco mosaic virus, with its rodlike shape, forms a nematic phase. In cholesterol the nematic phase is modified to a cholesteric phase characterized by continuous rotation of the direction of molecular alignment. An intrinsic twist of the cholesterol molecule, rather like the twist of the threads of a screw, causes this rotation. Since the molecular orientation rotates steadily, there is a characteristic distance after which the orientation repeats itself. This distance is frequently comparable to the wavelength of visible light, so brilliant colour effects result from the diffraction of light by these materials. Perhaps the first description of a liquid crystal occurred in the story The Narrative of Arthur Gordon Pym, by Edgar Allan Poe: I am at a loss to give a distinct idea of the nature of this liquid, and cannot do so without many words. Although it flowed with rapidity in all declivities where common water would do so, yet never, except when falling in a cascade, had it the customary appearance of limpidity. . . . At first sight, and especially in cases where little declivity was found, it bore resemblance, as regards consistency, to a thick infusion of gum Arabic in common water. But this was only the least remarkable of its extraordinary qualities. It was not colourless, nor was it of any one uniform colour—presenting to the eye, as it flowed, every possible shade of purple, like the hues of a changeable silk. . . . Upon collecting a basinful, and allowing it to settle thoroughly, we perceived that the whole mass of liquid was made up of a number of distinct veins, each of a distinct hue; that these veins did not commingle; and that their cohesion was perfect in regard to their own particles among themselves, and imperfect in regard to neighbouring veins. Upon passing the blade of a knife athwart the veins, the water closed over it immediately, as with us, and also, in withdrawing it, all traces of the passage of the knife were instantly obliterated. If, however, the blade was passed down accurately between two veins, a perfect separation was effected, which the power of cohesion did not immediately rectify The liquid described in this passage is human blood. In its usual state within the human body, blood is an ordinary disordered isotropic fluid. The disklike shape of red blood cells, however, favours liquid crystallinity at certain concentrations and temperatures. Optical properties Effect of liquid crystals on polarized light An understanding of the principal technological applications of liquid crystals requires a knowledge of their optical properties. Liquid crystals alter the polarization of light passing through them. Light waves are actually waves in electric and magnetic fields. The direction of the electric field is the polarization of the light wave. A polarizing filter selects a single component of polarized light to pass through while absorbing all other components of incoming waves. If a second polarizing filter is placed above the first but with its polarization axis rotated by 90°, no light can pass through because the polarization passed by the first filter is precisely the polarization blocked by the second filter. When optically active materials, such as liquid crystals, are placed between polarizing filters crossed in this manner, some light may get through, because the intervening material changes the polarization of the light. If the nematic director is not aligned with either of the polarizing filters, polarized light passing through the first filter becomes partially polarized along the nematic director. This component of light in turn possesses a component aligned with the top polarizing filter, so a fraction of the incoming light passes through the entire assembly. The amount of light passing through is largest when the nematic director is positioned at a 45° angle from both filters. The light is fully blocked when the director lies parallel to one filter or the other. ![]() ![]() ![]() ![]() Use of liquid crystals (liquid crystal display) as optoelectronic displays ![]() ![]() ![]() ![]() 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. 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):1–48 (May 1973). The Nobel Prize acceptance lecture by P.G. de Gennes, “Soft Matter,” Reviews of Modern Physics, 64(3):645–648 (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, 1:12–16 (1991). |
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