Liquid Crystals
Liquid crystals are a highly ordered liquid material that has a unique molecular structure which causes them to behave like a solid crystal. When first discovered by H. Reinitzer in 1889, liquid crystals were considered little more than a scientific curiosity It was not until fairly recently that scientists discovered how these molecules change position when an electrical charge is applied, and it is this discovery that has paved the way for the widespread use of liquid crystals, particularly in electronic displays.
As the name implies, liquid crystals are materials in a liquid state. In large amounts they appear to be rather cloudy, like honey, but are clear and nearly transparent when in thin layers (as they are normally found). Like solid crystals, their molecules are long and sausage-shaped, and they are very well-ordered within the liquid. There are three types of liquid crystal. The most organized variety are smetic crystals. Their molecules run parallel to each other and are structured into overlapping layers. The molecules in nematic crystals are also parallel to one another, but lack the layering found in smetic crystals. The most widely used type of liquid crystals are cholesteric crystals; in this type molecules are aligned in each layer, but the layers themselves are slightly tilted from one to the next. This produces a twisting effect throughout the liquid, not unlike the way a grain runs through wood.
Through experimentation it was found that, if an electrical charge is passed through a liquid crystal material, the molecules within the liquid will line up according to the direction of the electrical field. If such a field is applied to a cholesteric liquid, it has the effect of "untwisting" the twisted layers. This phenomenon forms the basis for most liquid crystal displays (LCDs). In an LCD, a very thin layer of liquid crystal (about 0.00099 in. or 0.0025 cm) is sandwiched between two sections of glass. Each piece of glass has been polarized--that is, it will allow light to pass through along only one axis. Because the polarization of each piece of glass is offset from the other, light would not normally be able to pass through; however, the presence of the liquid crystal twists the light rays (just as its molecular layers are twisted) so that light can pass through. In this state, the LCD appears clear.
In order to create a display (for example, numbers on a clock), a small electrical charge must be applied to the liquid crystal, untwisting its molecules. Without the twisted molecules to bend it, the light is unable to pass through the polarized glass; thus, the LCD turns dark. Engineers create numbers and letters by turning bars and blocks of the LCD on and off. All liquid crystals operate best within a well-defined temperature range, usually from 23° F (-5° C) to 149° F (65° C); at temperatures lower than 23° F the material is in a solid state, while at temperatures greater than 149° F will the material loses it crystalline properties. The range in which liquid crystals are most effective is called the mesophase, or the anisotropic state.
Because they scatter light rather than generate it, LCDs are the only form of electronic display that is actually more visible when exposed to direct sunlight. This makes them very useful in automobiles, where a driver must see the readout clearly in the daytime, yet not be blinded by it at night (the LCD is backlit by a dim light source). LCDs are also very economical, requiring about one thousandth the power of light-emitting diodes (LED) and other displays.
Beyond electronic display panels, liquid crystals continue to be used for different applications. In basic biochemical research they have been coupled with nuclear magnetic resonance (NMR) devices to determine the molecular structure of proteins and other compounds.
Very recently, liquid crystals have been used as a tool for understanding the phenomena of order and chaos. The French physicist Pierre-Gilles de Gennes (1932-) (sometimes described as the Isaac Newton of our age) began studies on phase transitions and other atomic changes in the 1950s, and in the 1960s first used liquid crystals in that research. While the true impact of his work is yet to be felt, it is considered to be important to the linking of molecular structure to general principles of physics. De Gennes was awarded the 1991 Nobel Prize in physics.
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