We all have seen the digital watches and calculators. These devices make use of liquid crystals for displaying various digits. Liquid crystals belong to a state of matter having the mixed properties of both the liquid and the solid states. Liquid crystals may be described as condensed fluid states with spontaneous anisotropy.
Liquid crystals are of two types: thermotropic liquid crystals prepared by heating the substance and lyotropic liquid crystals made by mixing two or more compounds. Some of the important liquid crystals that belong to the first category are p-azoxyanisole, cholesteryl nonanoate and p-n octyloxybenzoic acid. In the second category sodium stearate and alpha-lecithin are the two important liquid crystals. Liquid crystals are used in colour T.V. and electronic display tubes, sensitive tapes and electronic devices for digital clocks, watches and calculators.
Light emitting diodes are also used for this purpose but liquid crystals need lesser electric power and have great clarity even in the presence of bright light.
Liquid crystal displays work on two principles, viz dynamic scattering and field effects. In displays the liquid crystals are sandwiched between two transparent electrodes (glass coated with a metal or metal oxide film). The thickness of a liquid crystal film varies from 6 to 25 microns. The cell is sealed to eliminate oxygen and moisture.
In dynamic scattering, if no electric field is applied, the cell is transparent. On applying electric field to the crystal the cell becomes opaque. Digital displays are made by photo etching a seven-segment pattern on tin or indium-coated glass plate.
The field-effect displays are mostly used in watches and calculators. The output of quartz crystal is given to seven segments of liquid crystal in a watch. Polarizers are also used for liquid crystal cells. Electric field changes the molecular alignment by which liquid crystal becomes opaque on account of a mirror behind the second polarizer.
Liquid crystal displays have revolutionized a number of fields. Nowadays thermometers with digital displays are also available in the market.
