Gases and liquids are isotropic. This means for any gas or liquid, the value of any of the physical properties such as refractive index, coefficient of thermal expansion, electrical conductivity, speed of sound etc., is same in all directions. In contrast to this, a crystalline solid when examined as an individual crystal (or a single crystal) behaves in a different way. Depending upon the direction in which the crystal is kept during measurement, it may have a different value for its physical properties mentioned above. Such a single crystal is anisotropic. In some cases different faces of a crystal may show different catalytic activity. Another class of compounds which are anisotropic are liquid crystals. Some organic compounds often have two melting points. On heating such a crystal, it melts into turbid liquid at a definite temperature; and on heating further, the turbid liquid becomes clear at another temperature. The turbid liquid is called ‘liquid crystal’. These molecules have a length which is larger than breadth. In general, the arrangement of molecules in liquid crystal resembles a pile of cigars. Depending upon the structural pattern of molecules, liquid crystals can be classified as follows:
Smectic liquid crystals have molecules arranged in parallel layers or planes. These planes are at equal distances. The molecules in all the planes point to the same direction. That is, the molecules have same orientation. The only difference between a solid crystal and a smectic liquid crystal is that in the former, the particles are arranged at regular intervals within a plane; whereas in the latter it is not so.
Nematic liquid crystals have all the molecules with the same orientation .Unlike nature of a nematic liquid crystal in smectic type, the molecules are not arranged in planes in nematic liquid crystals. Depending on the way the molecules, Application of an electric field causes a change in the orientation of the molecules in nematic liquid crystal. A change in molecular orientation causes a change in optical properties. It is this anisotropic character that makes a nematic liquid crystal useful in LCD(liquid crystal display) watches and calculators.
Cholesteric liquid crystals have a multiple layer structure, but each successive layer is inclined or twisted slightly.
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