Liquid crystal (LC) can be considered as a state of matter in between the conventional liquid and crystalline solid states. A liquid can flow freely and is isotropic which means its properties are uniform in all directions. A crystalline solid is anisotropic which means its molecules are orientally packed and may not flow. A liquid crystal, however, may flow like a liquid, but its molecules may be oriented in a crystal-like way. In liquid crystals, optical, electrical conductivity and thermal properties may vary with direction. Nematicm, smectic and cholestric are the three major types of liquid crystals. Nematic liquid crystals have no ordered structure of molecules, and smectic liquid crystals have a layered molecular structure, whereas cholesteric liquid crystals have the molecules in a twisted and chiral arrangement.
Depending on how the liquid crystal phase is produced, liquid crystals can also be categorised as either thermotropic or lyotropic. Thermotropic liquid-crystals have a phase transition into the liquid-crystalline state as temperature is raised from their solid state or as temperature is lowered from their liquid state. Lyotropic liquid-crystals have a phase transition as a function of both temperature and concentration of the liquid-crystal molecules in a solvent. An everyday example of a lyotropic liquid crystal is soap. Among the thermotropic liquid crystals there are two different classes of substances, those are either enantiotropic or monotropic. Enantiotropic liquid crystals are able to enter the liquid crystal state both by cooling a liquid and heating a solid. Monotropic liquid crystals, on the other hand, can enter their liquid crystal phases by either cooling a liquid or heating a solid, but not both.