Thursday, 1 December 2016


Liquid Crystal and Their Uses

Liquid Crystals
(i) Definition: The turbid liquid phase of a solid that exists in between the melting and clearing temperature is called liquid crystal. 
Crystal ⇋ Liquid Crystal ⇋ Liquid
(ii) Discovery: In 1888, Frederick Reinitzer, an Austrian botanist discovered the liquid crystals. He was studying an organic compound Cholesteryl Benzoate. This compound turns milky liquid at 145⁰C and becomes a clear liquid at 179⁰C. When the substance is cooled, the reverse process occurs. This turbid liquid state was called liquid crystal. 
(iii) Characteristics: Liquid crystals have both properties of liquids and crystals (solids). Liquid like properties include viscosity, surface tension, and fluidity etc. Crystal like properties include optical properties and molecules have some orderly arrangement. We can say that the properties of liquid crystals are intermediate between those of crystals and isotopic liquids. A crystalline solid may be isotopic or anisotropic but liquid crystals are always anisotropic
(iv) Types: Those substances which make the liquid crystals are often composed of long rod like molecules. In the normal liquid phase, these molecules are oriented in random directions. In liquid crystalline phase, they develop some ordering of molecules. Depending upon the nature of ordering, liquid crystals can be divided into; Nematic, Smectic and Cholesteric
Uses of Liquid Crystals
(i) As Temperature Sensor: Like solid crystals, liquid crystals can diffract light. When one of the wavelengths of white light is reflected from a liquid crystal, it appears coloured. As the temperature changes, the distances between the layers of the molecules of liquid crystals change. Therefore, the reflected light changes accordingly. Thus liquid crystals can be used as temperature sensors. 
(ii) To Find Potential Failure/As Room Thermometers: Liquid crystals are used to find the point of potential failure in electrical circuits. Room thermometers also contain liquid crystals with a suitable temperature range. As the temperature changes, figures show up in different colours. 
(iii) Medical Diagnosis: Liquid crystals are used to locate veins, arteries, infections and tumors. The reason is that these parts of the body are warmer than the surrounding tissues. Specialists can use the techniques of skin thermography to detect the blockages in veins and arteries. When a layer of liquid crystal is painted on the surface of a breast, a tumor shows up as a hot area which is coloured blue. This technique has been successful in the early diagnosis of breast cancer. 
(iv) Electrical Devices: Liquid crystals are used in the display of electrical devices such as digital watches, calculators and laptop computers. These devices operate due to the fact that temperature, pressure and electromagnetic fields easily affect the weak bonds, which hold molecules together in liquid crystals. 
(v) Solvents in Chromatography: In chromatographic separation, liquids crystals are used as solvents. 
(vi) Oscillograph and TV Displays: Oscillograph and TV displays also use liquid crystal screens. 

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