LIQUID CRYSTALS

By: Aamarpali Puri

What is Liquid Crystal?

It is the fourth state of matter. It generally lies between the solid and liquid phase. It is formed from organic compounds.

It was discovered by Austrian Chemist, Frederich Reintzer (1888). He found that Cholesteryl Benzoate does not melt in the same manner as other compounds and has two melting points. When solid Cholesteryl benzoate is heated it melts sharply to form a turbid liquid and at higher temperature an equally sharp change to a clear liquid occurs. On cooling the changes take place in reverse direction at the same temperatures. The temperature at which the solid melts to form turbid liquid is usually called the transition point, and the higher temperature, at which the true liquid is obtained, is referred to as the melting point.

Liquid crystals are very important in the study of Optics, Chemistry and Polymer science. If an electric field is applied to liquid crystal the molecule will align in the same direction as the field. It exhibits unusual optical effects like it is doubly refracting and gives interference pattern in polarized light. There are three basic phases Smectics, Nematics and Cholesterics.

In Smectics normal liquid flow does not occur; the movement is of gliding nature, in one plane. Molecules are ordered in one direction in approximately parallel layers. This arrangement may be compared to number of hair brushes, in which bristles are parallel but set at random, placed one on top of another.

Nematics have mobile thread like structure. If viewed along the lines of the forces of a magnetic field the turbid liquid appears clear. Molecules are aligned but the layers are not there. The molecules can be compared to a loose bundle of cigars, so that individuals can move up and while remaining parallel.

Cholesterics are optically active compounds. They show marked coloured effects in polarized light as a consequence of layered structure.

Some uses of Liquid Crystals are in Flat Screen Television, Wrist watches, Laptop screens, Digital clocks, Thermometery etc.

Liquid Crystal Display televisions (LCD TV) use LCD display technology to produce images. LCD televisions are thinner and lighter than Cathode Ray Tube (CRTs) of similar display size. LCD televisions produce a black and coloured image by selectively filtering a white light.

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