The pure solids or crystalline solids melt sharply. The temperature remains constant at its melting point until all the solid melts. There are some solids which pass through turbid state before melting.
These turbid phases are called liquids crystals because they can flow as liquids. They have the properties like liquids as surface tension, viscosity etc. But it is very interesting to know that the molecules of such turbid liquid resemble crystals in certain optical properties. These turbid liquids are hence called liquid crystals. So, a liquid crystalline state exists between two temperatures i.e. melting temperature and clearing temperature.
The property of liquid crystals is intermediate between those of crystals and isotropic liquids. That is why they are called liquid crystals or mesomorphic states. They have the fluidity of the liquids and the optical properties of the crystals.
Uses of Liquid Crystals
Because of optical and electrical properties liquid crystals find many practical applications. Many organic compounds and biological tissues behave as liquid crystals. The important uses are given below:
- Liquid crystals can diffract light like solid crystals. When one of the wavelengths of white light is reflected, it appears colored. As the temperature changes, the distance between the layers of molecules changes. Therefore the colour reflected light changes accordingly. Thus liquid crystals are used in measurement of temperature sensors.
- 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.
- Liquid crystalline substances are used to locate the 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 blockages in veins and arteries. When a layer of liquid crystals is painted on the surface of the breast, a tumor show up as a hot area which is colored blue indicating breast cancer.
- Liquid crystals are used in the display of electrical devices such as digital watches, calculators and computers. By a very small change of the electrical field, the orientation of the molecule is changed.
- In chromatographic separations, liquid crystals are used as solvents. Oscillographic and TV displays also use liquid crystal screens.