Aug 2016

Liquid Crystals

Whenever we study the properties of crystalline solids, we come to know that the pure solids melt sharply. The temperature remains constant at the melting point until all the solid melts.

In 1888, Frederick Reinitzer, an Austrian botanist discovered a universal property. He was studying an organic compound cholesteryl benzoate. This compound turns milky liquid at 145 degree Celsius and becomes a clear liquid at 179 degree Celsius. When the substance is cooled, the reverse process occurs. This turbid liquid phase was called liquid crystal.

Up til now, it has been reported that there are many crystalline solids which melt to a turbid liquid phase before finally melting to a clear liquid. These turbid liquid phases 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 liquids posses some degree of order as well. It means that these turbid liquids resembles crystals in some properties and the most important properties are optical once. These turbid liquids are hence called liquid crystals. So, a liquid crystalline state exist between two temperatures that is the melting temperature and clearing temperature. A crystalline solid may be isotropic or anisotropic, but liquid crystals are always isotropic.

From 1888 to until about thirty years ago, they were largely are laboratory curiosity. But now they have found a large number of applications.

Those substances which make the them are often compose of long rod like molecules. In 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, it can be divided into:

1. Nematic.

2. Smectic.

3. Cholesteric.

The properties of liquid crystals are intermediate between those of crystals and isotropic liquids. They have the fluidity of the liquids and the optical properties of the crystals.

Uses Of Liquid Crystals:

Due to the remarkable optical and electrical properties, they have many practical applications. Many organic compounds and biological tissues behave as fluid crystals. The unique properties of crystals have intrigued the scientists since their discovery, nearly hundred years ago.

Some of their important uses are as follows:

1. Diffraction Of Light:

Like solid crystals, they can diffract light. When one of the wavelength of white light is reflected, from a liquid crystal it appears colored. As the temperature, changes the distances between the layers of molecules of liquid crystals change. Therefore, the color of reflected light changes accordingly. Thus they can be used as temperature sensors.

2. Detection Of Potential Failure:

They are used to find the point of potential failure in electrical circuits. Room thermometers also contain them with the suitable temperature range. As the temperature changes, figures show up in different colors.

3. Medical Uses:

These substances are used to locate the veins, arteries, infections and tumors. The reason is that these parts of the body are warmer than the surroundings tissues. specialists can use the techniques of skin thermography to detect the blockages in veins and arteries. When a layer of liquid crystals is painted on the surface of breast, a tumor shows up as hot area which is colored blue. This technique has been successful in early diagnosis of breast cancer.

4. Electrical Devices:

They are used in early display of electrical devices such as digital watches, calculators and laptop computers. These devices operate due to the fact that temperature, pressure and electro-magnetic fields easily affect the weak bonds, which hold the molecules together in liquid crystals.

5. Chromatographic Separations:

In chromatographic separations, liquid crystals are used as solvents.

6. Oscillographic and T. V:

Oscillographic and T. V displays also used liquid crystal screen.