Whenever we study the properties of crystalline solids, we come to know that pure solids melt abruptly. The temperature remains constant at the melting point until all the solid is melted.
In 1888, Frederick Reinitzer, an Austrian botanist discovered a universal property. He was studying an organic compound cholesterol benzoate. This compound turns milky liquid at 145 degrees Celsius and becomes a clear liquid at 179 degrees Celsius. When the substance cools, the reverse process occurs. This cloudy liquid phase was called liquid crystal.
Until now, it has been reported that there are many crystalline solids that melt into a cloudy liquid phase before finally melting into a clear liquid. These cloudy liquid phases can flow like liquids. They have properties like liquids like surface tension, viscosity, etc. But it is very interesting to know that the molecules of such cloudy liquids also have a certain degree of order. It means that these cloudy liquids are similar to crystals in some properties and the most important properties are optical once. These cloudy liquids are therefore called liquid crystals. So, a liquid crystalline state exists between two temperatures, which is the melting temperature and the cleaning temperature. A crystalline solid can be isotropic or anisotropic, but liquid crystals are always isotropic.
From 1888 until about thirty years ago, they were largely laboratory curiosities. But now they have found a host of applications.
Those substances that form them are often composed of long rod-like molecules. In normal liquid phase, these molecules are oriented in random directions. In the crystalline liquid phase, they develop a certain arrangement of molecules. Depending on the nature of the order, 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 liquids and the optical properties of crystals.
Uses of liquid crystals:
Due to their remarkable optical and electrical properties, they have many practical applications. Many organic compounds and biological tissues behave like fluid crystals. The unique properties of crystals have intrigued scientists since their discovery nearly a hundred years ago.
Some of its important uses are as follows:
1. Diffraction of light:
Like solid crystals, they can diffract light. When one of the wavelengths of white light is reflected, it appears colored from a liquid crystal. As the temperature changes, the distance between the layers of molecules in liquid crystals changes. Therefore, the color of the reflected light changes accordingly. Therefore, 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 proper temperature range. As the temperature changes, the figures appear in different colors.
3. Medical uses:
These substances are used to locate veins, arteries, infections, and tumors. The reason is that these parts of the body are hotter than the surrounding tissues. Specialists can use skin thermography techniques to detect blockages in veins and arteries. When a layer of liquid crystals is painted on the surface of the breast, a tumor appears as a hot blue area. This technique has been successful in the early diagnosis of breast cancer.
4. Electrical devices:
They are used in early displays of electrical devices such as digital clocks, calculators, and laptop computers. These devices work due to the fact that temperature, pressure, and electromagnetic fields easily affect the weak bonds that hold molecules together in liquid crystals.
5. Chromatographic separations:
In chromatographic separations liquid crystals are used as solvents.
6. Oscillography and TV:
Oscillographic and television screens also used liquid crystal displays.