Sunday, January 9, 2022

What are liquid crystals?

Liquid crystals (LCs) are a state of matter which has properties between those of conventional liquids and those of solid crystals.

For instance, a liquid crystal may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of liquid-crystal phases, which can be distinguished by their different optical properties (such as textures).

The contrasting areas in the textures correspond to domains where the liquid-crystal molecules are oriented in different directions. Within a domain, however, the molecules are well ordered. LC materials may not always be in a liquid-crystal state of matter (just as water may turn into ice or water vapor).

©SciTechDaily

Liquid crystals can be divided into thermotropic, lyotropic and metallotropic phases.
  • Thermotropic and lyotropic liquid crystals consist mostly of organic molecules, although a few minerals are also known.
  • Thermotropic LCs exhibit a phase transition into the liquid-crystal phase as temperature is changed.
  • Lyotropic LCs exhibit phase transitions as a function of both temperature and concentration of the liquid-crystal molecules in a solvent (typically water).
  • Metallotropic LCs are composed of both organic and inorganic molecules; their liquid-crystal transition depends not only on temperature and concentration, but also on the inorganic-organic composition ratio.

The various liquid-crystal phases can be characterized by the type of ordering.

One can distinguish positional order (whether molecules are arranged in any sort of ordered lattice) and orientational order (whether molecules are mostly pointing in the same direction), and moreover order can be either short-range (only between molecules close to each other) or long-range (extending to larger, sometimes macroscopic, dimensions).

Most thermotropic LCs will have an isotropic phase at high temperature. That is that heating will eventually drive them into a conventional liquid phase characterized by random and isotropic molecular ordering (little to no long-range order), and fluid-like flow behavior.
©Alternator

Examples of liquid crystals can be found both in the natural world and in technological applications. Widespread liquid-crystal displays use liquid crystals. Lyotropic liquid-crystalline phases are abundant in living systems but can also be found in the mineral world. For example, many proteins and cell membranes are liquid crystals. Other well-known examples of liquid crystals are solutions of soap and various related detergents, as well as the tobacco mosaic virus, and some clays.

Liquid crystals find wide use in liquid crystal displays, which rely on the optical properties of certain liquid crystalline substances in the presence or absence of an electric field.

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