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Liquid crystals

The liquid crystals discovered more than a century ago continues to be of great interest to physicists, chemists, and electronics engineers around the world.

In recent years, liquid crystals have become everyday materials. They are used in objects without which it would be difficult to imagine life today. Liquid crystals are found in digital watches, computer monitors and televisions. The total area of liquid crystal displays produced every year is already expressed in square kilometers. This rapid development of new technologies would not have been possible if it had not been preceded by scientific research on the nature of liquid crystals.

The three commonly known states of matter: solid, liquid and gas, are characterized by different physical properties.

liquid crystals -
Fig. 1. Changes that occur in solids under the influence of temperature in which the liquid crystalline state can occur [H. D. Ogrodnik]

In the solid state, atoms or molecules are regularly spaced to form a three-dimensional long-range arrangement.

In anisotropic crystals the ordering of molecules is asymmetrical and the physical quantity measured depends on the direction in which the measurement is made or the direction of the agent causing the phenomenon. Heating an anisotropic crystal to its melting point causes a phase change, the crystal becomes an isotropic liquid. Molecules have no long-range ordering, they are free to move and rotate.

In an isotropic phase, no physical quantity that characterizes a liquid or gas depends on the direction in which it is measured. Further increases in temperature result in a liquid-gas transition. Not all substances follow this pattern (Figures 1 and 2). There can be an intermediate phase in them characterized by liquidity typical for liquids, but at the same time it shows long-range ordering – typical for crystalline solids, entailing anisotropy of some physical properties (dielectric, optical). This phase is called the liquid crystalline phase or mesophase, from the Greek word “mesos” – indirect.

Thus, a group of compounds (usually organic), having both the solid state (crystalline phase), the liquid state (isotropic phase) and the intermediate state is referred to as liquid crystals.

Liquid crystals. Temperature-induced changes in the ordering of molecules.
Fig. 2. Temperature-induced changes in the ordering of molecules [H.D. Ogrodnik]

Liquid crystal substances have found applications in many areas of life. They are used in LCD displays and screens, various types of temperature indicators: medical, aquarium and room thermometers, indicators on bottles, etc. They are widely used in medicine to produce thermographic films, as well as in cosmetic industry as additives to creams.

Due to the degree of ordering of molecules of liquid crystalline substances, liquid crystals can be divided into three main types: nematic, cholesteric and smectic. These phases differ in the way the molecules are arranged and in the occurrence of layered structures.

In nematic liquid crystals the molecules remain parallel to each other and have no other restrictions on mutual movement.

In smectic liquid crystals, molecules are arranged in layers, and their location in the layer is parallel or deviated by some angle with respect to the normal to the layer.

In cholesterol liquid crystals, the molecules are arranged in pseudolayers, such that their long axes are parallel to the plane of the pseudolayers. The molecules in the next layer are twisted by some fixed angle, relative to the molecules lying in the previous layer.

The appearance of liquid crystalline phases (mesophases) is associated with a change in solution concentration, and thus with the interaction of the substance with the solvent. Such liquid crystals are called lyotropic. Thermotropic liquid crystals can be divided according to the shape of the molecules of which they are composed. Here we distinguish liquid crystals made up of rod-like, calamitic, disc-like and banana-shape molecules.

History of liquid crystals

The first liquid crystal discovered was cholesterol benzoate. In 1888, an Austrian botanist Friedrich Reinitzer heated this substance and found that at 145°C it melted into a milky white liquid, which under further heating at 179°C gave a clear liquid. Further research into this unusual phenomenon was undertaken by the German physicist Otto Lehmann, who showed that this liquid substance was also birefringent like a crystal.

Thus Otto Lehmann found the existence of optical anisotropy and proposed the name Flűssige Kristalle or liquid crystal for the newly discovered phase. The liquid crystalline phases generated by temperature changes are now called thermotropic liquid crystals. Lehmann also discovered that a mesomorphic phase is also formed during the dissolution of certain organic substances in inorganic solvents.

These findings are described in the Master’s thesis of Dołęga H., 2007: Study of phase polymorphism of a new liquid crystal antiferroelectric. Academy of Podlasie, Siedlce. Translation was done with the assistance of DeepL translator.