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The diagram shows the classification of liquid crystal phases.

Liquid Crystal Phases

Following over 120 years of research in liquid crystals, a large number of liquid crystal phases have been discovered. Their appearance depends on various criteria – their molecular structure and temperature, for example, as well as their concentration and the solvent.

The diagram shows the classification of liquid crystal phases.

Friedrich Reinitzer observed the phenomenon of liquid crystal phases in 1888 using the benzoate of cholesterol. He discovered a thermotropic liquid crystal. Soaps or tensides, for example, can form lyotropic phases which can consist of micelles or double layers.

The picture shows the structure of a crystal: the repeating primitive cell.
Liquid crystal phases have a range of different structures, but all have one thing in common: They flow in a similar way to viscous liquids, but show the physical behavior of crystals. However, there are also significant differences from the two aggregate states. A crystal can be described using a coordinate system. Each atom of a molecule has its specific position. The structure of a crystal can be reduced to a tiny unit, the primitive cell, which is repeated periodically in all three dimensions. This periodicity describes the long-range order of a crystal. A crystal is a highly ordered system in which the physical properties have different characteristics according to the viewing angle. This is called anisotropy. The properties of a liquid crystal phase are also anisotropic, although the structure can no longer be described in a coordinate system. The periodicity and thus the long-range order are lost. Molecules orient themselves by their neighboring molecules, so that only short-range order can be observed. In contrast, a liquid is a completely disordered system, in which the physical properties are isotropic, i.e. directionally independent. What a liquid crystal phase and a liquid have in common is fluidity.
Appearance of liquid crystal phases in a temperature profile

In LCD technology, the thermotropic nematic phase is by far the most significant phase. It is formed from rod-shaped (calamitic) molecules which arrange themselves approximately in parallel to each other. These molecules can also form smectic phases, which exist in multiple manifestations. Smectic phases are more ordered than nematic phases: as well as the parallel alignment of the molecules, they also form layers.

As the temperature rises, the order of a system decreases (see picture). The temperature at which a liquid crystal phase is converted into the isotropic liquid is called clearing point. A substance may form one or more liquid crystal phases if the structural conditions allow this. However, the appearance of liquid crystal phases is not necessarily a consequence of the molecular structure.