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