After some substances are in the molten state or dissolved by a solvent, although they lose the rigidity of solid substances, they gain the ease of fluidity of liquids, and retain the anisotropic orderly arrangement of some crystalline substances, forming a kind of both crystals. The intermediate state of some properties of liquid and liquid, the orientation-ordered fluid that exists in the process of transformation from solid to liquid is called liquid crystal. The definition is relaxed to include substances that can be liquid crystal phase in a certain temperature range and are normally crystalline at lower temperatures. For example, a liquid crystal can flow like a liquid (fluidity), but its molecules are oriented and ordered like a road (anisotropy). There are many different types of liquid crystal phases, which can be distinguished by their different optical properties such as birefringence. When a polarized light source is used, different liquid crystal phases will appear with different textures when viewed under a microscope. Different textures in the texture contrast region correspond to different liquid crystal molecules. However, the molecules are well oriented in order. And liquid crystal materials may not always be in the liquid crystal phase (just as water can turn into ice or water vapor).
Liquid crystals can be divided into thermotropic liquid crystals and lyotropic liquid crystals. Thermotropic liquid crystals refer to liquid crystals formed from a single compound or from a homogeneous mixture of a few compounds. Substances that usually show liquid crystal phase within a certain temperature range. The molecular weight of a typical long rod-shaped thermotropic liquid crystal is generally around 200-500 g/mol. Lyotropic liquid crystal: is a liquid crystal formed by two or more compounds including solvent compounds. The liquid crystal phase occurs when the concentration of solute molecules in the solution is within a certain range. Its solvent is mainly water or other polar molecular liquids. The main reason for the long-range order of molecular arrangement in such liquid crystals is the interaction between solute and solvent molecules, while the interaction between solute molecules is secondary. A lyotropic liquid crystal is a liquid crystal formed by two or more compounds including a solvent compound.