Liquid crystals are those substances that are in a mesomorphic (intermediate, intermediate) state between isotropic liquid and solid crystalline. These elements are fluid, able to stay in the form of drops. Together with the manifestation of these properties, liquid crystals exhibit anisotropy of magnetic, electrical, optical, and other properties due to ordering in molecular orientation. In other words, substances have multidirectional characteristics. In the absence of external influence, thermal conductivity, electrical conductivity, magnetic susceptibility, and dielectric constant are anisotropic in liquid crystals . Dichroism and birefringence are noted in substances.
Lyotropic and thermotropic liquid crystals are classified depending on the preparation. There is also a division into types according to the structure of the molecules.
Smectic Liquid Crystals
They were first found in soap (hence the name “smegma” - soap). The ends of the molecules seem to be fixed in planes perpendicular to their longitudinal axes. Liquid smectic crystals are characterized by a layered structure. These substances include aqueous soap solutions, ethyl ester of azoxybenzoic acid.
"Smectics" are considered the most extensive class of liquid crystals. Some of their varieties are also detected by ferroelectric ones (the presence of spontaneous polarization in a certain temperature range). High viscosity did not allow the widespread use of smectic liquid crystals in engineering.
Nematics
Liquid nematic crystals differ in the orientation of the longitudinal molecular axes in a certain direction. In other words, they are characterized by a long-range orientational order. The name of the crystals comes from the Greek definition of "dumb" - a thread. Disinclinations (threads) are highly mobile, in natural light they are clearly visible.
Cholesteric liquid crystals and their application
The molecular form of this type of substance is a parallel elongated plate. Cholesterics give propyl cholesterol ether, cholesterylcinnamate, other derivatives of cholesterol.
The thermal indicators of cholesteric liquid crystals are widely used in medical and technical dianostics. The sensitivity of these substances to temperature allows you to visualize the temperature distribution over the surface. This, in turn, is used in introscopy (observation of processes inside bodies that are optically opaque), in the detection of certain diseases, as well as in temperature sensors. These crystals form a temperature picture in the form of a color chart. Cholesterics can also be used to visualize microwave fields. For the production of indicators, the effect of dynamic light scattering is used. Liquid crystal displays use ambient light. This can significantly reduce power consumption. So, the power of liquid crystal indicators is an order of magnitude lower than in film and powder phosphors, LEDs, gas discharge indicators. Cholesterics are used at the base of conversion to a visible infrared image.
In a cholesteric liquid crystal (as opposed to nematic), dynamic light scattering can have a memory — a state that scatters light can persist even after the field is removed. Moreover, certain properties of cholesterol affect the duration of the condition. So, memory can be stored from several minutes to several years. In the initial state (non-scattering) cholesteric leads to alternating voltage. The specified property is used when forming memory cells.