Liquid crystals

Liquid crystals are substances thatare in the state of mesomorphic (intermediate, intermediate) between isotropic liquid and solid crystalline. These elements are fluid, able to stay in the form of droplets. Together with the manifestation of these properties, liquid crystals exhibit anisotropy of the magnetic, electrical, optical, and other properties due to the order in the molecular orientation. In other words, substances have multidirectional characteristics. In the absence of external influences, thermal conductivity, electrical conductivity, magnetic susceptibility, dielectric permeability are anisotropic in liquid crystals. Dichroism and birefringence are noted in substances.

Depending on the preparation, lyotropic and thermotropic liquid crystals are classified. There is also a division into types according to the structure of the molecules.

Liquid crystals smectic

They were first discovered in soap (hence the name -"smegma" is soap). The ends of the molecules seem to be fixed in planes perpendicular to their longitudinal axes. Smectic liquid crystals are distinguished by their layered structure. These substances include solutions of water soaps, ethyl ester of azoxybenzoic.

"Smectics" are considered the most extensive classliquid crystals. Some of their varieties are also found in ferroelectric (the presence of spontaneous polarization in a certain temperature range). High viscosity did not allow to find wide application of liquid crystals smectic in engineering.

Nematics

Nematic liquid crystals are differentorientation of the longitudinal molecular axes along a certain direction. In other words, they are characterized by a long-range orientation order. The name of the crystals came from the Greek definition of "Mama" - the thread. Dyssynclinations (filaments) have a high mobility, they are clearly visible in natural light.

Cholesteric liquid crystals and their application

Molecular form of substances of this typeis a parallel oblong lamina. Cholesterol gives propyl cholesterol ether, cholesteryl cinnamate, other cholesterol derivatives.

Thermo-indicators of liquid crystalscholesteric type are widely used in medical and technical diagnostics. The sensitivity of these substances to the temperature makes it possible to visualize the temperature distribution over the surface. This, in turn, is used in introscopy (observation of processes inside bodies, optically opaque), in the detection of certain diseases, as well as in temperature sensors. These crystals form a temperature pattern 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 allows you to significantly reduce the power consumption. Thus, the power of liquid crystal indicators is an order of magnitude lower than in film and powder phosphors, light-emitting diodes, gas-discharge indicators. Cholesterics are used at the heart of the transformation into a visible infrared image.

In a cholesteric liquid crystal (in contrast tonematic) dynamic light scattering can have a memory - a state that diffuses light can also be stored after the field is deleted. In this case, certain properties of the cholesteric affect the duration of the state. So, memory can last from several minutes to several years. The initial state (non-dispersive) of the cholesteric is given by an alternating voltage. The specified property is used when forming memory cells.