The term liquid crystal is used to indicate the intermediate phases (mesophases) between that of crystalline solid and that of isotropic liquid of some substances. A substance can present several mesophases with a decreasing order level and, for example, pass from the solid crystal phase (which corresponds to the highest three-dimensional order) to the smectic mesophase A (in which a layered order is preserved), to the mesophase nematic (in which a linear order is preserved), to the liquid phase (which has no sorting of any kind).
The name “crystal” is linked to the anisotropic optical (electric and magnetic) properties of these intermediate phases. In fact, the light, when it passes through a liquid crystal, undergoes a rotation in the polarization plane, which is proportional to the length of the light path in the crystal itself. Moreover, the liquid crystal, thanks to its properties of fluidity and coalescence in drops, has the behavior of a liquid whereby the rotatory power can vary with the variation of physical parameters such as temperature, electric and magnetic field, the concentration of impurities.
Classification of liquid crystals
Liquid crystals are classified into three main types:
- nematic liquid crystals;
- cholesteric liquid crystals;
- smectic liquid crystals.
The nematic liquid crystals have elongated molecules that are aligned with each other with the significant parallel axis. In the presence of an external electric field, if the electrical anisotropy is positive, the molecules are oriented parallel to the field. In the case of negative electrical anisotropy, they are oriented orthogonally to it. In applications (displays) the liquid crystal layer between two electrodes is chosen to rotate the light 90 degrees, in the absence of an electric field.
If the liquid crystal is between two crossed polarizers (at 90 degrees between them), then the light passes unchanged by locally illuminating a screen. When a voltage is applied between the electrodes, the rotation power is reduced, and in the limit canceled, since the electric field alters the orientation of the molecules; the light then passes through the liquid crystal without undergoing rotations. In this case, the crossed polarizers do not allow the light beam to emerge. The magnetic anisotropy is generally positive so that the molecules are oriented parallel to the magnetic field.
The cholesteric liquid crystals can be considered two-dimensional nematic systems, characterized by a structure ordered only on planes, resulting in the direction of molecular orientation varying from one plane to the next, to obtain a pitch helix configuration p. From the optical point of view, this resulting periodic structure with a period p/2 (since parallel and antiparallel molecules possess the same properties), if p/2 turns out to be the wavelength of visible light, p is generally a function of temperature, the correspondence between selectively reflected color and temperature is unambiguous. Medical thermoscopes are based on this property.
Smectic liquid crystals have a layered structure with a spatial arrangement within each layer. In the smectic mesophase A, the molecules usually are oriented to the lay of the layers. In the smectic mesophase B, there is a hexagonal arrangement within the layers. In the smectic mesophase C, the molecules are obliquely oriented to the planes according to an angle that is a function of temperature.