Liquid crystal Guide, Meaning , Facts, Information and Description
Liquid crystals are a class of molecules that, under some conditions, inhabit a phase in which they exhibit isotropic, fluid-like behavior – that is, with little long-range ordering – but which under other conditions inhabit one or more phases with significant anisotropic structure and long-range ordering while still having an ability to flow.Liquid crystals find wide use in liquid crystal displays, which rely on the optical properties of certain liquid crystalline molecules in the presence or absence of an electric field. In the presence of electric field, these molecules align with the electric field, altering polarization of the light in a certain way.
The ordering of liquid crystalline phases is extensive on the molecular scale but does not extend to the macroscopic scale as might be found in classical crystalline solids. The ordering in a liquid crystal might extend along one dimension, but along another dimension it might have significant disorder.
Liquid crystals are divided into two groups depending on the shape of the molecules. Calamitic liquid crystals consists of rod-like molecules and have order in the direction of the longer axes of the molecules. In contrast, discotic liquid crystals are composed of flat-shaped molecules which align in the direction of the shorter axes of the molecules.
Important types of calamitic liquid crystals include
- nematics (most nematics are uniaxial but biaxial nematics are also known)
- smectics (smectic A, smectic C, and hexatic)
- discotic nematicss
- columnar phases
When the molecules that form liquid crystals have asymmetric carbon atoms and when the system has not chirality but racemic modification, the orientation vector of the molecular axis of the liquid crystals changes continuously and a macroscopic spiral structure appears in the system as a result. The cycle of the spiral structure is different for each molecule, but each molecule has the property that it reflects the light corresponding to its cycle. From this property, the liquid crystals change color when the cycle of the spiral structure agrees with the visible rays of light. Some kinds of liquid crystals change the cycle of their spiral structure when the temperature changes. This principle is applied in liquid crystal thermometers.
Nematic liquid crystals, which have spiral structures, are called cholesteric liquid crystals. Cholesteric liquid crystals are not distinguished from nematic liquid crystals thermodynamically; hence cholesteric liquid crystals are sometimes called chiral nematic liquid crystals.
Effect of chirality
Although almost all chiral liquid crystals include asymmetric carbon atoms in their molecules, it has recently been discovered that macroscopic chirality appears in liquid crystals that consist of bent-core molecules which do not have asymmetric carbon atoms. However, the appearance mechanism of this macroscopic chirality is not yet clear.