Liquid Crystals 2
Introduction
As we know the common states of matter are: solid, liquid and gas; but there are many others states of matter, as plasma (just to say an example), that show different characteristics and behavior. Precisely we can think in a liquid crystal (LC) like a state of matter which is between solid and liquid and because that, a LC can present characteristics of both states, but also present new and interesting characteristics that have many applications in many brands of the technology that we use every day.
As we know a crystal is present when the matter has high order in the positions and also in the orientations of the particles. These isotropic properties are lost when the solid go to a liquid phase. Thinking in the liquid crystal, the molecules can flow as liquid and also may be oriented in a crystal-like way. Obviously, the symmetries present in a liquid are of higher order than a crystal. Therefore we can also think in a liquid crystal like a system which have intermediate symmetries between a crystal and a liquid. One of this intermediate symmetry could be respect with the orientation of the particles in the system.
Definition: A LC is a state of matter, of organic compounds, which present properties between solid and liquid state of a crystal. Talking about symmetries is a system which present intermediate symmetries between a crystal and a liquid.
Overview of properties
LC have the property that can flow as liquid, but also present domains with different orientation order. As we can appreciate in the figure above, each domain in this LC present different optical textures when is observed under polarized light. In each one of these regions the molecules are oriented in different directions.
Some crystalline solids can pass to liquid crystals due to a change in the temperature, these are call thermotropic liquid crystals. But the factors that determine the phase transition is not only temperature, also the concentration (generally in water) or actually pressure.
History
In 1888, Austrian botanical physiologist Friedrich Reinitzer, and his colleague von Zepharovich , working with some derivatives of cholesterol (cholesteric liquid crystals) found two different melting points in a derivative cholesteryl benzoate. With the help of the physicist Otto Lehmann found that the intermediate "fluid" was crystalline.
By that time, Reinitzer had discovered and described three important features of cholesteric liquid crystals: the existence of two melting points, the reflection of circularly polarized light, and the ability to rotate the polarization direction of light.
Lehmann continue with the studies of liquid crystals and in August of 1889 he had published his results in the Zeitschrift für Physikalische Chemie. The German chemist Daniel Vorländer continued the work of Lehmann and with the time he could synthesized most of the liquid crystals know.
In 1991, when liquid crystal displays were already well established, Pierre-Gilles de Gennes received the Nobel Prize in physics "for discovering that methods developed for studying order phenomena in simple systems can be generalized to more complex forms of matter, in particular to liquid crystals and polymers".