1. Introduction



What are Liquid Crystals?

Liquid Crystals (LCs) are state of matter intermediate between that of a crystalline and an isotropic liquid. They possess many of the mechanical properties of liquid, e.g., high fluidity, formation, and coalescence of droplets. At the same time they are similar to crystals in that they exhibit anisotropy in their optical, mechanical, electrical, and magnetic properties.

The characteristic feature of LCs is the presence of long-range orientational order in the arrangement of constituent molecules, and sometimes one- or two-dimensional quasi long-range translational or positional order. LCs exhibit a great variety of phases, which differ one from another by their structure and physical properties.

Although LCs, or mesophases, combine the properties of a solid and an isotropic liquid, they exhibit very specific electrooptical phenomena, which have no corresponding analogues in solids or in isotropic liquids.

Why Liquid Crystals?

The quintessential property of a LC is its anisotropy. The optical, mechanical, electrical and magnetic properties of LC medium are defined by the orientation order of the constituent anisotropic molecules. Due to the anisotropy of the electrical and magnetic properties, the orientation of the LC molecules is effectively controlled by weak electric or magnetic fields. As a result, changing the LC molecules orientation, it is possible to change optical, mechanical properties of the medium. All of these are important to the functioning of devices based on LCs: digital watches, calculators, flat TV-displays, thermometers and LC displays are all examples of what LC technology can achieve.

A brief history of Liquid Crystals

The discovery of an intermediate, liquid crystalline, state of matter is credited to Friedrich Reinitzer, an Austrian botanist. He described his experiment with the cholesteryl ester in a paper published in 1888. The name liquid crystal was first suggested by Otto Lehmann, a German physicist. A classification of LCs based on their structural properties was first proposed by Georges Friedel, a French mineralogist, in 1922. Experimental investigations by O. Lehmann and G. Friedel, together with the theory of LCs presented by the Sweden physicist Carl Oseen, formed the scientific bases of LCs research. It was not until in the 1950s that further progress was made in understanding liquid crystals (by F.C. Frank and others). Since middle 1960 the entire theoretical and experimental development has been influenced by Pierre-Gilles de Gennes, a French physicist.



Liquid Crystals