Liquid Crystals: Materials for Displays

It was about 100 years ago that an anomaly in the melting point of certain materials led to the discovery of liquid crystals. An Austrian botanist named Reinitzer had noticed that cholesteryl benzoate, a substance very similar to cholesterol, did not melt at a precise temperature as most organic materials do. This curious behavior was thought to be due to impurity in the material, but as the material was purified further, the anomaly in the melting behavior grew, and led to the discovery of the first liquid crystal. With this discovery of the cholesteric liquid crystal began the fascinating search for other liquid crystals. The word cholesteric probably has its origin in the fact that the material Reinitzer had investigated was a derivative of cholesterol, and in liquid crystal because the material has a state in between a liquid and a crystal. With discoveries that followed, it was learned that cholesteric liquid crystals are actually a sub-class of a more general class of materials called nematics in which the molecules possess no positional order, but have long range orientational order. By this we mean that the long axis of the molecules remains approximately parallel over long distances, but the position of the molecules does not form a periodic lattice. The local average direction of the long axis of the molecules is generally represented by a unit vector, n, called the director. The nematic liquids composed of chiral molecules or containing chiral additives possess a helical structure and are called cholesteric liquid crystals. Liquid crystals in general are sometimes called mesophases from the Greek word meso, which means intermediate. It is remarkable that such a humble beginning