LIQUID CRYSTALS

Liquid crystals (LCs) are distinctive functional soft materials with a combination of order and mobility on a molecular, supramolecular and macroscopic level. Hierarchical self-assembly in LCs offers a powerful strategy for producing nanostructured mesophases. Molecular shape, microsegregation of incompatible parts, specific molecular interaction, self-assembly and selforganization are important factors that lead to the formation of various LC phases. LCs are accepted as the fourth state of matter after solid, liquid and gas. This fourth state of matter is intermediate between the solid and the liquid state. For this reason, they are referred to as intermediate phases or mesophases. This is a real thermodynamic stable state of matter where the constituents of a mesophase are called mesogens. A rigid core of the mesogen (which often consists of aromatic rings) induces structural order whereas the flexible parts (e.g., alkyl chains) provide the necessary mobility within the LC phase.

The unique feature of LCs is the presence of both order and high degree of mobility in the mesophase that leads to the self-healing, adaptive and stimuli-responsive behaviour of these supramolecular systems and because of this, LCs have become the quintessential self-assembling molecular materials of the modern era. LCs have made huge impact on the development of the human societies. LCs are the advanced technological material found in lowpower- consuming LC displays (LCDs) which are being used in the last decades for the development of mobile data processing and communication tools. It is quite possible that LCDs might be replaced by other technologies in the future but, the fundamental knowledge gained with LCs can be used for the self-assembly of a huge variety of other materials.

In 1888, Friedrich Reinitzer, Professor of Botany and Technical Microscopy at the German Technical University in Prague found that the compound cholesteryl benzoate which he had extracted from carrots exhibited two melting points, one at 145.5 °C and other at 178.5 °C. Between these two melting points, there was a milky liquid phase. Above 178.5 °C, the phase became clear. He observed distinct violet and blue colour phenomena at both these two different melting points under polarizing optical microscope. After having similar observations with a further derivative cholesteryl acetate that has a monotropic cholesteric phase, he contacted physicist Otto Lehmann.