Write short note on Liquid Crystals and Glassy states.

Que.: Write short note on Liquid Crystals and Glassy states.

Ans.:

Liquid Crystals:

1. Liquid crystals (LCs) or mesomorphs are a state of matter which has properties between those of conventional liquids and those of solid crystals. For instance, a liquid crystal may flow like a liquid, but its molecules may be oriented in a crystal-like way.

There are many different types of liquid-crystal phases, which can be distinguished by their different optical properties (such as textures). The contrasting areas in the textures correspond to domains where the liquid-crystal molecules are oriented in different directions. Within a domain, however, , the molecules are well ordered. LC materials may not always be in a liquid-crystal phase (just as water may turn into ice or steam).

2. Liquid crystals can be divided into thermotropic, lyotropic and metallotropic phases. Thermotropic and lyotropic liquid crystals consists mostly of organic molecules, although a few minerals are also known.

Thermotropic LCs exhibit a phase transition into the liquid-crystal phase as temperature is changed. Lyotropic LCs exhibit phase transitions as a function of both temperature and concentration of the liquid-crystal molecules in a solvent (typically water).

Metallotropic LCs are composed of both organic and inorganic molecules, their liquid-crystal transition depends not only on temperature and concen-tration, but also on the inorganic organic composition ratio.

Glassy States:

1. Glass is a state of matter. Glasses combine some properties of crystals and some of liquids but are distinctly different from both. Glasses have the mechanical rigidity of crystals, but the random disordered arrangement of molecules that characterises liquids.

2. Glasses are usually formed by melting crystalline materials at very high temperatures. When the melt coils, the atoms are locked into a random (disordered) state before they can form into a perfect crystal arrangement.

3. As a liquid (at the melting temperature, Tm) is cooled from a high temperature, it may either crystallise or become super cooled. The particles (atoms, molecules or ions) forming crystalline materials are arranged in orderly repeating patterns, with elementary building blocks (unit cells extending to all three spatial

dimensions.

4.The structures of crystalline solids depends (predictably) on the chemistry of the material and the conditions of solidification (starting temperature and cooling rate, ambient pressures etc.) and can be described easily in detail. Super cooled liquids, on the other hand, demonstrate a rather intringing behaviour.

5. Upon further cooling below the T their particles progressively lose translational mobility, so that around the so called glass transition temperature (Ty) rearrangement to "regular" lattice sites is practically unfeasible; this behaviour is distinctive for the amorphous structures described as glasses or vitreous solids.

Que.: Write short note on Liquid Crystals and Glassy states.