Unit 1- Class 3- Liquid Crystals (Classification of liquid crystals)

The nematic phase
They exhibit long range molecular ordering but possess no positional ordering. The preferred direction varies from point to point in the medium, but there is a uniform alignment with respect to the long axis- director. The nematic phase is bifringent due to the anisotropic nature. They also exhibit dielectric anisotropy.


The cholesteric phase
In this structure the local molecular ordering is identical to that of the nematic phase, but are exhibited my molecules possessing chiral centers. Chiral nematic liquid crystals are also refereed as twisted nematic liquid crystals. Unlike in nematic phase where all the molecules stay parallel to one another, in chiral nematic phase, the molecules arrange themselves in such a way that a group of molecules align at different angles with respect to their adjacent groups i.e., the director is not fixed in space, but rotates through out the sample forming a helical pattern as it changes its direction. The distance traveled by the director as it completes one full turn is called the pitch of the liquid crystal. The twist present in chiral nematic crystal makes them to exhibit spectacular optical properties, which is made use of in displays. The most striking features of cholesteric mesophase is strong optical activity and selective light reflection. The pitch is temperature dependent and hence cholesteric phase is finding application in thermography.



The nematic phase
Substances that form smectic phases are soap-like. In smectic mesophase, there is a small amount of orientational order and also a small amount of positional order. The molecules tend to point along the director and arrange themselves in layers. Based on the orientation of the director there are many types of smectic phases. The interlayer force of attraction in smectic phase is weak as compared with the lateral forces between molecules. As a consequence the layers are able to slide over one another.

in the next class we will discuss about d or columnar liquid phases