# DoITPoMS

Summary

• Liquid crystals are characterised by their high orientational and low positional molecular order.
• Molecules capable of forming liquid crystals are always anisotropic – typically they will be calamitic (rod-shaped).
• There are three types of calamitic liquid crystal: nematic, smectic and chiral nematic. They are defined by their differing degrees of positional order.
• The degree of orientational order of a liquid crystal can be quantified using the $${\rm{order}}\;{\rm{parameter}}\;Q = {{(3\left\langle{{\cos }^2}\theta \right\rangle - 1)}} \;/\;{2}$$
• Defects in liquid crystals are given the name disclinations. Each type of disclination is assigned a positive or negative number; the magnitude indicates its strength whilst the sign indicates which disclinations can cancel each other out.
• Disclinations can be viewed directly by polarised light microscopy. For example, in a nematic they appear as schlieren brushes.
• Liquid crystals also exhibit birefringence when viewedthrough crossed polars.
• The most common modern commercial use of liquid crystals is in liquid crystal displays.