What are liquid crystals
Nematics
Smectics
History of liquid crystals
Recommended reading
The three common states of matter, solid, liquid and gas, are different because the molecules in each state have a different degree of order.
In the (crystalline) solid state there exists a rigid arrangement of molecules which stay in a fixed position and orientation with a small amount of variation from molecular vibration. To maintain this arrangement there are large forces holding the molecules in place and therefore a solid is difficult to deform. In the liquid phase the molecules have no fixed position or orientation and are free to move in a random fashion and the liquid state has less order than the solid state. The random motions of the molecules mean that the intermolecular attractive forces that kept a solid together are now only strong enough to keep the liquid molecules fairly close together. A liquid can therefore be easily deformed. In the gas state the random motion of the molecules has increased to overcome the intermolecular forces and the molecules eventually spread out to fill any container that holds them. The order in a liquid which derived from the closeness of the molecules has therefore been lost in a gas which consequently has less order than the liquid. The probability of molecules in a certain region being in a rigid arrangement and of the same orientation can be used to define a positional and orientational order which is greatest in the solid state and least in the gaseous state.
The differences between the three states can be attributed to the temperature of the substance. Temperature is a measure of the randomness of the molecules and therefore the higher the temperature the less order exists and increasing temperature will cause the transition from a solid to a liquid and then to a gas.
A thermotropic liquid crystalline phase occurs in some substances in a temperature region between the solid and liquid states. In this state the substance possesses some properties of both liquids and solids. A liquid crystal is a fluid like a liquid but is anisotropic in its optical and electro-magnetic characteristics like a solid. When the liquid crystal is formed from the isotropic state some amount of positional or orientational order is gained. It is this order that accounts for the anisotropies of the substance.
In the following sections the main phases of thermotropic liquid crystals are described.
