1. Liquid Crystals Thermotropic (temperature dependent)
Lyotropic (concentration dependent)
Metallotropic
Nematic
Smectic
Cholesteric
Various forms depending on concentration

2. Thermotropic Liquid Crystals
Flexible Tails
Rigid Core
Rod shaped molecule

3. Liquid Crystals are a Phase
For some molecules, there are extra steps in the phase change between solid and liquid.
Crystalline
Smectic
Nematic
Liquid
Liquid Crystalline
Increase in temperature Decrease in order.

4. Smectic Liquid Crystals
Less ordered than a crystalline solid.
Molecules have orientational order (they are all “facing” in about the same direction).
Molecules are in layers (some positional order, but they behave like liquids (i.e. can move around) within these layers.

5. Nematic Liquid Crystals
Less ordered than smectic phase and crystalline solid.
Molecules have orientational order (they are all “facing” in about the same direction).
Molecules have no positional order (there are no layers).

6. Liquid Crystal Displays
Use “twisted” nematic phase.
Liquid crystals form a helical twist as a result of perpendicular directors (polarising filters).
Liquid crystals act a guide for polarised light.

7. Cholesteric Phase (aka Chiral Nematic)
Some types of nematic liquid crystals spontaneously arrange themselves into a spiral arrangement
The molecules have a nematic arrangment within each layer.

8. Cholesteric Liquid Crystals and Temperature
The distance for a full twist in the helix is call the “pitch”.
Cholesteric liquid crystals reflect light approx. equal to the pitch.
The pitch depends on temperature.
At lower temp the pitch is longer.

9. Effect of Phase on Opacity