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

Thermotropic Liquid Crystals Flexible Tails Rigid Core Rod shaped molecule

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.

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.

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).

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. http://youtu.be/O3aITfU_UvE

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.

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.

Effect of Phase on Opacity