What are Liquid Crystals?

Liquid crystals are a phase of matter whose order is intermediate between that of a liquid and that of a crystal. The molecules are typically rod-shaped organic moieties about 25 Angstroms in length and their ordering is a function of temperature. The nematic phase, for example, is characterized by the orientational order of the constituent molecules. The molecular orientation (and hence the material's optical properties) can be controlled with applied electric fields. Nematics are (still) the most commonly used phase in liquid crystal displays (LCDs), with many such devices using the twisted nematic geometry.

Nematics LCD Display Nematic Director

The smectic phases, which are found at lower temperatures than the nematic, form well-defined layers that can slide over one another like soap. The smectics are thus positionally ordered along one direction. In the Smectic A phase, the molecules are oriented along the layer normal, while in the Smectic C phase they are tilted away from the layer normal. These phases, which are liquid-like within the layers, are illustrated below. There are also tilted phases which have hexatic in-plane ordering, like the Smectic I and Smectic F, as well as various crystalline smectics.

Smectic Ordering

Of particular interest are tilted phases of chiral molecules, which possess permanent polarizations and are thus ferroelectric. These ferroelectric liquid crystals (FLCs) respond much more quickly to applied fields than nematics do and can be used to make fast, bistable electro-optic devices called Surface-Stabilized Ferroelectric Liquid Crystals (SSFLCs).

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