Classification of liquid crystals
When classifying liquid crystal phases we first of all distinguish between two main types: one with nematic order and one with smectic order. In the nematic phase (of which the cholesteric phase is a special case) the molecules are free to move in all directions (i.e. there is no positional order of the centers of mass), but on the average they keep their long axes locally parallel (see figure 2). In a smectic state, of which a number of important structural variations exist (figure 6), we also have a positional order along one dimension (some smectic phases have positional order in more than one dimensions). A smectic is a layered structure with the molecules oriented parallell or tilted relative to the layer normal. Two smectic phases, called smectic A and smectic C, have acquired a special importance and are now relatively well understood. They are characterized by absence of positional order within the layers; the molecules have some freedom to move within the layers, while they, as in all smectic phases, are much less free to move between layers. These smectics can therefore be said to be stacks of two-dimensional fluids, but they behave as crystalline across the layers. The abscence of in-layer order contributes to their high potential for future electro-optic applications.