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thermotropic liquid crystal molecules exhibit temperature dependent liquid crystal behaviour, the
transition to liquid crystal is accomplished by a change of temperature
lyotropic liquid crystals are materials in which the liquid crystalline properties are induced by the
presence of a solvent, with mesophases 5 depending on solvent concentration, as well as temperature.
The main phases of liquid crystals are nematics, chiral nematics, smectics and chiral smectics.
Nematics
By decreasing the temperature from the isotropic phase, in which the molecules are randomly
positioned and oriented, to the nematic phase, the material gains an amount of orientational order but
no positional order. This orientational order makes it possible to define an average direction of the
molecules, called the director n [3].
Fig. 73 The director n
Chiral nematic (cholesteric)
In this phase the molecules prefer to lie next to each other in a slightly skewed orientation. This
induces a helical director configuration in which the director rotates through the material. The liquid
crystals consist of rod-like molecules, which take the form of the steps of a microscopically small spiral
staircase.
Fig. 74 Chiral nematic phase
An important characteristic of the chiral nematic phase is the pitch p. It is defined as the distance it
takes for the director to rotate one full turn in the helix. A by-product of the helical structure of the
5 Equilibrium liquid crystalline phases formed with order less than three dimensional (like crystals) and mobility less than that
of an isotropic liquid. Parallel orientation of the longitudinal molecular axes is common to all mesophases (long-range
orientational order)
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