Modulated Structures in Dielectric Liquid Crystals

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Abstract
Review of theoretical and experimental results concerned to formation of space-modulated orientational structures in liquid crystals is given. Properties of ideal liquid crystal dielectrics are considered. Origion conditions and properties of flexoelectric and flexoelectromagnetic structures in nematics, dielectric instabilities in cholesterics and smectics, ferroelectric structures in chiral smectics are discussed. It is well known that an electric field induces a modulation of the orientational structure of liquid crystals. There are two types of such modulated structures (MS) having different physical natures. The dissipative one is connected with the transport of electric charges and mass of the substance. The thermodynamic MS are not accompanied by the irreversible transport processes. Such MS can exist in ideal liquid crystal dielectrics. In fact there are always some ion impurities and as a result two types of MS are possible in liquid crystals. However it is possible to change the thresholds of these orientational instabilities in dependence on the concentration of impurities, anisotropy of transport parameters, boundary conditions and frequency of an external field. Thus one can exclude or induce the effects of first and second types. Below we shall consider the thermodynamic orientational transformations and conditions of their observation.