In this chapter we consider the orientational order from the microscopic point of view. A central role is played here by the orientational distribution function, which in general depends on all three Eulerian angles. In the case of a uniaxial medium like the nematic or smectic A phase, however, the dependence on one of these angles can be disregarded. Experimentally it is very difficult to determine the full orientational distribution function. For this reason one often resorts to an expansion of this function. The expansion coefficients form a set of microscopic order parameters, of which only a few can be determined experimentally. Several of the appropriate methods will be discussed in some detail. If one is prepared to accept certain models for the molecules, the microscopic order parameters can be related to the macroscopic tensor order parameter introduced in Sect. 5.2. In the last section scattering methods are discussed that, at least in principle, give information on the full orientational distribution function.
KeywordsNuclear Magnetic Resonance Electric Field Gradient Dipolar Coupling Nematic Phase Isotropic Phase
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