Propagation normal to the optic axis

Sackmann et have investigated the temperature variation of the pitch of a mixture of right-handed cholesteryl chloride and left-handed cholesteryl myristate by this method. At a certain temperature (7 ) there is an exact compensation of the two opposite helical structures and the sample becomes nematic. At this temperature only the central spot (zero order) is observed, while at the other temperatures, polarized diffraction maxima of higher order make their appearance. The inverse pitch varies almost linearly with temperature passing through zero at 7 (fig. 4.1.16). 

We now consider defect structures in the cholesteric liquid crystal. Treating the cholesteric as a spontaneously twisted nematic, 

P being the pitch. The free energy density is then expressible as 

Such disclinations are closely analogous to nematic wedge disclinations ( 3.5.1). The singular line is along the z axis (parallel to the twist axis) and the director pattern is given by 

As already indicated briefly in 3.5.8 the effect of elastic anisotropy has some interesting implications for cholesterics, especially for long-pitched structures. We have seen that disclination pairs in nematics have angular forces in the presence of elastic anisotropy. For all practical purposes, the solutions that were obtained for nematics will hold good for each nematiclike cholesteric layer, except that the layers now twist continuously in the 

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