Grain chiral nematics

The structures of phases such as the chiral nematic, the blue phases and the twist grain boundary phases are known to result from the presence of chiral interactions between the constituent molecules [3]. It should be possible, therefore, to explore the properties of such phases with computer simulations by introducing chirality into the pair potential and this can be achieved in two quite different ways. In one a point chiral interaction is added to the Gay-Berne potential in essentially the same manner as electrostatic interactions have been included (see Sect. 7). In the other, quite different approach a chiral molecule is created by linking together two or more Gay-Berne particles as in the formation of biaxial molecules (see Sect. 10). Here we shall consider the phases formed by chiral Gay-Berne systems produced using both strategies. 

There is a wide range of other chiral mesophases that are somewhat unconventional in terms of their structure and many of these are rather recent developments. Blue phases are cubic mesophases that have a double twist stracture. They are exhibited by highly chiral materials and appear at temperatures above the chiral nematic phase. These phases give characteristic platelet type textures that are often blue (hence the name) but they can also appear green, yellow and red. The twist grain boundary (TGBA ) phase also has a fmstrated complex helical stmcture. In this case the normal lamellar stmcture is... 

Now that we have discussed all of the structures of the phases that play a part in the formation of twist grain boundary phases, let us now consider the events that can occur at a nematic to smectic phase transition. At a normal chiral... 

The result obtained has very interesting consequences (i) to have well aligned SmA samples, very flat glasses without corrugation are needed (ii) even small dust particles or other inhomogeneities create characteristic defects in the form of semispheres (see Fig. 8.29b below) and weU seen under an optical microscope (iii) layers are often broken (not bent) by external factors in particular, strong molecular chirality may result in the formatimi of defect phases like twist-grain-boimdary phase (iv) the thermal fluctuations of director in smectic A phase are weak and the smectic samples are not as opaque as nematic samples. In fact there is a critical cell thickness for short-wave fluctuations. 

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