Twist elastic force

Fig. 3.21. Forces (a) and refractive indices (b) for a 5CB droplet between aligned bare mica sheets (T=27°C). (a) Force runs approaching the surfaces (filled circles and triangles) a residual repulsion is observed in retraction (open circles) a jump-out from the minimum of a layering oscillations is shown. The continuous lines represent the elastic twist force F D)/R = ttK220 /D (see [50] for details), calculated assuming an infinite anchoring strength with K22 = 6.5 x N. The twist...

Here F is the force of interaction between masses m and M, L the rod length, f the elastic parameter of the fiber, and cp the angle of the twist. 

The spatial and temporal response of a nematic phase to a distorting force, such as an electric (or magnetic) field is determined in part by three elastic constants, kii, k22 and associated with splay, twist and bend deformations, respectively, see Figure 2.9. The elastic constants describe the restoring forces on a molecule within the nematic phase on removal of some external force which had distorted the nematic medium from its equilibrium, i.e. lowest energy conformation. The configuration of the nematic director within an LCD in the absence of an applied field is determined by the interaction of very thin layers of molecules with an orientation layer coating the surface of the substrates above the electrodes. The direction imposed on the director at the surface is then... 

When a stress Le. a force per unit area) is applied to a solid, for example it is stretched, sheared, twisted or squashed, it deforms, i.e. changes its length, or shape. For small deformations, the amount of deformation is proportional to the applied stress. The material is said to behave elastically. Beyond a certain deformation (the elastic limit) the material ceases to be elastic, and the material no longer returns to its initial shape when the stress is removed. This is called plastic deformation. If the material is deformed further then it will eventually break. Some materials, such as rubber, are elastic for large deformations, while others, such as plasticine, have a relatively small elastic limit but can then undergo large plastic deformations. Brittle materials, such as china, can only withstand small deformations before they break. 

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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