Homeotropic nematics, external fields

The exact solution is obtained for describing the external field effects in the optically induced molecular reorientation and bistability in a homeotropic nematic liquid crystal. With a low external field applied transverse or parallel to the laser propagation direction, optical bistability can always be suppressed or enhanced and hence can be seen in all existing nematics at a low laser power (typically < 1 kW/cm ). The fields corresponding to the tricritical point are obtained. 

The model is qualitatively as follows. At high frequencies an electro-convective instability with a periodic velocity distribution in the plane of the layer appears in all samples (in the isotropic phase, in the planar and homeotropically oriented nematics, in cholesterics [111], in nematics with smectic order, and even in smectic A [77]), because of the nonuniform distribution of the space charge along the direction of the external field Ez)-... 

For chiral nematic liquid crystals, the method outlined previously for a planar nematic cell has been shown to be quite effective. For smectic-A the preparation method is similar to that for a homeotropic nematic cell. In this case, however, it helps to have an externally applied field to help maintain the homeotropic alignment as the sample (slowly) cools down from the nematic to the smectic phase. The cell preparation methods for a ferroelectric liquid crystal (FLC), smectic-C for surface stabilized FLC (SSFLC) operation, is more complicated as it involves surface stabi-lization. f On the other hand, smectic-A (Sm-A ) cells for soft-mode FLC (SMFLC) operation are easier to prepare using the methods described above. ... 

Figure 3.2. A homeotropic nematic liquid crystal with strong surface anchoring (a) external field off (b) external field on — only the bulk director axis is deformed.

Hydrodynamic effects on phase separations and morphologies are also important [144, 152, 153], Sulaiman et al. [153] have presented a lattice Boltzmann algorithm that includes hydrodynamics to describe the deformations of a drop of isotropic fluid immersed in a nematic Uquid crystal solvent, both in the absence and in the presence of an external field. The equilibrium shape of the drop is affected by the elastic constant K of a liquid crystal, the radius Rof a droplet, and the anchoring strength W of the director at the surface of a droplet, and the surface tension o. As W/a, or Io/Ro increases, the anchoring energy dominates over the surface tension and the droplet distorts to more easily allow homeotropic anchoring at its surface. 

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