Field effects cholesteric-nematic transition

Liquid-crystal electro-optic phenomena can be divided into two categories—those caused only by dielectric forces and those induced by the combination of dielectric and conduction forces. The two conduction-induced phenomena discussed later are dynamic scattering and the storage effect. Four of the dielectric phenomena, or field effects as they are sometimes known, are discussed first (1) induced birefringence, (2) twisted nematic effect, (3) guest-host interaction, and (4) cholesteric-nematic transition. 

Square grid effect Before the electric field becomes great enough to cause the cholesteric-nematic phase transition, a periodic deformation may appear in cholesteric liquid crystals. The layer undulation occurs in two orthogonal directions so that a square pattern is observed. This effect is more likely to happen for cholesteric liquid crystals of large pitch (about microns). 

Analytical approaches to understanding the effect of molecular flexibility on orientational order have concentrated on both the isotropic-nematic and the nematic-smectic transition [61, 62] and mean field theory has shown that cholesteric pitch appears not to depend on the flexibility of the molecule [63]. 

There are of course many open questions and further possibilities in the field. Some specific points were emphasized in the text. It should be remarked that up to now most researchers concentrated on the nematic phase. Although there are still many important aspects to be investigated even in this phase, the study of other mesophases looks very promising as well. We called attention already to the problem of optical reorientation in the cholesteric and smectic C phases. Regarding thermal effects we remind that the interesting point about nematics is the nearly critical behaviour near the nematic - isotropic phase transition. Similar phenomena can be expected to take place at other second-order phase transitions such as the smectic A - smectic C or some of the nematic- smectic A transitions. 

Contrary to cholesterics and nematics, not much work has been done regarding the effects of external fields on smectics. The possibility of a Freedericksz transition in a smectic liquid crystal by an external dc magnetic field has been considered by Helfrich, Rapini, Hurault, and Meirovitch et The results show that all transitions... 

The magnetic field also widens the temperature-concentration region of the existence of the LC phase. This effect is related to the cholesteric liquid crystal - nematic liquid crystal phase transition and the orientation of macromolecules in the direction parallel to the magnetic field lines. In this case, large supramolecular structures (domains) develop in solutions. The effect of magnetic field on the variation in LC transitions with the pwlymer concentration in solution shows an extremal pattern. Figures 15 and 16 demonstrate the concentration dependences of AT for HPCl- DMAA, HPC-3-DMAA, and HPC-l-water systems measured at various magnetic field intensities. 

---