Magnetic field effects cholesterics

IV. Effects of a Magnetic Field on tfie Cholesteric Structure... 

We next examine the effect of a magnetic field acting along the helical axis of a cholesteric film having a planar texture. If > 0 and boundary constraints are absent, there is a possibility of a 90° rotation of the helical axis because > Xi- on the other hand, boundary effects are... 

In two of his earliest papers on liquid crystals,Meyer investigated how electric and magnetic fields act on the anisotropy of the electric and magnetic susceptibility in a cholesteric liquid crystal. This effect is quadratic in the field. 

R.B. Meyer, Effects of electric and magnetic fields on the structure of cholesteric liquid crystals, Appl. Phys. Lett. 12(9), 281-282, (1968). 

It is interesting to observe that cholesteric copolyesters synthesized from i+,U -dihydroxyazoxybenzene and mixtures of (+) -3-methyl-adipic acid and dodecanedioic acid do not orient. Systematic variations with molecular weight and careful analysis in certain homologous series should give information on the influence of the intrinsic flexibility of the chains, the apparent viscosity of the melt and certain basic microscopic processes such as entanglement effects on the tendency for macromole cular chains to become aligned when placed in a magnetic field. 

It is reasonable to assume that the hne shapes shown in Fig. 13 reflect the presence of helical structures in cholesteric LCs even in the strong magnetic field, and it can safely be assumed that the helical structure directions were perpendicular to the magnetic field, as proposed by Meyer [14]. At first, Eq. (18), in which uniaxial rotation is assumed, was used to simulate the hne shapes in Fig. 13. According to the field effect on cholesteric LCs, in which, macroscopically, cholesteric LCs are spontaneously twisted nematics, Eq. (18) was integrated in terms of / l, which should have provided the pseudo-line shapes of helical structures in the magnetic field. However, it was very difficult or nearly impossible to reasonably reproduce the experimental line shape with our analysis. Therefore, the concept of biaxiality for... 

We have considered several examples which illustrate the diversity of the interesting effects associated with lightwave-liquid-crystal interaction in cells with a nonuniform initial director distribution. Many such cells can be constructed. We have already seen that their qualitative properties depend not only on the boundary conditions but also on the specific physical properties of the LC material. This fact should be stressed, since in cells with a uniform director orientation the differences in the Franck constants, say, from one LC to another lead merely to quantitative differences. In many cases, even the description of the equilibrium structure for nonuniform cells encounters serious mathematical difficulties. Conservation laws may provide a powerful technique for solving problems of this type. For instance, a theorem of E. Noether was used in Ref 23 to derive analytic expressions for the equilibrium structure of complex configurations such as homeotropic-planar oriented cholesterics and cholesterics in magnetic fields with a homeotropic orientation at the walls. 

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. 

R. B. Meyer, Effects of Electric and Magnetic Fields on the Structures of Cholesteric Liquid Crystals, Appl. Rhys. Lett., 12, p. 281 (1968). 

Electric Field. The preceding and the following discussions of magnetic field induced effect in cholesteric liquid crystals can be applied to the case of electric field if we replace Ax by Ae, and /f by E, as pointed out in the previous chapter. 

In the remainder of this section we will generalize Eq. [30] to include the effects of external magnetic and electric fields and also to take account of the finite pitch in the equilibrium state of cholesteric liquid crystals. 

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