Director realignments

Now we see that only fluctuations of the director in the y-direction are essential, bn = Hy, because - fluctuations are not related to the director realignment and rix -fluctuations are not seen by the transverse light wave incident from the x-direction. 

The most common experiments of this type involve reorientation of the sample in the magnetic field [20, 118-125]. Either a sudden reorientation of the initially magnetically aligned sample by a fixed angle about an axis perpendicular to the magnetic field, followed by observation of the development of the transient orientational distribution of the director, or continuous rotation of the sample can be used. Both types of experiments yield /i, the twist or rotational viscosity. In sudden reorientation experiments with 6q>k/4, spatially periodic patterns of the director reorientation form and the director realignment becomes inhomogeneous. In this case, NMR spectra can yield four of the five independent viscosity constants and the ratio of two of the three elastic constants of the nematic phase [121]. 

Electrical (capacitive) detection. Electric methods provide an alternative way of monitoring the director realignment. They are based on following the time dependence of the effective electric susceptibility of the nematic layer. The effective susceptibility (in general a complex quantity) across the initially planar nematic sample of thickness d is... 

Upon the application of an operating voltage just above the threshold voltage results in a modest realignment ( 10%) of the nematic director in the middle of... 

The application of an electric field between the electrodes results in a realignment of the nematic liquid crystal mixture and the dichroic dye molecules parallel to the electric field resulting in a lower optical density (absorption) and, theoretically, the disappearance of colour assuming an ideal order parameter (S = 1) of the nematic liquid crystal director and the dye molecules. A residual absorption in this state gives rise to a display with a strongly coloured background and weakly coloured information. 

The coefficients of friction for the director have the dimensions of viscosity and are particular combinations of Leslie coefficients, ji = aj,- (I2, Ji = 3 + 2-It is significant that only two coefficients of viscosity enter the equation for motion of the director. One (72) describes the director coupling to fluid motion. Eor example, if the director tumes rapidly under the influence of the magnetic held, then, due to friction, this rotation drags the liquid and creates flow. It is the backflow effect that will be described in more details in Section 11.2.5. The other coefficient (Yi) describes rather a slow director motion in an immobile liquid. Therefore, the kinetics of all optical effects caused by pure realignment of the director is determined by the same coefficient yj. However a description of flow demands for all the five viscosity coefficients. 

Assume that our capacitor is filled by a nematic mixture with 0 well aligned along the x-axis and let the same charge injection mechanism works. Then, in a dc regime, the periodic flow will inevitably interact with the director. The maximum realignment, i.e. the deflection of the director angle 9 in the z-direction, will be... 

As the previous sections have shown, nematic polymer liquid crystals may be oriented by surface forces and in electric fields. It has been shown recently that such field-induced changes in orientation may also be used to orient pleochroic dyes through the guest-host effect. In such an effect either guest dyes dissolved in a nematic polymer " host or side-chain dye moieties in a nematic copolymer system (where A is a nematic moiety and B is a dye in Fig. 2b) undergo a cooperative realignment as the nematic director responds to the applied field. Since the pleochroic dye has its absorption transition... 

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