Twist Cells

Figure 8.11 Illustration of Mauguin twisted nematic cell, reported in 1911. Substrates are thin mica plates, which are uniaxial with their optic axis parallel to plane of plates. Apparently, uniaxial crystal stmcture of mica produces strong azimuthal anchoring of nematic LCs of Lehmann, such that director is parallel (or perpendicular) to optic axis of mica sheets at both surfaces. Mauguin showed that method of Poincard could be used to explain optics of system if it was assumed that LC sample created layer of material with uniformly rotating optic axis in twisted cells.

In this way, the above two methods are very effective to control ee and choose one of the chiral domains. It is noted that both methods use no chiral molecular species. Instead, CPL and twist cell geometry are used as chiral stimuli, which act as a symmetry-degeneracy-breaking field. This field triggers and accelerates a preferential formation of one of the two possible chiral conglomerates, which is fixed in a B4-like phase. The monochiral films obtained by achiral molecules open... 

Fig. 4 (a) Applied voltage vs. transmission (V-T) curves of in twisted cells at 100 Hz and 25°C. a) Pure 5CB, b) 5CB-capped Pd, c) 5CB-capped Ag-Pd (1/9), d) 5CB-capped Ag-Pd (1/4), e) 5CB-capped Ag-Pd (1/1), f) 5CB-capped Ag-Pd (4/1), g) 5CB-capped Ag-Pd (9/1), and h) 5CB-capped Ag. (b) Cartoon of a liquid crystal cell filled with 5CB-sol containing metal nanoparticles. A self-assembly property of liquid crystal is speculatively perturbed by nanoparticles in the liquid crystal medium [301], (Copyright 2008, American Chemical Society)... 

Farrell, E.R. and Keshishian, H. (1998). Laser ablation of persistent twist cells in Drosophila Muscle precursor fate is not segmentally restricted. Development 126 273-2S0. 

R 347 I. Botos and A. Wlodawer, Cyanovirin-N A Sugar-Binding Antiviral Protein with a New Twist , Cell Mol. Life Sci., 2003,60,277 R 348 W. Braun, Developing Computational Tools for NMR The Early Days of Protein NMR , Magn. Reson. Chem., 2003,41( pec. Issue), S51 R 349 K. L. Bren, Recent Advances in NMR Spectroscopy of Cytochromes C , Chemtracts, 2002,15, 677... 

Measurements of the LC azimuthal anchoring energy coefficient W(f oi the PVCN substrates (Fig. 5.9) were performed by preparing twist cells with... 

Let us first concentrate on the flexodomains. With the help of the general theory e — es can be determined from the measurements of Uc and qc of the flexodomains if the elastic coefficients K and and the anisotropy are known. In this way the value of ei — es = 5.7 pC/m was obtained for BMAOB. Note that this value has been confirmed for twisted cell geometry. In a later study BMAOB doped with a swallowtail compound in concentrations of up to 4.55 mole% was analysed. The value of ei — es for the mixture was found to slightly increase with increasing concentration of the swallow-tail compound (see Fig. 6 in Marinov et... 

To illustrate the variation technique that is very useful for subsequent discussions of electro-optical effects, consider a simplest example. For a twist cell shown in... 

Fig. 8.9 Twist cell in the zero field geometry of the problem (a) and the calcnlated distribution of angle tp(z)...

It is instructive to calculate the value of the elastic energy (per unit area) of a typical twisted cell, discussed above. Using (8.23), the free energy is given by... 

For consistency we go back to the problem of the twisted cell discussed in Section 8.3.2, however, the director angles cp at the boundaries will be not constant but can be changed due to elastic and external torques. Let a nematic layer be confined by two plane surfaces with coordinates zj = —plane through angle cp (there is no tilt, the angle 9 = ti/2 everywhere, and the azimuthal anchoring energy is finite). 

Finally, we find the dependence of the twist angle on the z-coordinate in a twist cell with soft director anchoring at one boundary ... 

Fig. 11.8 A planar cell installed between crossed polarizers is non-transparent (a) whereas a twist cell rotates the linear polarization through tc/2 and transmits light (b). Polarizer P is parallel to the director at the input plate...

Fig. 12.16 Bistable twist cell. Right- and left-handed twist-structures of a nematic liquid crystal with the same elastic energy (a) and the angular dependence of total free energy (b)...

Berreman, D.W., Hefluer, W.R. New bistable liquid-crystal twist cell. J. Appl. Phys. 52, 3032-3039 (1981)... 

SURFACE ORIENTATION AND COMPLIANCE EFFECTS ON TWIST-CELL PERFORMANCE... 

In a recent article Nehring, Kmetz and Scheffer described effects of weak anchoring on equilibrium configurations of twist cells.1 In order to obtain tractable equations for analytic solution, the three bulk elastic constants were made equal, the field-and-strain-free orientation at the surface was assumed to be parallel to the surface, and only nematic liquids were discussed. That treatment gives useful insights into the nature of the problem but leaves a number of interesting questions unanswered. 

In this paper some results of numerical computations of dynamic configuration changes in twist cells will be described. 

Geometric parameters to describe a twist cell are illustrated in Fig. 1. In the absence of an applied voltage, the elastic strain energy of the bulk and at the surface is minimized at... 

Fig. Schematic diagram of a twist cell showing tilt angle 0 azimuth 3 cell thickness h and relative positions of polarizer and analyzer.

Five independent viscosity parameters are required to describe the flow of nematics and cholesterics. The set of five may be chosen in various ways.9 one particularly simple set determines resistance to the motions shown in Fig. 4. The first three, riii U2 and ri3, are like the viscosity of ordinary isotropic liquids but depend on the relative direction of shear and director orientation. These three may be regarded as principal axis of a viscosity ellipsoid. The fourth, ni2 is a measure of deviation from true ellipticity, giving a difference in viscosity as directors lean toward or opposite the shear flow. The final viscosity, Yl determines resistance to pure rotation without flow. In some cases a fair approximation to twist cell behavior may be obtained ignoring flow and considering only Yl When numerical computations are undertaken, however, one might as well do the problem correctly and include the flow.11 12... 

In order to compare the temperature dependence of the pretilt angle for the twisted and the non-twisted samples, twisted cells were made using the same treated glass plates which had been used for the measurements by the MCN method. Thus, as soon as the temperature dependence of the pretilt angle for the non-twisted cell was measured, the cell was taken apart, rinsed with an organic solvent to remove LC, and dried, then the twisted cell was reassembled. 

It is clear that the twisted cell has a weak decreasing tendency... 

The effect of the 90 twist on the temperature dependence of pretilt. Solid squares are data of the first measurement for non-twisted cell and triangles for twisted cell using same glass substrates. 

Tilt varies throughout the cell thickness In twisted cells due to a competition between the splay and bend elastic energies and the twist elastic energy. For small tilts and twists, 90 , this variation Is calculated to be small. Doping the liquid crystal with a cholesteric compound, as Is commonly done In TN-LCDs, stabilizes the twisted structure, thereby decreasing tilt variation. 

Consider a 90 twist cell with S3rmmetric surface director tilt between crossed polars after application of a relatively high AC field. Then 61=62 and Eq. (6) reduces to zero. Light intensity transmitted through this system is essentially zero for all values of 0. Figure 4 shows the experimental verification of this situation. The cell was known to have a twist of 90. ... 

In contrast, consider a 90 twist cell of unsymmetric surface director tilts. Such dissymmetry can be introduced by depositing SiO at different angles of Incidence on the two plates. Under this circumstance 61 62. Light intensity falls to zero only when 0=0 , i.e., when the front surface director lies in the incident plane of polarization. Light intensity increases with the absolute value of 0. Figure 5 shows the behavior of such a cell. 

D. W. Berreman, Optics in smoothly varying anisotropic planar structures application to liquid-crystal twist cell, J. Opt. Soc. Am., 63, 1374 (1973). 

D. W. Berreman, Liquid-crystal twist cell dynamics with backflow, J. Appl. Phys., 46, 3746 (1975). 

Figure 4.12 gives the dependences of the optical transmission of a twist cell for both the conventional geometry P L(0) and when the polarizer P forms an angle of 45° with respect to the orientation of the director at z = 0... 

Preparation of Twist Cells, Optical Properties at Zero Field... 

A twist cell is usually formed by placing orienting glasses on top of each other. Then twist directions at angles tt/2 and —tt/2 are equally probable. Regions with different signs of twist are observable by interference methods... 

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