Hybrid nematic cell

Our illustration of pretransitional behavior in frustrated NLC systems is restricted to fluctuations in the biaxial structure of the hybrid nematic cell with equal strengths of homeotropic and planar anchorings. The detailed analysis of the pretransitional dynamics of all five degrees of freedom around the bent-director configuration is somewhat more complicated because of the nonuniformity of the base tensors. 

The common feature of all ordered structures in a hybrid nematic cell is the repulsive character of the structural force. The repulsion is due to the antagonistic boundary conditions, which always lead to at least small deformations, and to the fact that within a certain ordered structure the frustration is stronger if the confining substrates are brought closer to each other. The magnitude of the force is tuned by the anchoring strength at both substrates. 

Ordering in a Frustrated Nematic Hybrid Nematic Cell... 

Fig. 8.9. (a) Director s tilt angle with respect to the 2 direction in a /i = 10-layer hybrid nematic cell as a function of MG temperature. Different lines correspond to the tilt angle in distinct layers 1 denotes the LG layer closest to the in-plane aligning substrate and 10 the layer nearest to the homeotropic substrate. Inset Temperature variation of ordering parameters in the two layers closest to the middle of the cell, (b) Temperature of the structural transition as a function of a cell thickness. 

Fig. 8.19 Walls. Hybrid nematic cell with planar and homeotropic director alignment at opposite bovmdaries with a wall between the two degenerate patterns differing by a tilt angle sign (a), and the top view on the different tilt areas surrounded by walls (b)...

It has also been used to study the second sound resonance in smectic A liquid crystals and measure the compression modulus. For measuring the flexocoefficients (ei — 63) and (ei + 63), hybrid-aligned nematic cells have been used extensively. AC techniques avoid problems associated with ionic impurities, but require elaborate numerical fitting of the data. Some observations on the pnblished measurements of flexo-coefHcients are made in Section 2.4, which ends with a few concluding remarks. 

Fig. 2.2. Schematic diagram of a hybrid-aligned nematic cell. The field-free director (shown by the continuous curved line) has a splay-bend curvature distortion in the xz plane. A DC field applied along the y axis rotates the polarization and the director (shown by the curved dashed line) acquires a 4>(z) profile. (Reproduced from Dozov et al. with the permission of EDP Sciences, http //publications.edpsciences.org.)...

S. Ponti, P. Ziherl, C. Ferrero and S. Zumer, Flexoelectro-optic effect in a hybrid nematic liquid crystal cell, Liq. Cryst. 26(8), 1171-1177, (1999). doi 10.1080/026782999204192... 

S.A. Jewell and J.R. Sambles, Fully leaky guided mode study of the flexoelectric effect and surface polarization in hybrid aligned nematic cells, J. Appl. Phys. 92(1), 19-24, (2002). doi 10.1063/1.1483392... 

P.R. Maheswara Murthy, V.A. Raghunathan and N.V. Madhusudana, An AC electro-optic technique of comparing the two anchoring energies of a hybrid aligned nematic cell, Liq. Cryst. 14(4), 1107-1124, (1993). 

A. Mazzulla, F. Ciuchi and J.R. Sambles, Optical determination of flexoelectric coefficients and surface polarization in a hybrid aligned nematic cell, Phys. Rev. E 64(2), 021708/1-6, (2001). doi 10.1103/PhysRevE.64.021708... 

The orientation interaction is considered for light waves and liquid crystals with a uniform initial director orientation (such crystals include twisted nematics and cholesterics, and crystals with hybrid or hyperhybrid structures). Conditions are found for critical behavior to occur for a nonthreshold interaction. A light-induced Frederiks transition is predicted for cells with a hybrid nematic orientation, in which radiation absorption may reorient the director. 

FIGURE 4.3. The Prederiks transition in different geometries (a) nematic between two coaxial cylinders (b) quasi-homeotropic orientation with opposite director tilt at the boundaries and (c) hybrid aligned (homeoplanar) nematic cell. 

Guillen, A.D. Mendoza, C.I. (2007). Influence of an electric field on the non-Newtonian response of a hybrid-aligned nematic cell under shear flow. The Journal of Chemical Physics, Vol. 126, pp. 204905-1-204905-9 Guillen, A.D. Mendoza, C.I. (2007). Erratum Influence of an electric field on the non-Newtonian response of a hybrid-aligned nematic cell under shear flow. The Journal of Chemical Physics, Vol. 127, pp. 059901-1-059901-2... 

Several types of spontaneous periodic director pattern yield information about elastic coefficients. Static stripe textures, as described by Lonberg and Meyer [45], appear in polymer nematics if the twist/splay ratio below the critical value of 0.303. Calculations of director fields and the influence of elastic constants and external fields on the appearance of these periodic patterns have been performed by several authors (e.g. [49-51]). In nematic cells with different anchoring conditions at the upper and lower cell plates (hybrid cells), other types of striped texture appear these are similar in nature, but involve different director deformations and elastic coefficients. For a description of various types of static periodic texture and their relationship to elastic coefficients see, for example, Lavren-tovich and Pergamenshchik [52]. In thin hybrid aligned films, a critical thickness is observed below which the director align-... 

Besides these two standard cell alignments, there are many other variations such as hybrid, twisted, supertwisted, fingerprint, multidomain vertically aligned, etc. Industrial processing of these nematic cells, as well as the transparent conductive coating of the cell windows for electro-optical device applications, is understandably more elaborate. 

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