Nematic homeotropic

As already mentioned, etched glass surfaces are conducive to a homeotropic orientation. Some crystalline cleavages (AI2O3, LiNbOa) also orient nematics homeotropically. However, the most popular technique for the homeotropic orientation is utilization of surfactants. 

This effect can be utilized in a liquid crystal matrix display. In the initial state all matrix elements are switched to the nematic homeotropic (dark) state by switching all column electrodes to a high potential and all row electrodes to zero potential. Immediately after this operation the column electrodes are brought to a sustaining voltage corresponding to a field strength between the two critical... 

In a representative liquid crystal mixture with a helix-pitch of about 2 /xm, the transformation from focalconic to nematic homeotropic mode needs 20 ms at 80 V. The sustaining voltage is about 10 V. The time required for the transition from the nematic to the focalconic mode is also about 20 ms. 

The special property of cholesteric substances with positive dielectric anisotropy is that for mixtures with large Ae the scattering focalconic texture is already converted to the transparent nematic homeotropic texture by a very short erase pulse of only approximately 50 jUS. The remaining erase time is required for the folding back process to a planar texture. Thus the stored image can be erased rapidly. 

FIG. 7 Same as Fig. 5, but for a nematic Gay-Berne film confined between homeotropically anchoring substrates (from Ref. 48). 

Stelzer et al. [109] have studied the case of a nematic phase in the vicinity of a smooth solid wall. A distance-dependent potential was applied to favour alignment along the surface normal near the interface that is, a homeotropic anchoring force was applied. The liquid crystal was modelled with the GB(3.0, 5.0, 2, 1) potential and the simulations were run at temperatures and densities corresponding to the nematic phase. Away from the walls the molecules behave just as in the bulk. However, as the wall is approached, oscillations appear in the density profile indicating that a layered structure is induced by the interface, as we can see from the snapshot in Fig. 19. These layers are... 

The classical scheme for dichroism measurements implies measuring absorbances (optical densities) for light electric vector parallel and perpendicular to the orientation of director of a planarly oriented nematic or smectic sample. This approach requires high quality polarizers and planarly oriented samples. The alternative technique [50, 53] utilizes a comparison of the absorbance in the isotropic phase (Dj) with that of a homeotropically oriented smectic phase (Dh). In this case, the apparent order parameter for each vibrational oscillator of interest S (related to a certain molecular fragment) may be calculated as S = l-(Dh/Di) (l/f), where / is the thermal correction factor. The angles of orientation of vibrational oscillators (0) with respect to the normal to the smectic layers may be determined according to the equation... 

Fig. 16a-c. Modes of alignment of I.c. a homogeneous b homeotropic c twisted nematic... 

Such polymers adopt, when affected by a mechanical field, an optically uniaxial homeotropic structure polymers B.1.2, B.1.7, B.1.8 (Table 8) have positive birefringence polymers B.1.1, B.1.8. (Table 9) have negative birefringence, which has not been reported to our knowledge, for low-molecular nematic liquid crystals. Although the authors do not comment on the cause for the observed phenomenon, the fact in itself is sufficiently uncommon. 

Fig. 27a and b. Scheme of thermorecording using the film of the homeotropic-oriented nematic LC polymer (a) and thermorecording of letters on the film of polymer XII at 105,5° (film thickness 40 pm) (b) I — initial homeotropic texture II — texture with the laser thermo-recorded letters... 

Fig. 3 Principle anchoring conditions of nematic liquid crystals at nanoparticle surfaces (a) planar anchoring, and (b) vertical (or homeotropic) anchoring...

The concept of local perturbations of the director around nanoparticles, often linked to homeotropic anchoring to the nanoparticle surface, is a concept often brought forward in discussions of thermal, optical and electro-optic properties of nanoparticle-doped nematic liquid crystals, which adds a slightly different perspective to the invisibility of smaller particles in aligned nematics. This appears to be of particular relevance for particles coated with either hydrocarbon chains or pro-mesogenic as well as mesogenic units. 

Moreover, segregation of nanoparticles at liquid crystal/substrate interfaces is also an effective way to promote or alter the alignment of thin nematic liquid crystal films. Depending on their concentration, size, and nature, several types of nanoparticles have been shown to induce homeotropic alignment as well as defects and remarkable defect patterns. 

Fig. 7 Examples of polyhedral oligomeric silsesquioxane (POSS) nanoparticles used to induce homeotropic alignment in nematic liquid crystals...

Fig. 8. Liquid-crystalline textures of didodecyl-PPE 12c. Bars represent 20 microns. These PPEs form homeotropic smectic phases the textures are typical for an edge-on view of lamellae that show apparent nematic ordering in this projection [46],...

Above 110 °C, this arrangement becomes mobile, and a smectic C liquid-crystalline phase is entered. Samples cooled down from the isotropic melt (140 °C) show Schlieren and banded textures when viewed under crossed polarizers (Figure 8). These textures look similar to nematic Schlieren textures, but from the X-ray diffraction data it is clear that 12c forms a homeotropically oriented smectic C phase. In a nematic phase, the small-angle diffraction peak would be absent, and a broad scattering feature, a nematic streak , would be observed. Polymer 12c was the first example of a PPE derivative for which three states of matter, i.e. crystalline, thermotropic liquid crystalline, and a highly viscous isotropic liquid, were accessible [46]. 

A suitable surface treatment results in a homeotropic orientation for a nematic mixture incorporating a dichroic dye of positive contrast and an amount of a chiral dopant insufficient to overcome the surface forces and generate a twisted structure in the nematic phase. 

The static homeotropically aligned guest-host nematic mixture (plus guest dye and chiral dopant) is optically transparent and, therefore, the display appears colourless in the non-activated state. The lower limit of the pitch for a given cell gap, before a twisted nematic structure becomes energetically more favoured than the homeotropic nematic structure, is determined by the djp ratio ... 

The first nematic guest-host prototype nematic GH-LCD reported by Heilme-ier and Zanoni " contained methyl red (157) as the dichroic dye dissolved in 4-butoxybenzoic acid as the nematic liquid crystal host. Other hosts investigated later included 4-methoxycinnamic acid and 4-ethoxy-4-aminoben-zonitrile (28), see Table 3.4. The melting point of these three single components is very high. Therefore, prototype GH-LCDs had to be operated and evaluated at very high temperatures. Thermal decomposition of the mixtures led sequentially to lower contrast, homeotropic orientation due to decomposition products and finally device breakdown. However, these initial experiments were sufficient to demonstrate the feasibility of this display type. 

Xiang Y., Lin Z. Y., Li T., Shi J., Huang J. W. and Ji L. N., A homeotropical Alignment Nematic Liquid Crystal Doped with FeTPPCl will Dramatically Decrease the Critical Magnetic Field for the Bend Reorientation in a Static Magnetic Field. Mol. Cryst. Liq. Cryst. 339 (2000) pp. 139-143. 

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