Homeotropic liquid crystal

M. Buczkowska and G. Derfel, Analysis of deformations of flexoelectric homeotropic liquid crystal layers with various anchoring strengths, Opto-Electronics Review 19(1), 56-60, (2011). doi 10.2478/sl 1772-010-0065-0... 

FIGURE 7.35 Layout of a homeotropic liquid crystal light valve. 

FIGURE 5.25. Diffraction of light by a homeotropic liquid crystal in the spatially nommiform harmonic field. Below Relative modulation characteristics max versus spatial frequency u)s of the field, u max and 0 )max characterize the resolution and sensitivity of the liquid crystal layer, respectively. Curves 1-6 correspond to different anchoring energies Ow = V0dlK33 (1) 0.133 (2) 0.665 (3) 1.33 (4) 6.65 (5) 66.5 (6) 133. 

Homeotropic liquid crystal cells remain important in liquid crystal deflec-toscopy (flow detection) [155, 167]. In this case, the controlling potential is created by the defect hole and could be approximated by item 3, Table 5.2. A typical homeotropic liquid crystal cell for testing dielectric films is shown in Fig. 5.27. The defect hole creates a nonuniform field, which becomes visible against a dark background made by crossed polaroids. 

FIGURE 5.27. A liquid crystal cell for testing dielectric films (1) Si substrate (2) Si02 dielectric film (3) homeotropic liquid crystal (4) teflon spacer (5) electrode (6) glass. 

Figure 32. Electrooptic transmission characteristic of a homeotropic liquid crystal cell between crossed polarizers versus applied voltage for incident monochromatic light.

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... 

Price and Wendorff31 > and Jabarin and Stein 32) analyzed the solidification of cholesteryl myristate. Under equilibrium conditions it changes at 357.2 K from the isotropic to the cholesteric mesophase and at 352.9 K to the smectic mesophase (see Sect. 5.1.1). At 346.8 K the smectic liquid crystal crystallized to the fully ordered crystal. Dilatometry resulted in Avrami exponents of 2, 2, and 4 for the respective transitions. The cholesteric liquid crystal has a second transition right after the relatively quick formation of a turbid homeotropic state from the isotropic melt. It aggregates without volume change to a spherulitic texture. This process was studied by microscopy32) between 343 and 355.2 K and revealed another nucleation controlled process with an Avrami exponent of 3. 

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. 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...

A typical experimental apparatus for studying photorefractivity in liquid crystals is illustrated in Fig. 2. Two coherent laser beams from an Ar+ laser are crossed in the sample, with a total of 5 mW of p-polarized output at 514 nm. The beams are unfocused and have a 1/e diameter of 2.5 mm. The liquid crystal composite is sandwiched between two ITO coated glass slides that are coated with octadecyl-silyl groups to induce the liquid crystal director to align perpendicular to the face of the glass slides, that is, homeotropic alignment [43], The cell thickness is determined by a Teflon spacer that is 12 to 100 p,m thick. A small electric field... 

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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