Liquid crystal, surface-induced

Keywords Atomic force microscopy Liquid crystal Surface-induced ordering Presmectic ordering Landau-de Gennes... 

The sensitivity of deuteron NMR to the molecular orientational order and to director field configurations turned out to be extremely useful in studies of liquid crystals confined into snbmicrometer pores. Moreover, the large surface-to-volume ratio of these composite systems render the interfacial and surface phenomena, induced by the liquid crystal-surface interactions, accessible even to an essentially integrative technique like NMR. Since the discovery of polymer dispersed liquid crystals (PDLCs) in 1986 [4], NMR of selectively deuterated liquid crystals was used to discriminate unambiguously among various director structures in cavities, resulting from an interplay between elastic forces, morphology and size of the cavity, and surface interactions. These structures include the escaped-radial, planar axial, planar-polar, and... 

There are several examples of experimentally observed anchoring transitions (see J6r6me [4] and Pieranski and Jerome [10], and references therein). Since anchoring transitions occurring at the surface of a liquid crystal cell induce a change of director configuration in the cell, these transitions can be used to switch a liquid crystal device between different states [24, 25],... 

As witli tlie nematic phase, a chiral version of tlie smectic C phase has been observed and is denoted SniC. In tliis phase, tlie director rotates around tlie cone generated by tlie tilt angle [9,32]. This phase is helielectric, i.e. tlie spontaneous polarization induced by dipolar ordering (transverse to tlie molecular long axis) rotates around a helix. However, if tlie helix is unwound by external forces such as surface interactions, or electric fields or by compensating tlie pitch in a mixture, so tliat it becomes infinite, tlie phase becomes ferroelectric. This is tlie basis of ferroelectric liquid crystal displays (section C2.2.4.4). If tliere is an alternation in polarization direction between layers tlie phase can be ferrielectric or antiferroelectric. A smectic A phase foniied by chiral molecules is sometimes denoted SiiiA, altliough, due to the untilted symmetry of tlie phase, it is not itself chiral. This notation is strictly incorrect because tlie asterisk should be used to indicate the chirality of tlie phase and not tliat of tlie constituent molecules. 

If we compare with figure C2.2.I I, we can see that this defonnation involves bend and splay of the director field. This field-induced transition in director orientation is called a Freedericksz transition [9, 106, 1071. We can also define Freedericksz transitions when the director and field are both parallel to the surface, but mutually orthogonal or when the director is nonnal to the surface and the field is parallel to it. It turns out there is a threshold voltage for attaining orientation in the middle of the liquid crystal cell, i.e. a deviation of the angle of the director [9, 107]. For all tliree possible geometries, the threshold voltage takes the fonn [9, 107]... 

The interaction of complex liquid crystal molecules with realistic surfaces is an area which is currently unexplored using electronic structure methods though, as stated earlier, the problem of surface-induced control of molecular orientation remains at the forefront of liquid crystal device technology. This problem is currently at the limits of practical capability of the most powerful computer systems. However treatment of a single mesogenic molecule on a... 

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 importance of surface characterization in molecular architecture chemistry and engineering is obvious. Solid surfaces are becoming essential building blocks for constructing molecular architectures, as demonstrated in self-assembled monolayer formation [6] and alternate layer-by-layer adsorption [7]. Surface-induced structuring of liqnids is also well-known [8,9], which has implications for micro- and nano-technologies (i.e., liqnid crystal displays and micromachines). The virtue of the force measurement has been demonstrated, for example, in our report on novel molecular architectures (alcohol clusters) at solid-liquid interfaces [10]. 

Charra F, Cousty J (1998) Surface-induced chirality in a self-assembled monolayer of discotic liquid crystal. Phys Rev Lett 80 1682-1685... 

Electro-optic effects induced by doping liquid crystals with one-dimensional metal nanoparticles were not only investigated in standard electro-optic test cells, but also in costume-made cells consisting of a thin layer of liquid crystal either deposited onto a thin film of alumina with embedded GNRs [443], or using rubbed polyimide alignment layers modified with solution-cast GNR [444]. In both cases, surface plasmon resonance frequencies of the GNR integrated into these liquid crystal cells could be electrically controlled. 

A standard TN-LCD consists of a nematic liquid crystal mixture of positive dielectric anisotropy contained in a cell with an alignment layer on both substrate surfaces, usually rubbed polyimide, crossed polarisers and a cell gap of 5- 0fim, see Figure 3.7. The nematic director is aligned parallel to the direction of rubbing in the azimuthal plane of the device. The alignment layer induces a small pretilt angle (6 1-3°) of the director in the zenithal plane. The... 

Schadt, M., Schmit, K., Kozenkov, V., and Chigrinov, V. Surface-induced parallel alignment of liquid crystals by linearly polymerized photopolymers. Jpn, J. Appl. Phys. 31, 2155 (1992). 

Stracke, A., Wendorff, J. H., Goldmann, D., and Janietz, D. Optical storage in a smectic mesophase thermal amplification of light-induced chromophore orientations and surface relief gratings. Liquid Crystals 2000, 27, pp. 1049-1057. 

Kim, D. Y., Kumar, J., and Tripathy, S. K. Liquid crystal alignment using optically induced surface relief gratings on azo polymer films. Polymer Preprints 1998, 319, pp. 1107-1108. 

Aoki, K., Kawanishi, Y., Seki, T., Sakuragi, M., Tamaki, T., and Ichimura, K. Reversible alignment change of liquid crystals induced by photochromic molecular films properties of azobenzene chromophores covalently attached to silica surfaces. Liq. Cryst., 19, 119 (1995). 

There has been little work done to date on the nature of the three-phase substrate-solid-liquid intersection, and there is little guidance in predicting the magnitude of the equilibrium contact angle 9 that enters into classical nucleation theory in the factor f(0). Considerably more work has been done on the contact angle in substrate-liquid-vapor equilibrium, which determines the wetting properties of the liquid for the substrate. It is possible that some of the techniques developed for that problem may be applicable to the study of surface-induced nucleation of crystallization. 

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