Effect Of The Oriented Layer States On Elastic Deformation And Alignment Defects Of Nematic Liquid Crystals

The surface anchoring energy of nematic liquid crystals (NLCs) on an alignmentlayer is of great importance in the fundamental physics of liquid crystals (LCs) andtheir applications to LC devices. Elastic deformation and defects of NLC induced bysurface alignment layer were investigated. The mainly studies were focused on thefollowing aspects:1、 Elastic deformation of NLCs induced by surface groovesExtending the work of Fukuda concerning the elastic deformation of nematicattributable to grooved surfaces, we have investigated the surface-groove-inducedazimuthal anchoring energy at a grooved interface with both isotropic and anisotropicpolar anchoring. When the anisotropic polar anchoring is strong enough, i.e.,2W1~qK22, with W1being the anisotropic anchoring strength coefficient and q beingthe wave number, we get the analytical expression of the azimuthal anchoring energy.The results show that the anisotropic polar anchoring strengthens the azimuthalanchoring of grooved surfaces. In the one elastic constant approximation,surface-groove-induced azimuthal anchoring energy is entirely consistent with theresult of Faetti and it reduces to the original result of Berreman with strong polaranchoring.A generalized form of anisotropic surface energy for the description of biaxial nematic liquid crystals is proposed, and the effects of weak anchoring on the elasticdistortion of the biaxial nematics on the surface grooves is investigated. With somereasonable approximations for the elastic constants based on the results of moleculartheory, we obtain a more general analytical expression for the additional elasticdistortion energy per unit area induced by the minor director m. The results showthat finite polar anchoring can provide an important contribution to the additionalelastic distortion energy.2、Surface-groove-induced boundary-roughness effects in nematicsBased on two models composed of a lower grooved surface with homeotropicanchoring and an upper flat surface with planar anchoring, and the director at theupper surface is either perpendicular to the direction of grooves (modelⅠ) or parallelto it (modelⅡ), model Ⅱallows twist deformation in the bulk while model Ⅰdoes not,we investigate the boundary-roughness effect on nematics induced by a groovedsurface within the Frank elastic theory. By comparison, we find that the twistdeformation strengths the effect of roughness. The effective anchoring strengthdepends on the ratio of the elastic constant.3、Elastic deformation of NLCs on a thin polymeric filmA generalized form of anisotropic surface energy for the description of twistedchiral nematic (TCN) samples with side-chain polymeric layers is proposed, and theanchoring properties of such samples are analyzed. Our results give a more generalexpression for the equivalent anchoring energy. The coupling of the polymer sidechains with the surface reduces the equivalent anchoring energy as well as thethreshold field.A simplified model of the nano-structured polymeric surface was proposed,characterized by a one-dimensional periodic stripe patterned surface with alternateplanar and homeotropic anchoring. We investigate the effect of both the coupling ofnematic liquid crystals with alignment layer polymers and the coupling of thepolymers with the substrate surface, on the anchoring of NLCs at such a surface,using the extended anisotropic surface energy form proposed by Alexe-Ionescu et al. Our results show that the coupling of the polymer with the surface will affect thedirector field of the nematic, and reduce the relaxation distance as well as the totalfree energy of the system.4、Surface induced defect in twisted nematicsThe defect structures in twisted nematic (TN) and twisted chiral liquid crystalshave been investigated within the Landau-de Gennes theory numerically, by usingrelaxation method. Our results show that there exists eigenvalue exchange across thedefect core of both the two models. The defect core is essentially biaxial and neverisotropic. The defect centre is uniaxial and is surrounded by a strong biaxial region.