Surface Disclination Research Articles

Response of a Hybrid-Aligned Nematic with a Surface Disclination Line to a Magnetic Field Pulse

Response of a Hybrid-Aligned Nematic with a Surface Disclination Line to a Magnetic Field Pulse

Interference oscillations in the optical response of a hybrid-aligned nematic with a surface disclination line to a pulsed magnetic field

The propagation of light through a hybrid-aligned nematic layer with a surface disclination line is investigated in a pulsed magnetic field. The experimental dependences of light intensity I on time t accompanied by interference oscillations are presented. The shift and expansion of the interference extrema as functions of the magnetic field pulse length during the reaction are shown. The I ( t ) dependence for the relaxation process is calculated with allowance for scattering. The calculated I ( t ) dependence agrees well with the experimental dependence over the entire hybrid-aligned nematic layer, except for the surface layer.

Light interference in a hybrid-aligned nematic layer with nonordered surface disclination lines

The propagation of a laser beam through a hybrid-aligned nematic layer with a surface disclination line has been investigated. A model of the light interference has been developed to consider the scattering by the structural inhomogeneities. The analytical expression that includes the factor characterizing an exponential decrease in the light scattering has been obtained. The dependence of the intensity of light transmitted through the layer on the magnetic field has been measured. The dependence has been accompanied by the interference oscillations. The theoretical expression is consistent with the experiment, which confirms the correctness of the model concepts.

The director field distribution with the strongly pinned alignment in nematic structures at the polymer surface

We investigate orientational nematic liquid crystal structures that form at the polymer surface due to adsorption, their stability against the schlieren texture and features of point and linear singularities in them. We demonstrate that surface disclination lines, which also occur due to molecular adsorption, can lead to azimuthal anchoring of a bulk layer and make the singularities with the charge m = ±1 stable. We consider the director field distribution in a nematic layer as a result of the combination of two aligning factors, which are the linear disclination that tend to impose uniform planar ordering and point defects that impart the radial configuration to the director. With the use of the calculation, we analyse the energies of elastic distortions of the director field for the radial–planar, radial–homeotropic and radial–radial configurations, depending on the nematic layer thickness.

Surface disclination cores in the B2 phase of bent-shaped molecules described by the Peierls–Nabarro model

The Peierls–Nabarro model, originally proposed to describe dislocation core in solids, is used to model core structures of 2π- and π-surface disclinations in the B2 phase of bent-shaped molecules. Structures of disclination cores are determined by surface anchoring. Core parameters as core widths (and positions of two partial π-disclinations as parts comprising 2π-disclination) are estimated using surface anchoring energy. Disclination core widths are usually narrow for strong anchoring. In such a case they are barely observable; nevertheless, they can exist.

Electric-field-induced deformation dynamics of a single nematic disclination

Disclinations in nematic liquid crystals usually adopt a straight shape in order to minimize their elastic energy. Once created in the course of a nonequilibrium process such as a temperature quench from the isotropic to the nematic phase, the topologically stable disclinations of half-integer strength either annihilate each other in pairs of opposite strength or form topologically unstable disclinations of integer strength. In this article, we demonstrate that the annihilation process can be inhibited and the defects can be deformed by an applied electric field. We study the disclination lines in the deep uniaxial nematic phase, located at the boundary between two different types of walls, the so-called pi wall (a planar soliton stabilized by the surface anchoring) and the Brochard-Léger (BL) wall stabilized by the applied electric field. By changing the electric voltage, one can control the energy of director deformations associated with the two walls and thus control the deformation and dynamics of the disclination line. At small voltages, the disclinations are straight lines connecting the opposite plates of the cell, located at the two ends of the pi walls. The pi walls tend to shrink. When the voltage increases above E(F), the Fréedericksz threshold, the BL walls appear and connect pairs of disclinations along a path complementary to the pi wall. At E>2 E(F), the BL walls store sufficient energy to prevent shrinking of the pi walls. Reconstruction of the three-dimensional director configuration using a fluorescent confocal polarizing microscopy demonstrates that the disclinations are strongly bent in the region between the pi and the BL walls. The distortions and the related dynamics are associated with the transformation of the BL wall into two surface disclination lines; we characterize it experimentally as a function of the applied electric field, the cell thickness, and the sample temperature. A simple model captures the essential details of the experimental data.

Chiral nematic liquid crystals in cylindrical cavities. A classification of planar structures and models of non-singular disclination lines

Abstract A part of homotopy theory is applied to classify planar structures in chiral nematic liquid crystals confined to cylindrical cavities. The resulting classification is exact in the approximation of undeformed chiral nematic surfaces. Within this approach the relative stability of possible planar structures with surface and bulk disclination lines is discussed. The number and the shape of these disclinations, which in some cases form spiral structures, are predicted. Further approximate analytical expressions for non-singular director fields close to disclination lines with integral strength are introduced. Our predictions, which are also in agreement with some previously suggested pictures of such director fields, are used to improve stability considerations of the confined planar chiral nematic structures in tubes and droplets.

Dielectric Properties due to Interfacial Interaction in Ferroelectric Liquid Crystal

Interfacial effects on the dielectric properties like a coercive field Ec, a residual polarization Pr and D–E hysteresis loop, are studied in ferroelectric liquid crystal DOBAMBC. The strong relation between the pitch Z of the surface disclination (the stripe pattern) and Pr is observed, that is, Pr increases with increase in Z. Ec becomes larger as the thickness of the cell d decreases and tends to be infinite toward d=0. This suggests the existence of the surface layer in DOBAMBC. Moreover no change of Z is observed for the change of the instantaneous field (a triangular wave) of 60 Hz, supporting that the stripe pattern can only repond to the low applied frequency. Z changes however when the effective field intensity is varied. From these results, it can be said that the <E>-dielectric properties of surface layer (and/or the surface disclination) are obtained from the study of Pr-<E> hysteresis.

Spontaneous Polarization and Electrical Field Response of Surface Disclination in Ferroelectric Liquid Crystals

Visual observations of a chiral smectic C phase were made simultaneously with the measurement of the spontaneous polarization to understand the relation between the stripe pattern (the surface disclination) and the polarization reversal process. The polarization reverse is not directly related to the dynamical change of the stripe pattern. During the polarization reverse, the bright and chaotic pattern were observed independent of the stripe pattern.

The disclinations= 1/2 at the air-nematic interface

Abstract The disclination s= + 1/2 in a nematic liquid crystal is observed to have some twist character so that it resembles half of a disclination s = +1 with circumferential directors at infinity and an escaped core. It therefore has helicity. When the disclination is trapped near the surface, its core lies just outside the surface. The helicity and the position of the core above or below the surface can both be determined by polarized light, or by decoration of the surface with dust or water droplets, combined with observation of the change of these patterns when the surface is gently blown parallel to the line of the disclination. The surface disclination line is sharply kinked at nodes where the helicity reverses. The director pattern at the surface almost always shows an asymmetry which (from geometrical necessity) is independent of the helicity, but always reverses at a node. It is possible by blowing on the surface to reverse the asymmetry of each segment of the line, so that the rule of reversal ...

INTERACTION BETWEEN SURFACE AND BULK DISCLINATIONS IN NEMATIC LIQUID CRYSTALS

The problem of the interaction between a surface disclination and a bulk disclination placed symmetrically with respect to it has been considered on the basis of the continuum theory. It is found that at large distances the bulk disclination is attracted by the surface disclination with a force that is independent of the sign of the former. The force of attraction is found to be nearly the same for bulk disclinations of strength ½, 1 and 2. The theory appears to qualitatively account for the experimental observations of Kleman and Williams.

Surface disclination lines observed in nematic liquid crystals when the surfaces induce homogeneously tilted alignment

2014 A topological study of the symmetry transformations on a surface inducing both tilted and homogeneous orientation in nematic liquid crystals is presented. It predicts the existence of two types of surface disclination lines, which have been observed experimentally. The evaluation of their strength (which does not satisfy the rule S = k 1/2 where k is an integer) required a special definition of this quantity. Their relative positions and interactions are well interpreted in terms of grain boundaries on the surface considered as an ordered two-dimensional medium. The evaluation of the elastic energies of those lines can only be very approximate, due to our ignorance of the anchoring energies. LE JOURNAL DE PHYSIQUE TOME 38, MAI 1977,

Anchoring energies and the nucleation of surface disclination lines in nematics

Abstract Observation of the nucleation of surface disclination lines in nematics shows their correspondence to emergences of walls at the boundary of the sample; these lines have no core discontinuity and their width gives an estimate of the anchoring energy of the molecules to the surface.

The interaction between a disclination in a nematic liquid crystal and a rubbed surface

For the case of a rubbed surface with a finite anisotropic surface tension, a model is developed for the structure and energy of a surface disclination and of an edge disclination near the surface. It is found that there is an energy barrier against detaching a disclination from the surface, and also that there is a critical distance within which a disclination is attracted, and beyond which it is repelled, by the surface. These properties may provide a measurement of the anisotropy of the surface tension.