Hybrid-aligned Nematic Cell Research Articles

Landau–de Gennes theory for a −1 boojum in a degenerate hybrid-aligned nematic cell

ABSTRACTWithin Landau–de Gennes theory, the texture of a cylindrically confined degenerate hybrid nematic cell containing a −1 boojum has been investigated using a three-dimensional finite-difference iterative method. We impose strong anchoring conditions, with the nematic director perpendicular to the upper plate and planar degenerate at the lower plate. The director distribution of a degenerate hybrid-aligned nematic cell with a −1 defect is known to have no axial symmetry. The axially symmetric characteristic of the biaxial distribution is surprising. The geometric confinement and boundary condition induce an order reconstruction. The influences of cell thickness on the frustrated cell and eigenvalue exchange solution for a sufficiently small thickness with order reconstruction nucleus have been analysed. Defect rings appear when cell thickness is sufficiently small.

New surface order reconstruction induced by electric field application in a nanoconfined HAN cell with a topological defect

In accordance with the 2D Landau–de Gennes tensorial formalism, we investigated the influence of an applied electric field E parallel to the defect line on the position and structure of a nematic line defect with topological charge M = − 1/2 in a hybrid alignment nematic cell with different cell gaps d. A new type of surface order reconstruction occurs in the cell as E is increased. Regardless of d, two biaxial layers can be achieved near the top and bottom substrates of the cell with different E values. This process involves double eigenvalue exchange across the cell. However, the structural transition processes vary for different d values.

Optical properties of hybrid aligned nematic cells with different pretilt angles.

The phase retardation difference, ΔΦ, is calculated for hybrid liquid crystal (LC) cells as a function of LC pretilt angles, θ0(1), θ0(2), on the opposite substrates of the cell for the case of an arbitrary angle of light incidence in the range from 0 to 90°. An LC director configuration is suggested for its application in optical compensators. Design and fabrication methods of hybrid aligned nematic (HAN) cells with an arbitrary LC pretilt angle are described. The LC pretilt angle is measured in the HAN cells with a given planar or vertical LC alignment on one of the substrates.

Measurement of the Sum (e 1 + e 3) of the Flexoelectric Coefficients e 1 and e 3 of Nematic Liquid Crystals using a Hybrid Aligned Nematic (HAN) cell

A hybrid aligned nematic cell (HAN) is often used to measure the sum (e 1 + e 3) of the flexoelectric coefficients e 1 and e 3. However, a HAN cell has the difficulty of an internal offset voltage (internal bias) caused by the homeotropic alignment. We present a method to determine the internal bias and then take it into account in the flexoelectric measurement.

High-Precision Measurement of Polar Anchoring Strength and Elastic Constant Ratio using Hybrid Alignment Nematic Cell

The surface polar anchoring strength is the most crucial element which controls the alignment distribution of a liquid crystal (LC) cell. We devised a new measurement method for the surface polar anchoring strength and the elastic constant using a hybrid aligned nematic LC (HAN) cell. We obtained these parameters from the measurement of the angle dependence of the phase difference and the amplitude ratio in the direction perpendicular to the alignment plane of a HAN cell and confirmed that these parameters can be measured with a high degree of accuracy.

Erratum: “Influence of an electric field on the non-Newtonian response of a hybrid-aligned nematic cell under shear flow” [J. Chem. Phys. 126, 204905 (2007)

Erratum: “Influence of an electric field on the non-Newtonian response of a hybrid-aligned nematic cell under shear flow” [J. Chem. Phys. 126, 204905 (2007)

Determination of the director profile in a nematic cell from guided wave data: an inverse problem

We consider an inverse problem: the estimation of the nematic director profile from experimental fully leaky guided mode data. This inverse problem is ill-posed in that small changes in the data may lead to large changes in the estimates of the director profile. The continuum equations for a nematic are exploited to stabilize the problem. We use experimental data drawn from a study of the dynamics of a hybrid-aligned nematic cell as an example.

Influence of an electric field on the non-Newtonian response of a hybrid-aligned nematic cell under shear flow

The authors study shear flow in hybrid-aligned nematic cells under the action of an applied electric field by solving numerically a hydrodynamic model. The authors apply this model to a flow-aligning nematic liquid crystal (4'-n-pentyl-4-cyanobiphenyl) and obtain the director's configuration and the velocity profile at the stationary state. The authors calculate the local and apparent viscosities of the system and found that the competition between the shear flow and the electric field gives rise to an interesting non-Newtonian response with regions of shear thickening and thinning. The results also show an important electrorheological effect ranging from a value a bit larger than the Miesowicz viscosity etab [Nature (London) 17, 261 (1935)] for small electric fields and large shear flows to etac for large electric fields and small shear flows. The analysis of the first normal stress difference shows that for small negative shear rates, the force between the plates of the cell is attractive, while it is repulsive for all other values of shear rates. However, under the application of the electric field, one can modify the extent of the region of attraction. Finally, the authors have calculated the dragging forces on the plates of the cell and found that it is easier to shear in one direction than in the other.

Light propagation and transmission in hybrid-aligned nematic liquid crystal cells: Geometrical optics calculations

The authors present a geometrical approach to calculate the transmission of light in a hybrid-aligned nematic cell under the influence of an applied electric field. Using the framework of geometrical optics they present results for the ray tracing as well as the transmission of light as a function of the applied low frequency voltage. Dispersion effects are included through a wavelength dependent dielectric function. Their results for the transmittance as a function of the applied voltage show oscillations that are in good qualitative agreement with previously obtained experimental measurements.

Modeling of dual frequency liquid crystal materials and devices

We present a director-based model of the dual frequency nature of liquid crystals based on a Debye-type relaxation of the permittivity in the direction parallel to the director. This relaxation is governed by a first order differential equation in terms of the polarization and electric field along the long axis. We demonstrate that this equation can be used as an extension to the well-known Eriksen-Leslie-Parodi theory. Since solution is in the time domain, the frequency response of the applied waveform need not be calculated. Consequently, the device response to arbitrary applied waveforms can be modeled. As an example, we present the switching response of a dual frequency addressed hybrid-aligned nematic cell, and suggest some optimization of the addressing scheme.

Two-Domain Hybrid-Aligned Nematic Cell Fabricated by Ion Beam Treatment of Vertical Alignment Layer

Organic materials for vertical alignment can be used to align liquid crystals horizontally by irradiating an ion beam on the surface under appropriate conditions. Using this technique, we can achieve easily a multidomain alignment for a hybrid-aligned nematic (HAN) cell. We found that a two-domain HAN cell obtained by ion beam exposure shows excellent electro-optical characteristics. The use of the same alignment material in a HAN cell could reduce the cost of production and generation of DC offset voltage.

P-181L: Late-News Poster: High-Precision Measurement of Polar Anchoring Strength and Elastic Constant using Hybrid Alignment Nematic Cell for Improvement of Response Property of VA-mode LCD

The surface polar anchoring strength is the most crucial element which governs the response property of VA-mode LCD. We devised a new measurement method for the surface polar anchoring strength and the elastic constant using a hybrid alignment liquid crystal cell. We measured the angle dependency of the phase difference in the direction perpendicular to the rubbing direction of HAN cells and demonstrated that these parameters can be measured with a high degree of accuracy.

Fully leaky guided mode study of the flexoelectric effect and surface polarization in hybrid aligned nematic cells

The fully leaky guided-mode optical-waveguide technique has been used to characterize the tilt profiles in two E7-filled hybrid-aligned nematic cells. The director distortion was measured with ac and dc voltages applied perpendicular to the substrates. By comparing the tilt profiles with model profiles produced using a free-energy minimization program, the flexoelectric coefficient of E7 at 20 °C has been measured as +1.5±0.2×10−11 C m. The effect of surface polarization in the cells has also been observed. This surface polarization is attributed to the influence of the homeotropic surface. The dipole alignment imbalance of the liquid crystal at the homeotropic surface has then been estimated.

Optical determination of flexoelectric coefficients and surface polarization in a hybrid aligned nematic cell.

We present an optical study of the influence of both the flexoelectric effect and surface polarization on a hybrid-aligned nematic cell using the half-leaky guided mode technique. Tilt angle profiles, obtained from fits of experimental data (reflectivity curves) taken under applied voltages, are compared with the ones derived by a complete theoretical model. Measurements with an applied alternating voltage allow the evaluation of the anchoring energy by solving the torque balance equation at the planar surface. From measurements with static fields, the sum of flexoelectric coefficients and the surface polarization are determined by numerical solution of Euler-Lagrange equations.

Simple method for the calculation of the deformation profiles in chiral nematic liquid crystal cells with asymmetric pretilt

We present a one-dimensional simulation methodology for determining the liquid crystal director distribution in a cell for arbitrary pretilt angles on two glass substrate surfaces. The method is illustrated for cases of an asymmetric twisted nematic cell, a hybrid alignment nematic cell, and a bend alignment nematic cell.