Gravitational field-induced orientational transition of aligned nematic liquid crystals

We demonstrate induced orientational transition of liquid crystal (LC) E7 between two optical fibre tips in a gravitational field as a result of minimising the free energy. The LC orientational transition is from a homeotropic state, with respect to fibre tip, to a tilted state as the spacing between two fibre tips is increased. The orientational distortion introduces birefringence in the optical fibre that changes the polarised direction of the output light. At short spacing, the undistorted homeotropic orientation of LC is preferred. However, at longer spacing, a distorted orientation is preferred. Once the LC director profile is known, a Finite Difference Time Domain method is used to calculate the optical properties, which agree well with the experimental results.     

Flow induced bistable anchoring switching in nematic liquid crystals

A hydrodynamically controlled switching of orientations of a nematic bistable surface anchoring is observed. A viscous torque moves surface lines between domains corresponding to bistable anchoring. An observed threshold for this viscous torque is attributed to relative spatial inhomogeneities of surface energy (∼10^-2).     

Analysis of cell gap-dependent driving voltage in a fringe field-driven homogeneously aligned nematic liquid crystal display

We have studied cell gap-dependent driving voltage characteristics in a homogeneously aligned nematic liquid crystal (LC) cell driven by a fringe electric field, termed the fringe field switching (FFS) mode. The results show that for the FFS mode using a LC with positive dielectric anisotropy, the operating voltage decreases as the cell gap decreases, whereas it increases with a decreasing cell gap when using a LC with negative dielectric anisotropy. The difference between LCs is explained by simulation and experiment.     

Quantification of the azimuthal anchoring of a homogeneously aligned nematic liquid crystal using fully-leaky guided modes

A novel optical guided mode technique, the fully-leaky guided mode technique, has been used to investigate the director distortion under the application of an in-plane electric field of a homogeneously aligned conventional cell filled with the nematic liquid crystal E7. The liquid crystal is aligned using polyimide rubbed along the direction of the gold electrode edges. A weak field is applied across a 3 mm gap between the gold electrodes to induce small changes in the twist angle of the director. These distortions are determined by fitting to the angledependent reflectivity and transmissivity data and are compared with continuum theory. From careful analysis of the results, both the twist elastic constant, k22, and the azimuthal anchoring strength, Wa, of the system are obtained. At 23.5 C for E7 on rubbed polyimide we find that k22=(6.50 +/- 0.05)x10-12N and Wa=(2.9 +/- 0.2)x10-5 J m-2.     

Analysis of cell gap-dependent driving voltage in a fringe field-driven homogeneously aligned nematic liquid crystal display

We have studied cell gap-dependent driving voltage characteristics in a homogeneously aligned nematic liquid crystal (LC) cell driven by a fringe electric field, termed the fringe field switching (FFS) mode. The results show that for the FFS mode using a LC with positive dielectric anisotropy, the operating voltage decreases as the cell gap decreases, whereas it increases with a decreasing cell gap when using a LC with negative dielectric anisotropy. The difference between LCs is explained by simulation and experiment.     

Polymer-dispersed liquid crystals From the nematic curvilinear aligned phase to ferroelectric films

Polymer-dispersed liquid crystals (PDLC) are widely used for electro-optic applications such as flexible displays, privacy windows or projection displays. Besides these applications, the confinement of a liquid crystal to small cavities is of fundamental interest. The present paper contains a review of the work on nematic and cholesteric PDLCs. Moreover, some very recent developments are summarized such as the use of ferroelectric liquid crystals for PDLC applications.     

Fast switching of vertically aligned negative liquid crystals by optically hidden relaxation

We propose a method for fast switching of vertically aligned (VA) negative liquid crystals (LCs) by hiding the relaxation process of LCs. During the turn-off process, a strong in-plane electric field is applied for a short duration of time instead of relying solely on the slow relaxation of LCs. The LC molecules are rotated to the transmission axis of one of the polarisers by the applied in-plane electric field, resulting in turn-off switching that is 5.8 times faster than that of a conventional VA cell. By applying an overdriving scheme, we experimentally obtained a total response time of 3.3 ms.     

Achieving a robust homogenously aligned liquid crystal layer with reactive mesogen for in-plane switching liquid crystal displays

Uniformly aligned liquid crystals (LCs) are of crucial importance in a practical application, such as displays, phase modulators and virtual reality devices. Although an alignment layer using polyimide-type polymers can almost perfectly align the LCs, polymer-stabilisation at the surface using pre-mixed monomers has been attempted to reduce the fabrication process, i.e. eliminating the alignment layer, thereby reducing the fabrication cost. Here, we propose an approach to achieve a homogeneous alignment of LCs by controlling the molecular structure of reactive monomers mixed in LCs without using conventional polyimide alignment layer. The result shows an excellent initial dark state and acceptable electro-optic performance which implies that the polymer stabilisation at the surface successfully anchors the homogenous orientation of LCs. We believe that the proposed fabrication method can contribute to cost-effective fabrication process by eliminating an alignment layer with no fabrication step of a surface treatment.     

Light-induced hydrodynamic reorientation of hybrid aligned nematic liquid crystals caused by direct volume expansion

The dynamics of the direct volume expansion mechanism of light-induced hydrodynamic motions in hybrid-oriented nematic liquid crystal (NLC) have been studied experimentally. It was shown that heating caused by the absorption of a laser beam results in expansion of a NLC volume and induces hydrodynamic flow in a planar capillary. The hydrodynamic flow reorients the NLC molecules and thus the optical properties of the NLC layer are changing. A change of the tilt direction of the NLC director is demonstrated experimentally, and it is shown that hybrid-oriented NLC could be considered as ‘flexible ribbon’. Velocity gradient brings about an increase of curvature when hydrodynamic flow velocity is directed out of the ‘flexible ribbon’s’ curvature. The ‘flexible ribbon’ reverses its curvature when velocity is directed into ‘flexible ribbon’s’ curvature.     

Carbazole nematic liquid crystals

A number of calamitic 2,7-diary-N-alkyl-substituted carbazoles with an enantiotropic nematic phase have been prepared. Branching of the aliphatic chain attached to the nitrogen atom in the carbazole ring leads to significantly lower liquid crystal transition temperatures. These new materials show a lower ionisation potential than fluorene analogues and blue photoluminescence in solution and as thin solid films.     

Three homeotropically aligned nematic liquid crystals: comparison of ultrafast to slow time-scale dynamics

The dynamics of two nematic liquid crystals, 4-(trans-4(')-n-octylcyclohexyl)isothiocyanatobenzene and 4-(4-pentyl-cyclohexyl)-benzonitrile, are investigated as a function of temperature both in the homeotropically aligned nematic phase and in the isotropic phase using optical heterodyne-detected optical Kerr effect experiments, which measures the time derivative of the polarizability-polarizability-correlation function (orientational relaxation). Data are presented over a time range of 500 fs-70 micros for the nematic phase and 500 fs to a few hundred nanoseconds for the isotropic phase. The nematic dynamics are compared with a previously studied liquid crystal in the nematic phase. All three liquid crystals have very similar dynamics in the nematic phase that are very different from the isotropic phase. On the slowest time scale (20 ns-70 micros), a temperature-independent power law, the final power law, t(-f) with f approximately 0.5, is observed. On short time scales (approximately 3 ps to approximately 1 ns), a temperature-dependent intermediate power law is observed with an exponent that displays a linear dependence on the nematic order parameter. Between the intermediate power law and the final power law, there is a crossover region that has an inflection point. For times that are short compared to the intermediate power law (approximately <2 ps), the data decay much faster, and can be described as a third power law, although this functional form is not definitive. The isotopic phase data have the same features as found in previous studies of nematogens in the isotropic phase, i.e., the temperature-independent intermediate power law and von Schweidler power law at short to intermediate times, and a highly temperature-dependent long time exponential decay that is well described by the Landau-de Gennes theory. The results show that liquid-crystal dynamics in the nematic phase exhibit universal behavior.     

Alignment and switching of nematic liquid crystals embedded in porous chiral thin films

Porous thin films with engineered microstructures have been fabricated using glancing angle deposition (GLAD). GLAD films with chiral microstructures have been previously shown to exhibit unique chiral optical response. The pores of these films were embedded with (non-chiral) nematic liquid crystals (LCs) to produce a new composite optical material wherein the GLAD film induces chiral nematic-like LC orientation. We demonstrate here reversible electro-optic switching of the LC component of these hybrid films. Unaddressed, cells of GLAD/LC hybrid films show enhanced chiral optic response compared with the unfilled GLAD film. When addressed, the chiral optic response vanishes.     

Highly birefringent nematic and chiral nematic liquid crystals

We report a simple interference method to determine the dispersion of the extraordinary refractive index and birefringence of highly conjugated and coloured nematic liquid crystals used as light-emitting materials in organic electroluminescent devices. The measurements are made in the nematic glass phase at room temperature. The birefringence is highly dispersive and values up to 1.1 are obtained. Chiral groups are incorporated into the end chains giving a chiral nematic liquid crystal with a very wide stopband in the visible region. The Berreman matrix method is used to simulate transmission through the chiral nematic liquid crystal cell using the refractive index parameters obtained experimentally. Excellent agreement between theory and experiment is found.     

Oligomeric rod-disc nematic liquid crystals

Six new oligomeric nematic liquid crystals are reported consisting of a triphenylene-based core attached to which are six 4-cyanobiphenyl units via flexible alkyl spacers.     

Highly birefringent nematic and chiral nematic liquid crystals

We report a simple interference method to determine the dispersion of the extraordinary refractive index and birefringence of highly conjugated and coloured nematic liquid crystals used as light‐emitting materials in organic electroluminescent devices. The measurements are made in the nematic glass phase at room temperature. The birefringence is highly dispersive and values up to 1.1 are obtained. Chiral groups are incorporated into the end chains giving a chiral nematic liquid crystal with a very wide stopband in the visible region. The Berreman matrix method is used to simulate transmission through the chiral nematic liquid crystal cell using the refractive index parameters obtained experimentally. Excellent agreement between theory and experiment is found.     

Spontaneous spreading of nematic liquid crystals

The spontaneous spreading of macroscopic drops of nematic liquid crystals on hydrophilic substrates has been investigated by interferometric techniques. There is a complex interplay between the elastic energy, due to antagonist anchoring at the interfaces, and the radial flow in the spreading drop. A relevant parameter appears to be the relative humidity of the atmosphere, because it controls the amount of water molecules adsorbed on the substrate and, therefore, the strength of anchoring defects. The spreading laws differ from the ones of simple wetting liquids, and contact line instabilities coupled to short- (anchoring) or large-scale (disclinations) defects of the nematic film are observed.     

Fast fringe-field switching of vertically aligned liquid crystals between high-haze translucent and haze-free transparent states

We report control of the haze value in a liquid crystal (LC) cell driven by a fringe electric field. When a fringe field is applied to a vertically aligned (VA) cell, a large spatial phase difference with a short grating period is induced in the LC layer. The average grating period of a VA cell driven by a fringe field is a quarter of the pitch of the interdigitated electrodes, which is half of the grating period of a VA cell driven by an in-plane field. Moreover, a sharper spatial phase profile is built around the edges of the interdigitated electrodes, which led to a high haze of 84.3% in the translucent state. The device was haze-free in the transparent state owing to the use of an LC layer without a polymer structure. To increase the haze value of the LC device while retaining a short response time, we developed an LC cell with crossed interdigitated electrodes where a large spatial phase difference is induced with little dependence on the azimuth angle. By applying a fringe electric field to a 20 μm thick LC cell using crossed interdigitated electrodes, we demonstrated a very high haze of 95.4% and a response time of less than 5 ms.     

Defect-induced melting in nematic liquid crystals

In this paper we employ a relatively simple theory to show how a nematic disclination line can act as a nucleation site for the growth of the isotropic phase. With this theory we are able to find analytical expressions for the critical temperature of nucleation and the behaviour of the core radius as a function of temperature. We are then able to compare these results with a previous numerical model of this effect.