Electrically induced anchoring transition in cholesteric liquid crystal cells with different confinement ratios

Reorientation of cholesteric liquid crystal induced by the electrically controlled ionic modification of surface anchoring within the cell with confinement ratio exceeding 1 has been studied. The change of homeotropic surface anchoring to the planar one on the electrode-anode substrate under the action of DC voltage causes the formation of the modulated hybrid-aligned cholesteric layer in the cell. Optical texture of the liquid crystal layer with such an orientation structure is the linear periodic stripes. Homogeneity of emerging optical texture depending on the confinement ratio as well as on the prehistory of voltage application has been considered. It has been found that the ionic modification of surface anchoring results in total transformation of the diffraction pattern observed after the laser beam passing through the sample.     

Stability of some structures in a cholesteric liquid crystal layer for tilted anchoring

An analysis of the permitted structures for a cholesteric liquid crystal based on the continuum theory as a function of the anchoring conditions and the thickness of the liquid crystal layer is presented. A new type of bistability is found. The stability domains of various structure types are given.     

Surface anchoring energy of cholesteric liquid crystals

ABSTRACT

In this paper, we propose a suitable surface energy expression for cholesteric liquid crystals. We show that there exists a symmetry allowed term for chiral nematics that doesn’t appear in the traditional Rapini-Papoular surface energy form. We discuss some consequences of this new surface anchoring term.     

Dynamics of surface anchoring breaking in a nematic liquid crystal

We present a simple dynamical model which describes the time dependence of the threshold electric field for breaking the surface anchoring of nematics. This model includes a surface friction, resulting from volume hydrodynamic dissipation. Experimental data with AC and DC pulsed fields are explained by this model. For DC, the threshold difference versus the field polarity is well-described by the flexo-electric effects in a uniform field for short times and in a non-uniform field for long times.     

Dynamics of surface anchoring breaking in a nematic liquid crystal

Abstract

We present a simple dynamical model which describes the time dependence of the threshold electric field for breaking the surface anchoring of nematics. This model includes a surface friction, resulting from volume hydrodynamic dissipation. Experimental data with AC and DC pulsed fields are explained by this model. For DC, the threshold difference versus the field polarity is well-described by the flexo-electric effects in a uniform field for short times and in a non-uniform field for long times.     

Robust liquid crystal droplets

The preparation and properties of cyanobiphenyl liquid crystal droplets encapsulated by the polymerizable lecithin 1,2-bis(10,12-tricosadiynoyl)-sn-glyero-3-phosphocholine (DC_8,9PC) are described. Under a wide variety of preparation conditions the droplets obtain a diameter of approximately 10 mum. These droplets are stable for periods of over one year at room temperature. Furthermore, they are stable upon temperature cycling between the nematic and isotropic phases and between the smectic A to nematic to isotropic phase transitions.     

Robust liquid crystal droplets

The preparation and properties of cyanobiphenyl liquid crystal droplets encapsulated by the polymerizable lecithin 1,2-bis(10,12-tricosadiynoyl)-sn-glyero-3-phosphocholine (DC_8,9PC) are described. Under a wide variety of preparation conditions the droplets obtain a diameter of approximately 10 mum. These droplets are stable for periods of over one year at room temperature. Furthermore, they are stable upon temperature cycling between the nematic and isotropic phases and between the smectic A to nematic to isotropic phase transitions.     

Director configurations in nematic droplets with tilted surface anchoring

The polymer dispersed nematic liquid crystal (LC) with the tilted surface anchoring has been studied. The droplet orientational structures with two point surface defects – boojums and the surface ring defect – are formed within the films. The director tilt angle α = 40° ± 4° at the droplet interface and LC surface anchoring strength W_s ~ 10^–6 (J m^?2) have been estimated. The bipolar axes within the studied droplets of oblate ellipsoidal form can be randomly oriented are oriented randomly relatively to the ellipsoid axes as opposed to the droplets with homeotropic and tangential anchoring.     

Transition radiation in cholesteric liquid crystal

We consider a constant velocity charged particle travelling in an arbitrary direction by a cholesteric liquid crystal. We calculate the time-dependent-induced polarisation in the cholesteric by the electric field generated by the charged particle. Thus, we express the radiation field originated by the induced dipole distribution in the cholesteric in terms of the cholesteric susceptibility. To simplify our procedure, we write Maxwell equations and the constitutive non-local equation for the cholesteric, in the Fourier space since in this representation the equations turn to be simple difference equations. We solve these equations iteratively by assuming small values for the cholesteric birefringence to find the first-order electric field produced by the charge particle immersed in the cholesteric. This allows us to obtain the dominant contributions of the radiation field one of which is the usual Cherenkov effect. We focus in the terms occurring for hypoluminic charged particle and calculate the radiated energy as a function of observing angle, frequency, velocity and direction with respect to the cholesteric axis.     

Polyurethane acrylate-stabilized cholesteric liquid crystal dispersions

Polymer stabilized cholesteric liquid crystals (PSCLCs) have been fabricated based on various structures of polyurethane acrylate (PUA) as well as conventional acrylate polymer, namely poly(1,6-hexanediol diacrylate). For PUA stabilized films, the transmittance spectra of cured films were essentially identical to those of uncured films, with no 'dead' reaction; whereas, using acrylate polymer; reflection greatly decreased upon curing and showed significant 'dead' reaction after the cessations of irradiation. The structure of PUA also had significant effects on the electro-optic performance, and these effects are interpreted in terms of elasticity and interfacial interaction between the polymer and LC.     

Interface modification of polymer stabilized cholesteric liquid crystal

Films were made by incorporating various quantities of fluoroalkyl acrylate (FA) into the chain termini of polyurethane acrylate oligomer of polymer stabilized cholesteric liquid crystals (PSCLCs). The effects of switching voltage, scanning electron and polarized optical microscope morphologies, texture transition and reversibility, and contact angle of the films were then studied. It was found that the switching voltage of the film decreased with the addition of FA up to 0.8 wt%, beyond which it increased. The decrease and increase were respectively explained in terms of decreased interface energy as noted from the contact angle of the LC on polymer surface, and too small droplet size as noted from the SEM. Copyright © 2008 John Wiley & Sons, Ltd.     

Structure of liquid crystal droplets with chiral propeller texture

We experimentally studied a nematic liquid crystal whose molecules form twisted head-to-head H-bonded dimers. We observed that when the material transformed from the isotropic to nematic phase, it formed droplets with chiral propeller textures. We carried out a computer simulation to investigate the liquid crystal director configuration inside the droplets and to study the effects of elastic constants and chirality on the droplet texture. Results of our study show it is likely that the material in the droplets had nonzero chirality due to spontaneous chiral phase separation.     

Surface anchoring of nematic liquid crystal 8OCB on a DMOAP-silanated glass surface

Dynamic light scattering spectroscopy has been used to determine the temperature dependence of the anchoring strength of the nematic liquid crystal 8OCB on DMOAP-silanated glass surfaces inducing homeotropic alignment. Wedge-type glass cells with known thickness profile starting from 150 nm to several microns have been used in the experiments. The relaxation rates of the nematic fluctuations with the wave vector perpendicular to the confining surfaces have been measured as a function of the cell thickness. Fitting of the thickness dependence of the relaxation rate allows for straightforward determination of the surface extrapolation length and therefore also the strength of the surface anchoring, which is 1×10^-4 J m^-2. The overall experimental accuracy of the experiments is discussed.     

Effect of an electric field on defects in a nematic liquid crystal with variable surface anchoring

We report experimental studies on defects in a nematic liquid crystal with negative dielectric anisotropy mounted in a cell with perfluoropolymer-coated surfaces. The sample exhibits a discontinuous anchoring transition from planar to homeotropic on cooling at zero or a small electric field, and above a cross-over voltage a continuous ‘inverse Freedericksz transition’, at which the director starts tilting in opposite directions at the two surfaces. Defects of strength ±1/2 are either annihilated or expelled when the director tilts. On the other hand, disclination lines of ±1 which end in partial point defects (boojums) at the surfaces in the planar alignment regime acquire point defects of strength ±1 at the midplane of the cell when the director tilts. At a low enough temperature, the homeotropic anchoring becomes strong, and an electric field above the Freedericksz threshold generates the usual umbilic defects, which follow the dynamic scaling laws found in earlier studies.     

Control of surface anchoring properties of liquid crystal by thermo-transfer printing of siloxane oligomers

We presented the results for the surface anchoring properties of a nematic liquid crystal (LC) in a wide range of the substrate wettability through thermo-transfer printing of siloxane oligomers. For the modification of the surface energy, poly(dimethylsiloxane) (PDMS) oligomers were directly transferred onto a glass substrate from a bulk PDMS plate by contact printing at elevated temperatures. The hydrophobicity of the PDMS-modified surface was found to increase with the temperature of the substrate during transfer printing. The LC alignment on the PDMS-modified substrate exhibited continuous variations of the pretilt angle from nearly 0° to 90° and the polar anchoring strength according to the surface energy.     

DNA hybridization-induced reorientation of liquid crystal anchoring at the nematic liquid crystal/aqueous interface

Interactions between DNA and an adsorbed cationic surfactant at the nematic liquid crystal (LC)/aqueous interface were investigated using polarized and fluorescence microscopy. The adsorption of octadecyltrimethylammonium bromide (OTAB) surfactant to the LC/aqueous interface resulted in homeotropic (untilted) LC alignment. Subsequent adsorption of single-stranded DNA (ssDNA) to the surfactant-laden interface modified the interfacial structure, resulting in a reorientation of the LC from homeotropic alignment to an intermediate tilt angle. Exposure of the ssDNA/OTAB interfacial complex to its ssDNA complement induced a second change in the interfacial structure characterized by the nucleation, growth, and coalescence of lateral regions that induced homeotropic LC alignment. Fluorescence microscopy showed explicitly that the complement was colocalized in the same regions as the homeotropic domains. Exposure to noncomplementary ssDNA caused no such response, suggesting that the homeotropic regions were due to DNA hybridization. This hybridization occurred in the vicinity of the interface despite the fact that the conditions in bulk solution were such that hybridization did not occur (high stringency), suggesting that the presence of the cationic surfactant neutralized electrostatic repulsion and allowed for hydrogen bonding between DNA complements. This system has potential for label-less and portable DNA detection. Indeed, LC response to ssDNA target was detected with a lower limit of approximately 50 fmol of complement and was sufficiently selective to differentiate a one-base-pair mismatch in a 16-mer target.     

Optical behaviour of cholesteric liquid crystal cells with novel photoisomerizable chiral dopants

In order to investigate the effect of photoisomerization of E-Z structures on the optical behaviour of cholesteric liquid crystal (ChLC) cells, a series of novel azo derivatives was synthesized. Molecular structures were identified using ¹H NMR, ¹³C NMR, FTIR and elemental analysis. Thermal properties and the specific rotation of the synthesized chiral azo derivatives were estimated. Rubbed polyvinyl alcohol coated on the inner surface of substrates was used to control the liquid crystal alignment in cells. The effect of chiral dopants on the reflection band of ChLC cells was investigated, as well as the dependence of polarizing optical microscope textures on temperature. The stability and reproducibility of the effect of UV irradiation on the cell reflection band and real image recording were confirmed. Real image recording of the ChLC cells fabricated in this investigation was also studied; a photoinduced image through a mask is given. Photoirradiated and non-irradiated areas appear as different reflected colours leading to the formation of an image. Stacking of the ChLC cells was found to intensify the brightness of the reflection band.     

Optical behaviour of cholesteric liquid crystal cells with novel photoisomerizable chiral dopants

In order to investigate the effect of photoisomerization of E–Z structures on the optical behaviour of cholesteric liquid crystal (ChLC) cells, a series of novel azo derivatives was synthesized. Molecular structures were identified using ¹H NMR, ¹³C NMR, FTIR and elemental analysis. Thermal properties and the specific rotation of the synthesized chiral azo derivatives were estimated. Rubbed polyvinyl alcohol coated on the inner surface of substrates was used to control the liquid crystal alignment in cells. The effect of chiral dopants on the reflection band of ChLC cells was investigated, as well as the dependence of polarizing optical microscope textures on temperature. The stability and reproducibility of the effect of UV irradiation on the cell reflection band and real image recording were confirmed. Real image recording of the ChLC cells fabricated in this investigation was also studied; a photoinduced image through a mask is given. Photoirradiated and non‐irradiated areas appear as different reflected colours leading to the formation of an image. Stacking of the ChLC cells was found to intensify the brightness of the reflection band.     

AFM investigations of a glassy cholesteric liquid crystal with hydrophobic aerosil particles

AFM investigations of a glassy heterogeneous system consisting of an oligomeric cholesteric liquid crystal and the hydrophobic aerosil R812 were carried out. With increasing aerosil concentration, a suppression of the characteristic cholesteric surface pattern was observed. Typical separated aerosil aggregates appear in the samples. Their size and form change from small lumps through bigger rod-like entities to large crystallite-like aggregates of aerosil particles. This matches with observations of light scattering of systems with low molecular mass liquid crystals and of the memory effect. The pitch of the cholesteric fingerprint pattern slightly decreases with increase in the aerosil concentration.