New method for measuring polar anchoring energy of nematic liquid crystals

A new method for measuring a polar anchoring energy of nematic liquid crystals (LCs) is proposed. A variation of LC tilt angle on the surface with an applied electrical field was determined by a reflective method. The twisted LC cell configuration was selected to compensate a contribution of the induced birefringence in the reflective spectra. The electrical field controlled reflectance was used to analyse the potential form of the polar anchoring energy and to define the anchoring strength. The proposed method is applicable for 2–5 μm thick LC cells.     

The polar anchoring energy of nematic liquid crystals with polyvinylidene fluoride alignment layer

It has been proved that the high dielectric polyvinylidene fluoride (PVDF) could be used as an alignment layer in liquid crystal device. In this paper, the transmittance, pretilt angle and the polar anchoring energy of the substrates with PVDF alignment layer were researched. Theoretical results and the experimental results about the reflectivity R_SS (polarisation-conserving signals) recorded by full-leaky guided mode in liquid crystal technique are analysed to evaluate the anchoring energy of PVDF alignment layer. The result shows that the polar anchoring energy between PVDF alignment layer and liquid crystal molecules is 2.80 × 10^?4 J/m².     

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).     

The effect of a finite anchoring energy on the transient periodic structures in nematic liquid crystals

Non-equilibrium transient periodic structures have been widely observed in thermotropic and lyotropic nematic liquid crystals. So far only the case of strongly anchored nematics has been considered. Here we investigate the influence of a finite anchoring energy on the non-equilibrium transient pattern in a twist geometry. Both the domain wavelength and the threshold field for the non-equilibrium pattern have been calculated for different values of the anchoring energy.     

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.     

Disclinations dynamics in confined nematic liquid crystals: strong anchoring

We have investigated the dynamics of pair annihilation of disclination lines in strong anchoring. This work is based on the Frank free energy. The director angle, φ(x, y, z), is obtained by the continuous theory. We show that the form of the viscous force in a confined nematic liquid crystal and in strong anchoring is a function of the initial distance between the two disclination lines. The asymptotic velocity, v _asy, is also a function of the initial distance. Our theoretical result on the asymptotic velocity is in good agreement with previous experimental results.     

Disclinations dynamics in confined nematic liquid crystals: strong anchoring

We have investigated the dynamics of pair annihilation of disclination lines in strong anchoring. This work is based on the Frank free energy. The director angle, φ(x, y, z), is obtained by the continuous theory. We show that the form of the viscous force in a confined nematic liquid crystal and in strong anchoring is a function of the initial distance between the two disclination lines. The asymptotic velocity, v _asy, is also a function of the initial distance. Our theoretical result on the asymptotic velocity is in good agreement with previous experimental results.     

Influence of weak anchoring on flow instabilities in nematic liquid crystals

We analyse the homogeneous instabilities in a nematic liquid crystal subjected to plane steady Couette or Poiseuille flow in the case when the director is prealigned perpendicular to the flow plane taking into account weak anchoring at the confining surfaces. The critical shear rate decreases for decreasing anchoring strength and goes to zero in the limit of torque-free boundary conditions. For Poiseuille flow two types of instability arise depending on the values of the azimuthal (W _a) and polar (W _p) surface anchoring strengths. The critical line in (W _a,W _p) space which separates the two instabilities regimes is obtained.     

Influence of weak anchoring on flow instabilities in nematic liquid crystals

We analyse the homogeneous instabilities in a nematic liquid crystal subjected to plane steady Couette or Poiseuille flow in the case when the director is prealigned perpendicular to the flow plane taking into account weak anchoring at the confining surfaces. The critical shear rate decreases for decreasing anchoring strength and goes to zero in the limit of torque-free boundary conditions. For Poiseuille flow two types of instability arise depending on the values of the azimuthal (W_a) and polar (W_p) surface anchoring strengths. The critical line in (W_a,W_p) space which separates the two instabilities regimes is obtained.     

The formula of anchoring energy for a nematic liquid crystal

The interface energy for a nematic liquid crystal (NLC) is considered as the sum of potential energy between LC molecules and molecules of the substrate surface, and a formula for anchoring energy is derived by elementary principles. The anchoring energy for a NLC should have two terms, the first term is the same as the Rapini–Papoular expression, the second is related to the normal of interface and resultes from the biaxial property of a NLC induced by interface. Hence there are two anchoring coefficients, W ₁ and W ₂. We demonstrate that W ₁ is equal to the tilt angle strength A_θ , and W ₂ corresponds to the difference between A_θ and the azimuthal strength A_? . Thus A_θ –A_? is due to the biaxial property of the NLC near the interface. Applying this formula to the twisted NLC cell, we discuss the threshold and saturation field, as well as the maximal tilt angel θ _m with respect to A_θ /A_? . Previously proposed formulae are discussed from our point view.     

The formula of anchoring energy for a nematic liquid crystal

The interface energy for a nematic liquid crystal (NLC) is considered as the sum of potential energy between LC molecules and molecules of the substrate surface, and a formula for anchoring energy is derived by elementary principles. The anchoring energy for a NLC should have two terms, the first term is the same as the Rapini-Papoular expression, the second is related to the normal of interface and resultes from the biaxial property of a NLC induced by interface. Hence there are two anchoring coefficients, W₁ and W₂. We demonstrate that W₁ is equal to the tilt angle strength A_θ, and W₂ corresponds to the difference between A_θ and the azimuthal strength A_ϕ. Thus A_θ-A_ϕ is due to the biaxial property of the NLC near the interface. Applying this formula to the twisted NLC cell, we discuss the threshold and saturation field, as well as the maximal tilt angel θ_m with respect to A_θ/A_ϕ. Previously proposed formulae are discussed from our point view.     

Slow relaxation processes in nematic liquid crystals at weak surface anchoring

We present new results of experimental investigations of azimuthal director reorientation dynamics for a nematic liquid crystal on solid substrates. Two types of substrate with weak anchoring were studied: glass/polystyrene and glass/UV-activated dye. Slow and fast relaxation processes were observed in both cases under the action of a strong 'in-plane' electric field. The slow surface reorientation and memory effects were controlled by two parameters: the electric voltage and the excitation time. It was established that the increase of the excitation time results in a slowing of the relaxation of the system to the initial state after turning off the electric field. A phenomenological model of a gliding of easy axes is proposed to explain the slow relaxation process.     

Orientation and anchoring effects in stretched polymer dispersed nematic liquid crystals

The induction of liquid crystal orientation through mechanical stretching was investigated for polymer dispersed liquid crystals (PDLCs) by means of infrared dichroism. Using a nematic liquid crystal BL006 and polyacrylic acid as the polymer matrix, it was possible to stretch the PDLC films with BL006 in either the isotropic or the nematic phase. After cooling the films under strain to room temperature, the molecular orientation of BL006 was found to be much higher for films that contained isotropic liquid droplets of BL006 at the time of stretching than for films that had nematic droplets. Stretching PDLC films with isotropic droplets results in no molecular orientation, but the orientation is induced during the subsequent cooling when BL006 goes through the isotropic-to-nematic phase transition. Interestingly for PAA/BL006, the nematic director orients along the long axes of the elongated droplets despite liquid crystal anchoring perpendicular to the polymer interface.     

Orientation and anchoring effects in stretched polymer dispersed nematic liquid crystals

The induction of liquid crystal orientation through mechanical stretching was investigated for polymer dispersed liquid crystals (PDLCs) by means of infrared dichroism. Using a nematic liquid crystal BL006 and polyacrylic acid as the polymer matrix, it was possible to stretch the PDLC films with BL006 in either the isotropic or the nematic phase. After cooling the films under strain to room temperature, the molecular orientation of BL006 was found to be much higher for films that contained isotropic liquid droplets of BL006 at the time of stretching than for films that had nematic droplets. Stretching PDLC films with isotropic droplets results in no molecular orientation, but the orientation is induced during the subsequent cooling when BL006 goes through the isotropic-to-nematic phase transition. Interestingly for PAA/BL006, the nematic director orients along the long axes of the elongated droplets despite liquid crystal anchoring perpendicular to the polymer interface.     

Temperature induced anchoring transition in nematic liquid crystals with two-fold degenerate alignment

The two-fold degenerate alignment of nematic liquid crystals provided by obliquely evaporated SiOx layers was studied as a function of temperature. A temperature induced anchoring transition from tilted to planar alignment with more than 80 degrees between their preferred directions was found. The director in the two-fold degenerate region seems to follow a circular sector from the uniform tilted to the uniform planar anchoring, as the temperature increases. The anchoring transition is reversible and reveals the existing coupling between the azimuthal and polar angles of the preferred directions of alignment. The applicability of the two-fold degenerate alignment and related anchoring transitions of nematic liquid crystals for devices is briefly discussed.     

Electric field transport of biphilic ions and anchoring transitions in nematic liquid crystals

Anchoring interaction of MBBA and MBBA + 5CB nematic layers with monomolecular films of CTAB on ITO glass supports is studied by videomicroscopy in the presence of an electric field. Planar-planar or homeotropichomeotropic electrode substrates are used to make the nematic cells. The substrate symmetry is broken by coating only one electrode with self-assembled CTAB film. In a DC electric field we observe some new effects; (i) polarity-dependent breaking of anchoring and switching to two oblique states in dielectrically stable planar cells; (ii) a polarity-dependent flow-induced metastable anchoring transition in homeotropic cells to a planar or tilted alignment after the field is switched off. These results are discussed in terms of a surface transition assisted by electric transport of biphilic CTAB ions and by a surface memorization of the flow-induced planar alignment.     

Polar anchoring energy and order parameter at a nematic liquid crystal-wall interface

By measuring the integrated birefringence versus thickness of homogeneous and distorted nematic liquid crystal configurations, we have obtained the anchoring energy and the change of the order parameter at the orientating layer. The measurements were made for planar and tilted orientating SiO layers and for rubbed polyimide. The tilted orientating SiO layer shows the most significant decrease of the order parameter near the interface, which can be explained by its microscopically inhomogeneous alignment.     

Experimental determination of the flexoelectric coefficients of some nematic liquid crystals

Abstract

We report measurements of the temperature variation of the flexoelectric coefficient (e ₁–e ₃) of a number of nematic liquid crystals like phenylcyclohexanes, cyanobiphenyls, etc. We have also measured (e ₁+e ₃) of a few systems using appropriate methods of applying an electric field gradient to the sample. In most of the systems, (e ₁–e ₃)/k, where k is a curvature elastic constant, is found to be positive and independent of temperature, as expected. However, in 4-heptyl-1-(4-cyanocyclohexyl)cyclohexane and a few other compounds with relatively flexible parts, |(e ₁–e ₃)/k| increases with temperature. We discuss the possible molecular origin of the sign and temperature dependence of the flexoelectric coefficients of the systems studied.     

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.     

Invited Article Experimental techniques for the investigation of ferroelectric liquid crystals

A review is given of several experimental methods for the measurement of the performance parameters of ferroelectric liquid crystals. We discuss (a) the technique for measuring the spontaneous polarization, the elastic modulus and dielectric susceptibility for the tilt of the director (the pyro-electric method), (b) the measurement of the tilt angle and the optical anisotropy (the electro-optical method) and (c) the determination of the non-linear susceptibility of the chiral smectic C* phase (the optical second harmonic generation technique).