Anchoring Strength Coefficient Research Articles

Enhancement of effective polar anchoring strength and accelerated electro-optic switching in a two-dimensional hexagonal boron nitride/polyimide hybrid liquid crystal device.

The monolayer hexagonal boron nitride (h-BN) nanosheet is transferred onto an indium tin oxide (ITO)-coated glass substrate. This h-BN slide is placed together with a conventional planar-aligning polyimide (PI) slide to fabricate a liquid crystal (LC) cell, in which the LC achieves uniform planar alignment. The effective polar anchoring strength coefficient of this h-BN-based hybrid LC device is found to increase significantly compared to that of a standard PI/PI LC device. The presence of the monolayer h-BN nanosheet as an alignment agent increases the planar anchoring energy through the epitaxial interaction between the LC and the honeycomb structure of the two-dimensional h-BN lattice in this hybrid device. The amplified polar anchoring energy is found to accelerate the electro-optic response time of the LC in this h-BN-based hybrid device.

Enhancement of polar anchoring strength in a graphene-nematic suspension and its effect on nematic electro-optic switching.

A small quantity of monolayer graphene flakes is doped in a nematic liquid crystal (LC), and the effective polar anchoring strength coefficient between the LC and the alignment substrate is found to increase by an order of magnitude. The hexagonal pattern of graphene can interact with the LC's benzene rings via π-π electron stacking, enabling the LC to anchor to the graphene surface homogeneously (i.e., planar anchoring). When the LC cell is filled with the graphene-doped LC, some graphene flakes are preferentially attached to the alignment layer and modify the substrate's anchoring property. These spontaneously deposited graphene flakes promote planar anchoring at the substrate and the polar anchoring energy at alignment layer is enhanced significantly. The enhanced anchoring energy is found to impact favorably on the electro-optic response of the LC. Additional studies reveal that the nematic electro-optic switching is significantly faster in the LC-graphene hybrid than that of the pure LC.

Comparative study of the anchoring strength of reactive mesogens and industrial polyimides used for the alignment of liquid crystals.

We measured the Rapini-Papoular polar anchoring strength coefficient W for 4^{'}-pentyl-4-cyanobiphenyl (5CB) on alignment layers formed by the reactive mesogen photopolymers RM 257, RM 82, and RM 84 [4,4^{'}-bis(acryloyl)biphenyl] (by Merck). These materials are commonly used for the photostabilization of the liquid crystal (LC) director in the bulk as well as at the surface of the LC layer via the formation of a loose polymer network that captures the director orientation. We developed a method of fabrication of alignment layers from these polymers, and estimated W from the measurements of the optical retardation as a function of applied voltage in uniformly aligned cells. We found that RM 257 yielded W of about 6×10^{-4}J/m^{2}, whereas RM 82 and RM 84 provided anchoring strengths of about 2×10^{-4} and 4×10^{-4}J/m^{2}, respectively. Subsequent heating of the sample either destroyed the alignment layer, or substantially decreased W to about 1×10^{-4}J/m^{2}, which was comparable to the anchoring strength of weakly rubbed commercial polyimides.

扭曲向列相薄盒中线缺陷的研究

Within the Landau-de Gennes theory,the order reconstruction of s =±1/2 twist disclinations in a twisted nematic cell is investigated,using the two-dimensional relaxation iterative method.The biaxial structure of the defect core as the cell gap decreasing is explored.At a critical value of dc*≈ 9ξ(here ξ is the characteristic length for order-parameter changes),the exchange solution is stable,while the defect core solution becomes metastable,where the system starts to stretch the defect structure and the biaxiality starts to propagate inside of the cell.Comparing to the case with no initial disclination,the value at which the exchange solution becomes stable increases relatively.At a critical separation of dc≈ 7ξ,the system undergoes a biaxial transition,and the defect core merges into a biaxial wall with large biaxiality.The force reaches a maximum at d≈ 9ξ,and a local minimum at d≈7ξ.For weak anchoring boundary conditions,because of the weakened frustration,the asymmetric boundary conditions repel the defect to the weak anchoring boundary as the anchoring strength coefficient decreasing.

Surface topography and rotational symmetry breaking

The surface electroclinic effect, which is a rotation of the molecular director in the substrate plane proportional to an electric field E applied normal to the substrate, requires both a chiral environment and C(2) (or lower) rotational symmetry about E. The two symmetries typically are created in tandem by manipulating the surface topography, a process that conflates their effects. Here we use a pair of rubbed polymer-coated substrates in a twist geometry to obtain our main result, viz., that the strengths of two symmetries, in this case the rub-induced breaking of C(∞) rotational symmetry and chiral symmetry, can be separated and quantified. Experimentally we observe that the strength of the reduced rotational symmetry arising from the rub-induced scratches, which is proportional to the electroclinic response, scales linearly with the induced topographical rms roughness and increases with increasing rubbing strength of the polymer. Our results also suggest that the azimuthal anchoring strength coefficient is relatively insensitive to the strength of the rubbing.

Mechanically generated surface chirality: Control of chiral strength

A substrate coated with an achiral polyimide alignment layer was scribed with the stylus of an atomic force microscope having a line-to-line force profile FAFBFCFAFBFC…. The strength of the resulting chiral surface was examined using the nematic liquid crystal electroclinic effect induced by the surface. The magnitude of the electroclinic effect was found to increase with increasing scribing force, which suggests a method for controlling the chiral strength. Additionally, the electroclinic magnitude divided by the rms surface roughness was approximately constant with scribing force, suggesting that the azimuthal anchoring strength coefficient is nearly independent of the scribing force.

Direct visualization and measurement of the extrapolation length on cooling toward the nematic–smectic-Aphase transition temperature

A herringbone "easy axis" pattern is scribed into a polyimide alignment layer for liquid-crystal orientation using the stylus of an atomic force microscope. Owing to the liquid crystal's bend elasticity K33 , the nematic director is unable to follow the sharp turn in the scribed easy axis, but instead relaxes over an extrapolation length L=K33/W2φ, where W2φ is the quadratic azimuthal anchoring strength coefficient. By immersing a tapered optical fiber into the liquid crystal, illuminating the fiber with polarized light, and scanning the fiber close to the substrate, a visualization and direct measurement of L are obtained on approaching the nematic-smectic- A phase transition temperature T NA from above. L is found to exhibit a sharp pretransitional increase near T NA, consistent with a diverging bend elastic constant.

Effective anchoring energy in dipolar organic film on metals surfaces

The influence of electron injection from the metal electrode into organic liquid crystal dipolar film on the effective anchoring energy (EAE) of the polar organic film is discussed from the energy point of view. It is shown that the accounting for the injected carrier in organic film results in a polynomial function for the EAE expanded up to the fourth order in cos θ s, where θ s is the polar angle of the director n ˆ at the film/metal interface. It is also shown that in a certain range of the location of centroid of the injected carrier z ¯ the destabilizing surface polarization mechanism may lead to destruction of the linear anchoring strength coefficient w 1. The strong influence of z ¯ on the quadratic term w 2 also has been demonstrated.

Relation between the microscopic and macroscopic descriptions of the effect of a solid-substrate surface on a homeotropically aligned nematic liquid crystal

A simple microscopic mean-field model is proposed for a homeotropically aligned planar nematic liquid crystal (NLC) in contact with a solid-substrate surface. The intermolecular interaction in the NLC is simulated with the anisotropic McMillan potential, and the orienting effect of the substrate surface on the molecules in the NLC is described as that of an external field acting only on the first surface molecular layer of the sample. This model is used to describe the deformation of the director field of the sample caused by the external field and to determine the anchoring strength coefficient W, which is employed to macroscopically describe the orienting effect of the solid substrate on the NLC. The dependence of this coefficient on the strength of the short-range orienting field of the substrate surface used in the proposed microscopic model is found, and a unique correspondence between W and the profile of the orientational order parameter near the substrate surface is established. The temperature dependence of the anchoring strength coefficient is derived and found to agree well with experimental data for the MBBA NLC.

Polar Anchoring Strength of a Tilted Nematic: Confirmation of the Dual Easy Axis Model

The polar anchoring strength coefficient W and polar pretilt angle theta0 were measured simultaneously for the liquid crystal pentylcyanobiphenyl at a rubbed polyimide alignment layer that is ordinarily used for vertical alignment. It was found that W proportional theta(2)0 over the range 0 degrees < or =theta0 less or similar to 35 degrees . The results provide a confirmation of the dual easy axis model, wherein the liquid crystal director adopts an equilibrium orientation theta0 at the substrate that is determined by competition between a pair of preferred orientation directions.

Chiral-Induced Polarization at a Tilted Nematic—Substrate Interface

An ac electric field at frequency ω is applied to a chiral nematic liquid crystal whose director is tilted by an angle θ i with respect to the substrate normal. The nematic director is observed to oscillate at frequency ω azimuthally about the substrate normal, indicating the existence of a component of polarization Pi at the substrate that is perpendicular to the director and parallel to the substrate. Pi, the anchoring strength coefficient, and the bulk viscosity are obtained by measurements of the oscillation amplitude as a function of ω. In particular, it is found that the interfacial polarization, when scaled to a volumetric value, is two orders of magnitude smaller than in the same material in the smectic-C* phase.

Step-wise Fréedericksz transition in a nematic liquid crystal

The stylus of an atomic force microscope is used to rub a polyimide-coated substrate with a spatial period of 10 μm. A spatial periodicity is thereby introduced into the anchoring strength coefficient, resulting in a shoulder in the intensity–voltage curve above the Fréedericksz transition threshold of a nematic liquid crystal.

Transverse surface-induced polarization at the interface between a chiral nematic liquid crystal and a substrate.

A chiral nematic liquid crystal that is tilted by an angle theta(i) with respect to a substrate is subjected to an ac electric field at frequency omega applied parallel to the substrate. The nematic director is found to oscillate azimuthally about the normal to the liquid crystal-substrate interface at frequency omega, indicating that a nonzero polarization perpendicular to the molecular tilt plane exists at the interface. The interfacial polarization, anchoring strength coefficient, and bulk viscosity are obtained by measurements of the oscillation amplitude as a function of omega.

Surface Order Parameter and Polar Anchoring Energy in Nematic Liquid Crystals

The trends of temperature dependence of the surface order parameter and of the anchoring strength coefficient are theoretically evaluated in the Landau-de Gennes phenomenological theory for nematic liquid crystals. The cases of planar, homeotropic and tilted alignment at a planar limiting surface are considered. It is shown that the cases with surface order parameter greater than the bulk one or smaller than this can occur in function of the phenomenological coefficients in the surface free energy expression. The anchoring strength coefficient, W, diminishes when temperature approaches the nematic-isotropic point, after passing through a maximum in most of cases. The possibility of a non-zero anchoring strength at temperatures above the transition temperature is revealed. The obtained trends of W(T) describe well a large variety of experimental results from literature.

Surface-anisotropy-induced linear electro-optic effect in a nematic liquid crystal.

A liquid-crystal cell treated for homeotropic alignment with different surfactants at the two surfaces was subjected to an electric field E in the plane of the cell. The differential optical retardation \ensuremath{\delta}\ensuremath{\alpha} was found to be linear in field over a frequency range 10100 000 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, where d(\ensuremath{\delta}\ensuremath{\alpha})/dE\ensuremath{\propto}${\mathrm{\ensuremath{\omega}}}^{\mathrm{\ensuremath{-}}1}$. A model is proposed that explains this result and facilitates the determination of the anchoring strength coefficient W. \textcopyright{} 1996 The American Physical Society.

A simple and reliable method for measuring the liquid crystal anchoring strength coefficient

Abstract By measuring the electric Freedericksz transition threshold in a wedged capacitance cell, we have developed a simple method for determining the anchoring strength coefficient for tilt of the director relative to the substrate normal. This technique requires neither a knowledge of the absolute cell thickness nor a knowledge of the optical birefringence. Moreover, it applies to both the homeotropic orientation for Δχ 0, where Δχ is the dielectric susceptibility anisotropy.

Anchoring strength coefficient of a monomer and its dimer at a polymer-coated interface

Abstract The dialkoxyphenylbenzoate monomer ‘5005’ (C5H11OC6H4COOC6H4OC5H11) and its dimer were aligned parallel to a buffed polyimide-coated glass substrate. A magnetically induced Freedericksz transition in both thick and thin cells was used in conjunction with a capacitance technique to determine the anchoring strength coefficient Was a function of temperature in the nematic phase. It was found that for both monomer and dimer, Wincreases with decreasing temperature. The anchoring coefficient for the dimer, however, was found to be an order of magnitude larger than for the monomer at comparable reduced temperatures. The splay elastic moduli were also determined for both species, and found to be of comparable magnitude, consistent with previous results.

Behaviour of the anchoring strength coefficient near a structural transition at a nematic–substrate interface

Abstract A nearly symmetric alkoxyphenylbenzoate monomer is found to exhibit a transition from perpendicular to tilted alignment at a lecithin-treated glass substrate several degrees below the nematic-isotropic phase transition. By means of the Freedericksz transition the coefficient B of the anchoring energy has been obtained. It is found that B rapidly decreases, and the tilt susceptibility increases on approaching this transition.

Temperature dependence of the anchoring strength coefficient at a nematic liquid crystal-wall interface

The anchoring strength coefficient B is measured as a function of temperature using a Freedericksz technique in a very thin (l La force d'ancrage du MBBA en fonction de la temperature est mesuree par la methode de la transition de Freedericksz dans une cellule mince traitee par un surfactant

ANISOTROPIC INTERACTIONS BETWEEN MBBA AND SURFACE-TREATED SUBSTRATES

Easy axis Θ and anchoring strength coefficient Bθ were measured on the interfaces between MBBA and substrates with various surfactants layers, by using wall effects on the Freedericksz transition. Experimental results indicate that steric, dispersive and polar interactions play individual roles in the molecular orienting forces of surface-treated substrates. Bθ values were found to be rather small. MBBA surface tension anisotropy was estimated to be of the order of 10-2 dyn/cm.