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.     

Threshold spatially periodic structure of the director in a nematic flexoelectric cell with finite anchoring energy

The influence of the finiteness of the anchoring energy of a director and the value of flexoelectric polarization on the threshold of a spatially periodic reorientation of the director and the period of the arising structure is considered for a planar nematic cell. The threshold and the period are calculated numerically and the corresponding analytical expressions are obtained for the case of a strong anchoring of the director. It is shown that for a finite azimuthal anchoring energy the range of admissible values of the flexoelectric parameter ν widens, while for a finite polar anchoring energy this range narrows as compared to the case of an absolutely rigid orientation of the director at the cell surface.     

Liquid Crystal Channel Waveguides: A Monte Carlo Investigation of the Ordering

We present detailed Monte Carlo (MC) simulations of a nematic cell with homeotropic boundary conditions at the four confining surfaces. The simulations are based on the Lebwohl-Lasher lattice spin model with boundary conditions chosen to mimic the cell anchoring. We have investigated the model using a standard Metropolis Monte Carlo method to study the optical transmission and the ordering through the cell.     

Monte Carlo Study of Slot-waveguide Liquid Crystal Phase Shifters

Abstract

We present detailed Monte Carlo simulations of a simple model of a nematic liquid crystal slot waveguide shifter, investigating the effect of an applied electric external field. The simulations are based on the Lebwohl-Lasher lattice spin model with boundary conditions chosen to mimic the planar alignment as in Silicon Organic Hybrid waveguides and the homeotropic anchoring appropriate for Polydimethylsiloxane polymer walls. The external field is modeled by adding a term to the Hamiltonian which describes its coupling to the mesogenic molecules. We have investigated the effect of the external field on the optical transmission and the ordering across the cell.     

Helical structure and threshold voltage of cholesteric-nematic phase change at different anchoring conditions

The helical structure of cholesteric mixed systems and their electro-optical behaviour are studied at planar and pretilted surface orientation. The results indicate that the influence of the forces of wall orientation is decreased by the helical structure. To prove this model strong and weak anchoring conditions of the molecules in the cholesteric mixture at the substrate surface are realized. Also the behaviour of the electro-optical threshold voltage at different surface conditions supports the assumptions concerning the helical structure.     

Spatially periodic reorientation of liquid crystal director at planar Freedericksz transition

Conditions for the formation of a spatially periodic structure in a liquid crystal cell at the threshold planar-planar director reorientation in a dc electric field are considered. The dependences of the threshold and the wave number of the arising structure on (i) the polar and azimuthal anchoring energy at the surface of the cell substrates, (ii) the ratio r of the elastic constants of the liquid crystal, and (iii) the value of the flexoelectric parameter v are calculated. It is shown that the range of the parameters r and v corresponding to the spatially periodic director reorientation depends strongly on the anchoring energy. The range of allowable values of the parameter r narrows with a decrease in the azimuthal anchoring energy and expands with a decrease in the polar anchoring energy, while the range of allowable values of the parameter v widens with decreasing azimuthal anchoring energy and depends nonmonotonically on the polar anchoring energy.     

Temperature Induced Surface Transitions in Liquid Crystal Planar-Tilted Hybrid Cells

Temperature induced surface transitions in liquid crystal hybrid cells with asymmetrical anchoring, planar to one surface and tilted to the other, are experimentally studied. The results are theoretically explained by an elastic theory and the resulting values for the effective splay-bend elastic constant for a given liquid crystal, as well as the parameters characterising the anchoring on the boundary surface with tilted anchoring are compared with data from previous experiments, with good agreement.     

Nanomesh Aluminum Films for LC Alignment. Theoretical and Experimental Modeling

A porous system for LC alignment is reviewed. Fabrication of nanomesh aluminum films and their porous structure are described. Methods of the nanomesh parameters for optimal LC alignment are discussed. A model of the LC alignment in a porous system is proposed. The LC orientation type is determined by the free anchoring energy and the micropore diameter. The difference between planar and homeotropic anchoring energies appears to be lower than the interaction energy by two orders of magnitude.     

Computer Simulation Studies of Anisotropic Systems,III. Two-Dimensional Nematic Liquid Crystals

We have studied a two-dimensional ensemble of cylindrically symmetric particles interacting via a weak anisotropic potential using the Monte Carlo technique of computer simulation. The calculation is simplified by confining the particles to the sites of a triangular lattice. The internal energy, specific heat, second rank orientational order parameter and second rank orientational pair correlation function were calculated at various temperatures. The variation of the order parameter and pair correlation function shows that the system exhibits a transition from an orientationally disordered to a partially ordered phase. The temperature dependence of the specific heat suggests that the transition is second order or higher.

The possibility of the existence of order-disorder transitions in two dimensions is discussed. The results of the simulation are then compared with the predictions of a molecular field theory of orientational phase transitions. As expected the theory is found to be in poor agreement with the calculations.     

Birth-and-spread growth on the Kossel and a non-Kossel surface

A birth-and-spread growth model is derived for an anisotropic crystal surface and fitted to growth rate data obtained from Monte Carlo simulations of an isotropic and anisotropic Kossel (0 0 1) surface and a non-Kossel (0 0 1) surface. Only the step free energy is used as a fit parameter. All growth rate sets are nicely fitted by the new expression and the fitted values of the step free energy are in the physically relevant regime.     

Effects of alloy composition on the As desorption from and adsorption on strained InxGa1−xAs surfaces

The specular reflectivity of strained In_xGa_1−xAs surfaces grown by molecular beam epitaxy on InAs (100) substrates is measured with reflection high-energy electron diffraction (RHEED). A discontinuous change in the surface reflectivity is observed as the substrate temperature is increased above the transition point where As desorbs from the surface. A clear hysteresis loop is revealed as the substrate temperature is decreased. The substrate temperature required for desorption of surface As increases with Ga composition. A comparison between experimental results and theoretical calculations based on a Monte Carlo simulation shows that the average vertical interaction is increasing with Ga fraction. Fluctuations in alloy composition across the surface result in In-rich domains from which As is preferentially desorbed. The sudden loss of As, corresponding to a first order phase transition, occurs when the As desorbed domains attain a critical size. The metastability of the phase transition is shown to be a minimum for In_0.5Ga_0.5As layers.     

Surface and Magnetic Orientation in a Type II Nematic Lyomesophase

Abstract

A type II nematic lyomesophase formed by a quaternary system (K decanoate/water/decanol/KCI) has been studied by X-ray diffraction (XD) and optical microscopy (OM) at room temperature. Samples in capillaries of various thicknesses, materials and geometries have been studied under the influence of wall effects, residual magnetic orientation and orientation in presence of magnetic fields. Results evidence that surface orientation extend typically up to 0.5 mm. The planar diffracting units have a characteristic distance in the director direction of 36 Å. The analysis of the compromise between surface and magnetic orientation by XD allowed to determine a critical field of 200 G for a 0.5 mm thickness; the elastic constant is estimated as 10^?-⁷ dynes. Analised by OM, the sample is axially positive, but shows also a weak biaxiality.     

Numerical investigation of magnetic field effect on pressure in cylindrical and hemispherical silicon CZ crystal growth

The effect of axial magnetic field of different intensities on pressure in silicon Czochralski crystal growth is investigated in cylindrical and hemispherical geometries with rotating crystal and crucible and thermocapillary convection. As one important thermodynamic variable, the pressure is found to be more sensitive than temperature to magnetic field with strong dependence upon the vorticity field. The pressure at the triple point is proposed as a convenient parameter to control the homogeneity of the grown crystal. With a gradual increase of the magnetic field intensity the convection effect can be reduced without thermal fluctuations in the silicon melt. An evaluation of the magnetic interaction parameter critical value corresponding to flow, pressure and temperature homogenization leads to the important result that a relatively low axial magnetic field is required for the spherical system comparatively to the cylindrical one.     

The effect of concentration and molecular weight of polycarbonate on the elastic constants of a nematic liquid crystal solvent

A polarized upconverting luminescence in a novel upconverison nanorods (UCNRs)/liquid crystal network polymer (LCN) composite film is proposed. The UCNRs were fabricated by hydrothermal method, which were further modified to be introduced into LCN precursors with good dispersion. The orientation of UCNRs in LCN precursors was found to be strongly influenced by the configuration of LC molecules which could be modulated by external electric field. The UCNRs/LCN composite films in planar and homeotropic configuration were fabricated by UV photopolymerization. It is demonstrated that upconverting fluorescence emission from the UCNRs/LCN film with planar orientation is polarization-dependent, while the emission from the UCNRs/LCN film with homeotropic alignment is not polarized. These observations are ascribed to the directional arrangement of the UCNRs enabled by the LC molecules alignment and the crystal symmetry of the UCNR matrix. This work paves a different avenue to the construction of the upconversion nanoparticles/polymer composite system with prospective application value in anti-counterfeiting optical films.     

晶体生长机制和生长动力学的蒙特卡罗模拟研究

采用动力学蒙特卡罗方法,对在完整光滑界面上低过饱和度溶液中的晶体生长机制和动态过程进行计算机模拟,得到了晶体生长速率与溶液过饱和度之间的关系以及晶体生长的表面形态.对以二维成核为主要生长机制的动力学生长规律进行分析,发现了二维成核生长的生长死区以及单核生长转变为多核生长时的过饱和度临界值,讨论了热粗糙度、表面扩散、台阶平均高度以及表面尺寸对晶体平均生长速率的影响.     

Refracting properties of lyotropic nematic calamitic mesophase under magnetic field

The present study found the time-dependent extraordinary refractive index and birefringence of lyotropic nematic calamitic mesophase for different temperatures during its orientation under magnetic field. Based on the experimental results, the average angle between the mesophase directors and magnetic field was calculated to estimate the orientation dynamics of the mesophase during the director rotation from planar to homeotropic orientations. On the other hand, the textural properties were investigated during the orientation. The ordinary and extraordinary refractive indices were also measured depending on temperature to obtain the orientational order parameter of the mesophase. Using the macroscopic refractive index results, the microscopic polarizability was studied as well.     

Effect of surface-Induced Anchoring on Nic Light Scattering Characteristics

The effect of surface anchoring energy on NLC light scattering cross-section and temperature dependence of anchoring parameter are discussed.     

Growth and Spectrometric Tests on Bi4Ge3O12 Crystals

A resistance-heated furnace with a good control of temperature gradients is used for the Czochralski growth of precipitate and colour free Bi₄Ge₃O₁₂ (BGO) crystals up to 30 mm in diameter and 80 mm in length from a 45 mm diameter crucible. The quality of the grown crystals is discussed in connection with the growth conditions. Spectrometric tests made on scintillators obtained from BGO crystals are presented. A Monte Carlo program is used in order to compute the full energy peak efficiency of the detector for different energies and geometries. The computed values are compared to the experimental results.     

EFFECT OF AZIMUTHAL SURFACE LAYER ANCHORING ON FIELD-INDUCED LAYER REORIENTATION IN A SHORT PITCH FERROELECTRIC LIQUID CRYSTAL

Abstract

The concept of the azimuthal surface layer anchoring (ASLA) is introduced to describe the field-induced layer reorientation in a short pitch ferroelectric liquid crystal (FLC). It was found that for the larger ASLA, the smaller layer reorientation occurs and saturates in the high field regime, especially near both surfaces. The ASLA tends to renormalize the surface interactions which produce the asymmetric director switching. It is suggested that the ASLA acts as a hindered restoring force for the layer reorientation.     

Comparison of thermodynamic and kinetic models of single‐layer crystal–mother‐phase interface

Simple analytical kinetic model of single‐layer crystal–mother‐phase interface is proposed, which provides results that can be compared directly with thermodynamic Jackson model. Both models are based on zeroth order approximation known from lattice‐gas models, in which solid and fluid growth units are treated as mixed randomly in the interface layer. It is shown that the kinetic and thermodynamic approaches can lead to very similar predictions about growth mechanism. The parameters characterising growth conditions, obtained from these models, are significantly different from those obtained from Monte Carlo simulations applied to study stable states of single‐layer interface. Monte Carlo calculations describe crystal growth in detail and their results can predict characteristic parameters for real experiment. Observed differences seem to be strongly influenced by the use of zeroth order approximation. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)