Photoinduced Ordering Transition in Microdroplets of Liquid Crystals with Azo‐Dendrimer

Ordering transitions are observed in azo‐dendrimer‐dissolved nematic liquid crystal (NLC) droplets dispersed in a glycerol matrix. The dendrimer molecules are spontaneously attached at the interface between NLC and glycerol, so that the nematic directors orient perpendicular to the interface, in the radial configuration. Photoisomerization makes the directors be tangential to the interface, in a photoinduced ordering transition from radial to bipolar structure. Similar experiments are conducted both in cholesteric (Ch) and smectic‐A (SmA) LC droplets. Complicated photoinduced ordering transitions are also observed in Ch and SmA droplets, and the associated molecular orientation changes are discussed. Photoisomerizable azo‐dendrimer molecules provide a possible way to control orientation, even in microdroplet systems in which the interface cannot be treated by conventional surface agents.     

Study of intrinsic anchoring in nematic liquid crystals based on modified Gruhn-Hess pair potential

A nematic liquid crystal slab composed of N molecular layers is investigated using a simple cubic lattice model, based upon the molecular pair potential which is spatially anisotropic and dependent on elastic constants of liquid crystals. A perfect nematic order is assumed in the theoretical treatment, which means the orientation of the molecular long axis coincides with the director of liquid crystal and the total free energy equals to the total interaction energy. We present a modified Gruhn-Hess model, which is relative to the splay-bend elastic constant K₁₃. Furthermore, we have studied the free nematic interfacial behavior (intrinsic anchoring) by this model in the assumption of the perfect nematic order. We find that the preferred orientation at the free interface and the intrinsic anchoring strength change with the value of modification, and that the director profile can be determined by the competition of the intrinsic anchoring with external forces present in the system. Also we simulate the intrinsic anchoring at different temperatures using Monte Carlo method and the simulation results show that the intrinsic anchoring favors planar alignment and the free interface is more disordered than the bulk.     

Influence of bulk nematic orientation on the interface between a liquid crystalline polymer and a flexible polymer

The structure of the interface between a liquid crystalline polymer (LCP) and a flexible polymer was studied using the three-dimensional bond fluctuation model in Monte Carlo simulations. Orientation in both phases in the neighborhood of the interface is sensitive to the far-field nematic orientation. The more diffuse interface caused by a homeotropic far-field orientation in the LCP results in a substantial reduction in the calculated interfacial tension relative to that for a planar far-field orientation.     

Exposure and temperature dependences of contact angle of liquid crystals on photoaligning surface

We report on first studies of wetting of liquid crystal on photoaligning surface. We observed strong light-induced variations of a contact angle of a liquid LC 5CB on a photoaligning surface of fluoro-polyvinyl-cinnamate (PVCN-F) and we connected these variations with changes of PVCN-F polarity. We also present drastic changes of the contact angle of a nematic liquid crystal (MLC-6080) on the PVCN-F surface in a vicinity of temperature anchoring transition of liquid crystal from homeotropic orientation to planar one. We did not find any peculiarities in temperature dependence of the contact angle of isotropic liquid (glycerol) on PVCN-F and of nematic liquid crystal on pure glass in the same temperature range. It allows us to suggest that rearrangement of LC molecules and flexible fragments in the LC-polymer interface are responsible for the change of surface tensions both of LC and polymer and the observed jump of the contact angle.     

Dielectric and electrooptical properties of hockey-stick-shaped liquid crystal with a negative dielectric anisotropy

We investigated dielectric properties of a hockey-stick-shaped liquid crystal (HLC). Two dielectric relaxation modes were observed at 0.91 kHz and 4.51 MHz. The low frequency relaxation modes in isotropic, smectic, and nematic phases are related to the motion of ions, collective tilt fluctuation, and rotation around the long molecular axis, respectively. Meanwhile, the high frequency relaxation modes in nematic and smectic phases was due to the rotation around the short axis of the molecules and hindered by the resistance of the ITO layers. We also examined the electrooptical response of the 5.0 wt% HLC-doped commercial nematic liquid crystal (LC) mixtures. The birefringence of the LC mixture was slightly increased, while the falling time and the rotational viscosity was decreased in the nematic phase.     

玻璃与液晶非线性光学界面反射特性的研究

本文系统地研究了由玻璃与液晶构成的非线性光学界面。用一台调Q的红宝石激光器研究了由玻璃与处于各向同性液相的液晶所构成的非线性界面。在不同入射角时非线性界面由内部全反射跃变到部分透射的阈值光强与Kaplen的平面波理论计算结果相一致。在T-T_c=2.5℃条件下测量了时间分辨的非线性界面的反射率。观察到了反射率的滞后迴线。非线性界面的这种反射率的滞后迴线可以归之于液晶分子退取向的弛豫过程。用Ar⁺激光器研究了玻璃与向列相液晶所构成的非线性光学界面,观察到了由相变而产 关键词：     

Hydrodynamics of nematic liquid crystals due to direct volume expansion

Hydrodynamics of planar nematic liquid crystal molecules under the influence of horizontal pressure gradient was studied, theoretically. The pressure gradient is due to nematic liquid crystal volume expansion which is a consequence of the incompressibility of the liquid. The resulting director reorientation which is due to coupling between the director orientation and fluidity of the liquid is calculated numerically for the hard and weak anchoring conditions at the cell walls.     

Transversal structure of liquid crystal in a light wave field

The change in the orientation of the director of a nematic liquid crystal in a light wave field under a uniform illumination is analyzed. A system of equations for the distribution of the director angle of inclination is derived for the case of normal incidence. This system accounts for the finite energy of interaction between the nematic and substrate. The dependence of the director distribution profile on the incident light intensity and cohesion energy is studied at fields considerably above the threshold.     

A biaxial nematic liquid crystal composed of matchbox-symmetric molecules

By means of Monte Carlo simulations in the isothermal-isobaric ensemble, we investigate the structure and phase behaviour of a thermotropic liquid crystal composed of matchbox-symmetric (or board-like) molecules. Besides the isotropic phase the liquid crystal exhibits also uniaxial and biaxial nematic phases. The interaction potential is derived through an expansion in terms of Stone's rotational invariants [A. J. Stone, Mol. Phys. 78, 241–256 (1978).] that can be reexpressed in terms of Cartesian tensors. This latter formulation is particularly well suited for computer simulations. We analyse the orientation distribution function which allows us to distinguish between intrinsic and extrinsic biaxiality. In addition, we study the orientation-dependent correlation functions. In the limit of large intermolecular separations, the value of the orientation correlation function corresponds to the uniaxial and biaxial order parameters which are coupled in a complex fashion.     

Nematic-crystal transitions in liquid crystals

A lattice theory of phase transitions in a system of rod-like molecules is presented with application to liquid crystals. The first order transitions between nematic and crystal phases as well as between isotropic and nematic phases are obtained.     

Dichroism of absorption by impurity molecules with internal rotation in nematic liquid crystals

Expressions are obtained for the components of the permittivity tensor in the optical region for a nematic liquid crystal with impurity molecules having the internal rotation conformational degree of freedom. The dependence of the intensity and dichroism of the impurity absorption bands in isotropic and nematic phases on the parameters of conformational, orientation, and mixed conformation-orientation orders of molecules is found. The influence of the mutual correlation of conformational and orientation degrees of freedom of molecules on these parameters is studied. The dependence of the oscillator strengths of molecular transitions on the phase state of a medium and the orientation order of the impurity subsystem is analyzed. The self-consistent nature of molecular, structural, and spectral changes upon the nematic-isotropic liquid phase transition is demonstrated.     

Nematic order-disorder state transition in a liquid crystal analogue formed by oriented and migrating amoeboid cells

In cell culture, liquid crystal analogues are formed by elongated, migrating, and interacting amoeboid cells. An apolar nematic liquid crystal analogue is formed by different cell types like human melanocytes (=pigment cells of the skin), human fibroblasts (=connective tissue cells), human osteoblasts (=bone cells), human adipocytes (=fat cells), etc. The nematic analogue is quite well described by i) a stochastic machine equation responsible for cell orientation and ii) a self-organized extracellular guiding signal, E₂, which is proportional to the orientational order parameter as well as to the cell density. The investigations were mainly made with melanocytes. The transition to an isotropic state analogue can be accomplished either by changing the strength of interaction (e.g. variation of the cell density) or by influencing the cellular machinery by an externally applied signal: i) An isotropic gaseous state analogue is observed at low cell density (melanocytes/mm^2) and a nematic liquid crystal state analogue at higher cell density. ii) The nematic state analogue disappears if the bipolar shaped melanocytes are forced to become a star-like shape (induced by colchicine or staurosporine). The analogy between nematic liquid crystal state analogue formed by elongated, migrating and interacting cells and the nematic liquid crystal phase formed by interacting elongated molecules is discussed. Received 2 August 1999 and Received in final form 5 January 2000     

The effect of magnetic fields and boundary conditions on the Couette flow of nematics

The paper discusses the theory of Couette flow of a nematic liquid crystal. The apparent viscosity, orientation and velocity profiles are computed forp-azoxyanisole as functions of shear rate and magnetic field for symmetric and asymmetric molecular alignments at the boundaries and for different relative radii of the cylinders. For symmetric homeotropic boundary condition an azimuthal field exhibits a threshold analogous to a Freedericksz transition. An expression is also derived for the Freedericksz threshold in the hydrostatic case.     

Temperature dependence of the effective anchoring energy for a nematic-ferroelectric interface

Specific features of the anisotropic interaction between a nematic mixture and a polar surface of a ferroelectric triglycine sulfate crystal have been studied over a wide temperature range including the substrate's Curie point T_c. The mixture was composed of two nematic liquid crystals, 60% of p-methoxybenzylidene-p-n-butylaniline (MBBA) and 40% of p-ethoxybenzylidene-p-n-butylaniline (EBBA), and doped with a small amount of a dichroic dye. The temperature dependence of the polarized components of optical density D_j of the dye absorption band for the nematic and isotropic phases of the MBBA+EBBA mixture has been obtained using polarization optic techniques. The temperature-induced structural changes in the nematic layer near T_c were found to be related to the changes in the orientational part of the tensor order parameter Q_ik. The experimental data have been interpreted using the model, in which the dispersive van der Waals forces of the substrate stabilize the planar orientation of the nematic in the bulk competing with the short-range anchoring forces in the vicinity of T_c. At the same time, the anisotropic part of the surface energy has two terms with the orthogonal easy axes. The nature of the surface electric field and its effect on the director alignment at the interface have been clarified. Taking into account the known relation between anchoring strength and the nematic order parameter, the effective anchoring energy w_eff for the studied system has been determined as a function of temperature.     

Molecular orientations and photorefractive effects in C60-doped liquid crystals with capillary-flow-induced alignment

Molecular orientations and photorefractive effects of C₆₀-doped nematic liquid crystals were investigated with layer-structured liquid crystal cells where a nematic phase was injected. Regardless of the surface treatments of the layers, liquid crystals injected with nematic phase showed a higher degree of director orientation and photorefractivity than those injected with an isotropic phase. The nematic alignment of liquid crystals in an untreated cell is generated by a capillary flow of liquid crystal during the nematic injection. However, it was observed that the grating formation was delayed a little longer with injection of nematic phase than with injection of isotropic phase. Received: 11 March 2002 / Revised version: 22 April 2002 / Published online: 12 July 2002     

Photo-orientation at liquid crystal—polymer interfaces

It is suggested that liquid crystal—polymer interfaces are coupled systems, in which the components mutually influence the orientational state of each other. The photo-orientation process at liquid crystal-polymer interfaces provides a striking example of such a coupling. Experiments show that the anisotropic structure generated by polarised light at a polymer surface is strongly affected by the phase of the liquid crystal covering the polymer. Photo-orientation is significantly more efficient when the liquid crystal is in the isotropic phase than when it exhibits orientational order. The observations are interpreted by assuming that in the smectic and nematic phases the liquid crystal stabilises to a large extent polymer chain-segments aligned parallel to the director, while it blocks the photo-induced formation of chain-segments in the perpendicular direction. Other situations, in which the coupling between the liquid crystal and the polymer can be important, are also discussed briefly.     

The effect of magnetic fields and boundary conditions on the shear flow of nematics

The flow of a nematic liquid crystal between plane parallel plates, with one plate moving with uniform velocity relative to the other, is discussed. The apparent viscosity, orientation and velocity profiles are computed forp-azoxyanisole as functions of shear rate and magnetic field for symmetric and asymmetric molecular alignment at the plates. For symmetric homeotropic boundary condition, a magnetic field applied along the flow direction exhibits a threshold reminiscent of a Freedericksz transition in the hydrostatic case. In general the apparent viscosity for the asymmetric boundary condition is less than that for the symmetric case.     

Fluctuations of orientational order in a uniaxial nematic liquid crystal with biaxial molecules and its response to an external field

Homogeneous thermal fluctuations of the orientational order parameters S and G of biaxial molecules in a uniaxial nematic liquid crystal are investigated in the framework of the molecular-statistical theory. It is demonstrated that the molecular biaxiality significantly affects the order parameters S and G, their temperature dependences in the nematic phase, the amplitude and the temperature dependence of the order parameter fluctuations in the nematic and isotropic phases, and the character of the transition from the nematic phase to the isotropic liquid phase. It is established that the fluctuations of the parameters S and G in the nematic phase are related to the temperature dependences of S and G and the susceptibilities χ_S and χ_G of the nematic liquid crystal to external fields, which leads to a change in the parameters S and G at a fixed director orientation. Explanations are offered for the known experimental data on the orientational ordering of biaxial molecules under the action of external fields in the isotropic phase of nematic liquid crystals.