Field-induced transition from homeotropic to planar geometry in the SmC* phase of an electroclinic liquid crystal

The optical and electrical behavior was investigated of a symmetric liquid crystal (LC) cell: ITO–silane–LC–silane–ITO. The silane layer induces a perfect homeotropic alignment of the molecules of the studied electroclinic liquid crystal (ELC) material, BDH 764E. A field-induced transition from the perfect homeotropic to planar orientation in the chiral smectic C (SmC*) and smectic A (SmA) phases of the ELC was observed. Optical and dielectric studies were performed for both alignment (geometry) modes. The field-induced transition from the homeotropic to planar orientation was studied vis-à-vis the high negative dielectric anisotropy obtained in the studied material. Such an ELC with large negative dielectric anisotropy and perfect homeotropic alignment may have important implications for modern LC display technology.     

Orientation of discotic and ferroelectric liquid crystals in a macroporous silicon matrix

Macroporous silicon with deep regular channels 3–4.5 μm in diameter was infiltrated with discotic and ferroelectric liquid crystals (LCs) at the temperature of the isotropic phase, and then, the system was slowly cooled to room temperature, with the liquid crystalline mesophase formed. The orientation of the LC molecules in the porous matrix was studied by FTIR spectroscopy. The alignment of LCs was ascertained by comparing the behavior of various vibrational bands of a liquid crystal introduced into the porous matrix with that for LC inside the bulk cells of planar and homeotropic alignment. The molecules of the discotic LC show a planar orientation of their column’s axis with respect to the surface of the macroporous silicon wafer; i.e., they are perpendicular to the channel axis. The long molecular axis of the ferroelectric LC is aligned with the pore walls, having homeotropic orientation with respect to the wafer surface. In a macroporous silicon matrix, both kinds of LCs show unexpected enhancement of the low-frequency vibrational bands. From Fizika Tverdogo Tela, Vol. 44, No. 6, 2002, pp. 1145–1150. Original English Text Copyright ? 2002 by Perova, Astrova, Tsvetkov, Tkachenko, Vij, Kumar. This article was submitted by the authors in English.     

Influence of the flow on the anchoring of nematic liquid crystals on Langmuir-Blodgett monolayers studied by optical second-harmonic generation

The influence of capillary flow on the alignment of the nematic liquid crystal 5CB on fatty acid Langmuir-Blodgett monolayers was studied by optical second-harmonic generation (SHG). The surface dipole sensitivity of the technique allows probing the orientation of the first liquid crystal monolayer in the presence of the liquid crystal bulk. It was found that capillary flow causes the first monolayer of liquid crystal molecules in contact with the fatty acid monolayer to be oriented in the flow direction with a large pretilt (78 degrees), resulting in a quasi-planar alignment with splay-bend deformation of the nematic director in the bulk. The large pretilt angle also suggests that the Langmuir-Blodgett film itself is affected by the flow. The quasi-planar flow-induced alignment was found to be metastable. Once the flow ceases, circular domains of homeotropic orientation nucleate in the sample and expand until the whole sample becomes homeotropic. This relaxation process from flow-induced quasi-planar to surface-induced homeotropic alignment was also monitored by SHG. It was found that in the homeotropic state the first nematic layer presents a pretilt of 38 degrees almost isotropically distributed in the plane of the cell, with a slight preference for the direction of the previous flow. Received 8 November 2000 and Received in final form 12 March 2001     

Anchoring and structural transitions as a function of molecular length in confined liquid crystals

Using deuteron nuclear magnetic resonance to study liquid crystals confined to cylindrical pores, an anchoring transition has been found. The transition exhibits an unexpected sharp dependence of the anchoring strength on cyanobiphenyl liquid crystal molecular length. A structural transition from a parallel axial to a planar radial configuration occurs due to an anchoring transition from planar to weakly homeotropic orientation at the walls. The anchoring strength is at a minimum near the decylcyanobiphenyl (10CB) liquid crystal length. Long chain liquid crystal configurations depend on thermal cycling and on the equilibrium atmosphere leading to a bistable SmA structure. Orientational order wetting in the isotropic phase also depends on molecular length.     

Effect of graphene oxide dispersion in antiferroelectric liquid crystal mixture in the verge of SmC* to SmCA* phase transition

The continuous growing demand for nanoscience applications and the improvement in the performance of liquid crystal based devices has been extensively required by the technological world. Recent progress in the field of liquid crystals has found its practical implementation in various display and non display devices which experiences obstacle due to impurity effects that reduces its performance. The dispersion of nanoparticles in liquid crystal medium helps in the reduction of impurity ions and thus improving the performance of liquid crystal based devices. The present work is based on the collective dielectric relaxation processes that have been observed in antiferroelectric liquid crystal (AFLC) mixture W1000 dispersed with 0.1% wt/wt and 0.3% wt/wt concentrations of graphene oxide. Graphene oxide itself favors vertical alignment and the coupling of AFLC W1000 mixture with graphene oxide affects its molecular ordering. This has been confirmed from the polarizing optical micrographs. The dielectric relaxation modes have been observed with and without the application of bias voltage in SmC* to SmC_A* phase transition during cooling cycle. The appearance and disappearance of P_L, P_H and X modes have been observed and are explained on the basis of molecular interactions. Graphene oxide dispersed system favors homeotropic alignment (dark state) and the application of bias field will convert it into homogenous alignment (bright state). Graphene oxide dispersion find prospective applications in good contrast display devices, supercapacitors, electronic gadgets, rechargeable batteries. Electro optical results unveil the faster response time, decreased rotational viscosity and spontaneous polarization with no change in tilt angle for the dispersed system. These observations can be exploited in photonic switches with sub millisecond response time which are required for fabricating faster liquid crystal devices.     

Orientation of a nematic liquid crystal by suitable boundary surfaces

The adsorption of a monolayer of hexadecyltrimethylammonium bromide (CTAB) on glass-slides allows the preparation of a monocrystal of p-n-methoxybenzilidene-p-butylaniline (MBBA) oriented either prependicularly to surfaces (homeotropic anchorage) or in a direction parallel to surfaces (planar anchorage). These orientations are dictated by the surface density and the structure of the adsorbed CTAB monolayers and confirm a model of orientation of a liquid crystal by physico-chemical coupling.     

Self-organization processes and topological defects in nanolayers in a nematic liquid crystal

Atomic force microscopy is used to study the self-organization processes that occur during the formation of topological defects in nanomolecular layers in a nematic liquid crystal with the homeotropic orientation of its molecules with respect to the substrate. In this case, a smectic monolayer with a thickness of one molecule length (about 2.2 nm) forms on the substrate, and a nanomolecular layer of a nematic liquid crystal forms above this monolayer. In such virtually two-dimensional layers, numerous different nanoclusters, namely, hut structures, pyramids, raft structures with symmetry C _nm (where n = 2, 4, 5, 6, 7, …, ∞), cones, and nanopools, form [1]. They have a regular shape close to the geometry of solid crystals. Modulated linear structures and topological point defects appear spontaneously in the nanopools and raft structures.     

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.     

Light propagation in periodic photonic structures formed by photo-orientation and photo-polymerization of nematic liquid crystals

Propagation of linearly polarized light beams in a nematic liquid crystal cell with distinguished regions of different molecular orientation has been analyzed. Specifically, combination of the planar/homogenic and homeotropic alignment, forming thus spatially limited regions characterized by a different LC molecular orientation, has been tested, as achieved by means of the photo-orientation and photo-polymerization processes, independently. An influence of molecular orientation on the light beam propagation has been checked for different directions of the linear polarization. Thanks to the molecular reorientation induced by the low frequency external electric field and also to the reorientational nonlinearity taking place in NLCs, propagation direction of the light beam can be additionally controlled by the electric bias and/or optical power, respectively. Proposed structural solutions and techniques, related to the photo-orientation and photo-polymerization processes described in this communication, give rise to the novel LC geometries and structures. The latter act as promising candidates for new practical photonic applications as they are expected to be of a particular importance for integrated optic elements and devices.     

Relaxation dynamics and power spectra of liquid water: a molecular dynamics simulation study

We present molecular dynamics simulations of liquid water at normal and supercooled conditions. Autocorrelation functions (ACFs) of several structural quantities and their fourier transforms are obtained and analysed. Structural correlations and relaxation times increase linearly with degree of supercooling. Power spectra of ACFs show increase in librational motion of liquid water with cooling. These modes intensify with supercooling because of structuring and ordering of water molecules. Overall, liquid water structure is homogenous over the temperatures and pressures studied and undergoes fluctuation–dissipation in its local-density variations [English and Tse, Phys. Rev. Lett. 106, 037801 (2011)].     

Magnetic-field control of the transmission of a photonic crystal with a liquid-crystal defect

The method of modulation of transmittance of a multilayer photonic crystal with a nematic liquid-crystal defect upon the orientational transition from the homeotropic to the planar state is investigated. The orientation of the director of the nematic is controlled by a magnetic field in the B-effect mode. The method of recurrent relations is used for numerical simulation of transmission spectra of the photonic crystal structure under investigation.     

Modeling light propagation in liquid crystal devices with a 3-D full-vector finite-element beam propagation method

A full-vector finite-element beam propagation method in 3-D is introduced for the simulation of light propagation in liquid crystal (LC) devices. The three electric field components are expressed in terms of mixed finite elements, providing the correct enforcement of boundary conditions. Moreover, the optical dielectric tensor of the medium can have all its nine elements nonzero, thus allowing the LC director to have an arbitrary orientation. A photonic crystal fiber with a LC infiltrated core and a homeotropic to multi-domain cell are analyzed. Comparison with other existing simulation techniques is provided, in order to validate the accuracy of the proposed method.     

First-order Fréedericksz transition above the nematic-smectic-a phase transition

The Fréedericksz transition is investigated for thin homeotropic cells of a negative dielectric anisotropy liquid crystal above its nematic-smectic-A phase transition temperature in a spatially uniform electric field. For very thin cells, a crossover from second- to first-order Fréedericksz behavior is observed; no additional fields are necessary. The results are consistent with the theory of Shi [Liq. Cryst. 29, 121 (2002)]].     

Influence of thermal treatment on the dynamics of the Goldstone mode in ferroelectric liquid crystal gel

ABSTRACT

Broadband dielectric spectroscopy was applied to investigate the effect of gel-former agent on the dynamics of the collective modes in a ferroelectric liquid crystal mixture. In the investigated range of gel concentrations, the dynamics of the Goldstone mode is significantly reduced due to the formation of a hydrogen bonding network of the gel-former agent in ferroelectric liquid crystal. The observed changes in dynamics of the collective processes are sensitive to the history of the thermal treatment of the samples.     

Determination of the tilt angles at surfaces of substrates in liquid crystal cells

A method to determine tilt angles of nematic liquid crystal directors at the surfaces of parallel substrates is described for homeotropic and homogeneous orientation as well as for TN-cells. Graphs which can be used to evaluate the tilt angles for a cyano-biphenyl mixture from simple transmission measurements are given. The direction of extreme optical path difference between plane polarized modes tilts much more than the tilt angle of the molecules.     

On the stability of homogeneous orientation in the plane-parallel cell of a liquid crystal doped with nanoparticles

We consider the problems of homogenous orientations stability of molecules in a nematic liquid crystal (NLC) doped with a small amount of cholesteric LC. We studied LC cells with homeotropic and twisted orientation of molecules on the cell boundaries. The problems are solved using the Noether theorem for LC. According to this theorem, invariance of free energy expression with respect to translation leads to conservation of the momentum flux, while invariance with respect to the rotation group leads to conservation of the angular momentum flux. These conservation laws are used to determine thresholds of stability of homogenous orientation of molecules in the above-mentioned mixture and the influence of nanoparticles on these thresholds.     

The orientational relaxation of methane molecules in the solid phase II at low temperatures

A model for the dynamics of the coupling between the orientations of the ordered CH₄ molecules in phase II of solid methane at low temperatures is proposed. The model is equivalent to the dynamics of disordered solid hydrogen. The effective interaction strength is determined by the overlap of the librational ground states in the molecular field potential and vanishes in the classical limit. An approximate expression for the effective interaction strength is derived, showing an exponential dependence on the uncertainty of orientation in the librational ground states. This parameter is estimated from the experimental values of the tunnel energies. The second moments of the spectral densities of several anisotropic operators are evaluated in the infinite temperature limit. The resulting gaussian approximations for the spectra are applied in a derivation of the spin lattice relaxation time. The calculated values of the spin lattice relaxation time are compared to experiment.     

Molecular dynamics simulations of the formation for NaCl cluster at the interface between the supersaturated solution and the substrate

Molecular dynamics simulations of supersaturated aqueous NaCl solution including the Pt(100) or NaCl(100) crystal surfaces have been performed at an average temperature of 298 K. The behavior of the NaCl cluster produced in the solution have been studied through the consideration of the water dielectric property near the crystalline surfaces for understanding the role of crystal growth on the surface. The surfaces in the solutions greatly influence heterogeneous nucleation in crystallization process. Density profile of the supersaturated solution and polarization of water molecules was calculated in order to describe the effect of the surfaces on the solution structure at the solid–liquid interfaces. The formation levels of NaCl clusters heavily depended on the water orientation at the interfaces. NaCl clusters were easily formed near the Pt(100) surface compared with the NaCl(100) surface owing to a different construction of water molecules between the platinum and NaCl surface.     

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.