The director distribution in a spinning nematic mesophase subject to a statis magnetic field

We have determined the probability distribution function for the director in a spinning nematic mesophase subject to a static magnetic field using electrons resonance spectroscopy. For low spinning speeds the director is found to be inclined to the magnetic field as predicted by a continum theory analysis of the system; there is however a small spread in the angle made by the director with the field. This is inhomogeneity in the director alignment increases smoothly as the angular velocity of the mesophase is increased until at high speeds the director adopts an isotropic distribution mesophase which account for the dispersion in the director produced by sample rotation.     

Light-controlled reorientation of nematic liquid crystal driven by an electric field

The joint influence of optical and (quasi-)static electric fields on the orientation of liquid crystal gives rise to peculiar effects. In this article we report on the generation of transient domains in liquid crystals, which are an order of magnitude larger than the size of the optical field profile. The formation of such a domain is due to the fact that the initially present optical field reverses the pre-tilt, and the voltage that is then applied gives rise to an amplification of the tilt angle. The resulting reorientation of the director strongly depends on the starting conditions of the preliminary present optical field. We demonstrate different switching conditions, depending on the relation between the incident angle of the beam and the pre-tilt angle. The resulting refractive index profiles give rise to lensing effects.     

Control of particle alignment in water by an alternating electric field

We attempted to align a large number of silica particles dispersed in aqueous solution by controlling the alternating electric field between the two electrodes (400 microm apart). Relatively large particles (9.9 microm) were found to align forming strings in the direction parallel to the electric field while relatively small particles (2.0 and 4.9 microm) were observed to align making stripes in the direction perpendicular to the field. The number of stripes formed by particles between the electrodes increased with increasing frequency of the alternating field. This peculiar perpendicular particle alignment appeared when the contribution to particle alignment of electroosmotic flow exceeded that of dielectric polarization and the osmotic flow was found to be stronger around the particles than in the vicinity of the electrode surface.     

Phase diagram of a two-frequency chiral nematic mixture in an electric field

The behaviour of a chiral nematic mixture with low frequency dielectric relaxtion was studied between two electrodes, with homeotropic anchoring, in the presence of an AC electric field E. The phase diagram in the parameter plane (d/p, E) was determined for this mixture which could be dielectrically negative or positive in different frequency ranges. The determination was carried out by changing the sample thickness d and both the amplitude and frequency of the electric field at a constant cholesteric pitch p.     

Theory of an electric field induced periodic phase in a nematic film

The elastic theory of nemtatic liquid crystals is used to determine whether it gives support for recent experimental observations of a novel periodic phase occurring in homeotropically aligned films when an electric field is applied in the plane of the film. The director remains in the plane defined by the film normal and the field, and the wavevector of the periodicity is parallel to the field. It is found that the theory is consistent with experiment.     

Self-consistent field study of the alignment by an electric field of a cylindrical phase of block copolymer

Self-consistent field theory is applied to a film of cylindrical-forming block copolymer subject to a surface field which tends to align the cylinders parallel to electrical plates, and to an external electric field tending to align them perpendicular to the plates. The Maxwell equations and self-consistent field equations are solved exactly, numerically, in real space. By comparing the free energies of different configurations, we show that for weak surface fields, the phase of cylinders parallel to the plates makes a direct transition to a phase in which the cylinders are aligned with the field throughout the sample. For stronger surface fields, there is an intermediate phase in which cylinders in the interior of the film, aligned with the field, terminate near the plates. For surface fields which favor the minority block, there is a boundary layer of hexagonal symmetry at the plates in which the monomers favored by the surface field occupy a larger area than they would if the cylinders extended to the surface.     

Electrohydrodynamic instability in some nematic cyanobiphenyls in an a.c. electric field

Abstract

Electrohydrodynamic instability in homeotropically oriented nematic samples of 4′-n-octyl-4-cyano-biphenyl and 4′n-alkyloxy-4-cyanobiphenyl, (n = 8.9) have been studied in an a.c. electric field. The domain patterns during the instability in these compounds in a very low frequency a.c. field are very similar to those in a d.c. field. The domain patterns observed at higher frequencies have been identified as ‘maltese crosses’ or ‘crossed isogyres’. The electro-convective ‘isotropic’ flows near the electrode play an important role in the observed instability.     

Electrohydrodynamic instability in some nematic cyanobiphenyls in an a.c. electric field

Electrohydrodynamic instability in homeotropically oriented nematic samples of 4'-n-octyl-4-cyano-biphenyl and 4'n-alkyloxy-4-cyanobiphenyl, (n = 8.9) have been studied in an a.c. electric field. The domain patterns during the instability in these compounds in a very low frequency a.c. field are very similar to those in a d.c. field. The domain patterns observed at higher frequencies have been identified as 'maltese crosses' or 'crossed isogyres'. The electro-convective 'isotropic' flows near the electrode play an important role in the observed instability.     

Novel bisindenocarbazole derivative exhibiting a nematic mesophase

In this letter we describe the synthesis of the first liquid crystalline bisindenocarbazole derivative. The novel bisindenocarbazole exhibits a broad nematic mesophase between 180 and 250 °C, which was characterized by polarizing microscopy and small angle X-ray scattering. The material shows an excellent electrochemical stability and a strong blue fluorescence with a quantum yield of 49% in solution.     

Strong deformations induced by a DC electric field in homeotropic flexo-electric nematic layers

Elastic deformations of homeotropic nematic liquid crystal layers subjected to a DC electric field were studied numerically in order to determine their development under increasing voltage. Both signs of the dielectric anisotropy, Δ?, and of the sum of flexo-electric coefficients, e ₁₁ + e ₃₃, were considered, as were also low, moderate and high ion concentrations. The electrical properties of the layer are described in terms of a weak electrolyte model. Quasi-blocking electrode contacts and a finite surface anchoring strength were adopted. Director orientation, the electric potential and the ion concentrations were each calculated as a function of the coordinate normal to the layer. The director distributions turned out to depend not only on the sign of Δ? but also on the sign of e ₁₁ + e ₃₃, due to the difference in the mobility of anions and cations. The importance of ion content was also confirmed.     

Light scattering from a nematic monodomain in an electric field Twist elastic constant and viscosity coefficient of nematic polymer-solvent mixtures

The light scattering technique was used to investigate the viscoelastic parameters characterizing director twist distortions in miscible nematic mixtures of 5CB (pentacyanobiphenyl) with two side chain liquid crystal polymers and a main chain liquid crystal polymer. By applying an AC electric field to homeotropically-aligned nematic monodomains of the mixtures, the field-dependent scattering intensities and director orientation fluctuation relaxation rates yield, respectively, the twist elastic constant K₂₂ and viscosity coefficient γ₁. The results directly demonstrate that the addition of liquid crystal polymers causes substantial decreases of the relaxation rates for dynamic light scattering from the twist mode and these changes are due to small decreases in K₂₂ coupled with large increases in γ₁. The decrements in K₂₂ are comparable for both side chain and main chain liquid crystal polymers. The relative increase in the twist viscosity for the side chain liquid crystal polymers is much smaller than those of main chain polymers. A theoretical model is used to qualitatively interpret the difference between the viscous behaviour of the twist mode for both side chain and main chain liquid crystal polymers in a nematic solvent.     

Orientational instability of nematic liquid crystal in a homeotropic cell with boundary conditions controlled by an electric field

We study the orientational instability of the director in a homeotropic nematic liquid crystal (NLC) cell in a DC electric field. The electric field is applied along or perpendicular to a cell surface depending on whether anisotropy of dielectric permittivity of NLC is positive or negative. The easy axis on one of the cell polymer substrates is allowed to deviate in perpendicular to the substrate plane due to the influence of the NLC and the electric field. It was established that the orientational instability of the director can have a threshold as well as be thresholdless which depends on the character of the coupling of the easy axis with the electric field. The temporal behaviour of a director and the easy axis during transition to the stationary state after turning on electric field and returning to the initial homogeneous state after turning off the field was investigated. In the case of the negative anisotropy of the static permittivity, the comparison of experimental and calculated time dependences of the easy axis reorientation angle let us to estimate values of the mobile easy axis viscosity coefficient and coupling parameter describing the coupling between the easy axis and the electric field.     

Effect of an electric field on defects in a nematic liquid crystal with variable surface anchoring

We report experimental studies on defects in a nematic liquid crystal with negative dielectric anisotropy mounted in a cell with perfluoropolymer-coated surfaces. The sample exhibits a discontinuous anchoring transition from planar to homeotropic on cooling at zero or a small electric field, and above a cross-over voltage a continuous ‘inverse Freedericksz transition’, at which the director starts tilting in opposite directions at the two surfaces. Defects of strength ±1/2 are either annihilated or expelled when the director tilts. On the other hand, disclination lines of ±1 which end in partial point defects (boojums) at the surfaces in the planar alignment regime acquire point defects of strength ±1 at the midplane of the cell when the director tilts. At a low enough temperature, the homeotropic anchoring becomes strong, and an electric field above the Freedericksz threshold generates the usual umbilic defects, which follow the dynamic scaling laws found in earlier studies.     

Wave mixing in electric field biased nematic thin films

Abstract

Wave mixing due to molecular reorientation in electric field biased nematic thin films is studied. By using the continuum theory, general formulae have been derived for the orientational angle distribution. The concept of a thin film grating is utilized to describe the diffraction efficiency. Experimental results which exhibit the behaviours of the transmission beam and diffraction beams have been obtained as predicted by the numerical calculation.     

Novel freebase and zinc bilinone dimers with optically active peripheral groups. Synthesis and application to chiral nematic induction in a nematic mesophase

In order to investigate effective dopants to induce chiral nematic liquid crystalline phases, novel freebase (FbBL) and zinc bilinone (ZnBL) derivatives bearing optically active aliphatic groups ((S)-3,7-dimethyloctyls) at the peripheral positions were prepared. From the CD spectra, it was confirmed that M-helicity in the bilinone frameworks was modestly enriched for ZnBLs, whereas helicity was hardly induced for FbBLs except for the o-xylylene-spaced dimer. When N-(4-methoxybenzylidene)-4-butylaniline (MBBA) was doped with the bilinone derivatives, the chiral nematic phase was effectively induced, and the helical twisting powers (β_Ms) ranged from ?95 to ?159 μm^?1. The control experiment using (S)-3,7-dimethyl-1-phenyloctane (β_M = +14 μm^?1) clearly showed that the induced chiral helical frameworks of FbBL and ZnBL predominantly contribute to chiral nematic induction of MBBA.     

Wave mixing in electric field biased nematic thin films

Wave mixing due to molecular reorientation in electric field biased nematic thin films is studied. By using the continuum theory, general formulae have been derived for the orientational angle distribution. The concept of a thin film grating is utilized to describe the diffraction efficiency. Experimental results which exhibit the behaviours of the transmission beam and diffraction beams have been obtained as predicted by the numerical calculation.     

Rotational diffusion and alignment of short gold nanorods in an external electric field

We present measurements of the polarized extinction of gold nanorod suspensions exposed to an external electric field. By employing an amplitude modulated field in combination with lock-in detection we resolve changes in the optical density as low as 10(-6) in an integration time of 10 s. This sensitivity allows us to probe the partial alignment of small gold nanorods with an aspect ratio of 2.5 and a width ranging from 13 nm to 28 nm. The degree of orientation scales as the square of the electric field strength, as expected for an induced dipole moment in an external field. By varying the modulation frequency we measure the rotation diffusion constant of different samples, which are in excellent agreement with the calculated values for a short cylinder.     

Dynamics of electric field induced deformations in flexoelectric nematic layers

The dynamics of deformations induced by DC electrical fields in homeotropically aligned layers containing a flexoelectric nematic material with negative dielectric anisotropy has been studied numerically. The rise time constants, characterising the development of deformations after switching on the external voltage, and the decay time constants, describing the decay of deformation after switching off the voltage, were calculated as a function of the parameters essential for the behaviour of the layer. In particular, the influence of flexoelectricity was studied. It was found that the stronger the flexoelectric properties of the nematic, the lower is its viscosity, the higher is the bias voltage, the weaker is the surface anchoring, the thinner is the layer and the higher is the ion concentration, the more rapid was the onset of deformation. Similarly, the lower the viscosity, the thinner is the layer, the stronger the anchoring and the larger the ion content, the more rapid was the decay of deformation. Neither the voltage previously applied nor the flexoelectric properties were found to affect the decay time.     

Biaxial order and a rotation of the minor director in the nematic phase of an organo-siloxane tetrapode by the electric field

Biaxiality in the nematic phase for a liquid crystalline tetrapode made up of organo-siloxanes mesogens is investigated using polarized infrared spectroscopy. An ordering of the minor director for the homeotropically aligned sample is found to depend on the amplitude of the in-plane electric field. On increasing the in-plane electric field, the minor director, lying initially along the rubbing direction, rotates to the direction of the applied field. The scalar order parameters of the second rank tensor are found to depend significantly on the strength of the electric field. A most significant increase is found in the nematic order parameter and in the parameter that characterizes the phase biaxiality.     

Probing the electric field alignment of a thermotropic liquid crystalline polymer by synchrotron radiation

The orientation of a cyclic side-chain thermotropic liquid crystalline material in an AC field was monitored in real-time using synchrotron radiation. Monitoring the realignment processes in the millisecond-to-minute time-scale was made possible by the high X-ray flux. Orientation parameters and response times were calculated as a function of temperature and frequency. Response times decreased exponentially with temperature due to a decrease in the viscosity. Very little dependence of the response time on frequency was observed, except at low temperatures, where a switch from homeotropic to planar alignment of the molecules was detected. This reorientation of the director was studied in real-time and the resulting complex diffraction patterns were due to equal but opposite director rotations from an alignment parallel to the applied electric field to an alignment perpendicular to the applied electric field. The orientation parameters were highest in the central portion of the mesophase temperature range. At temperatures near clearing, the net degree of orientation diminished. Cooling through the mesophase with an applied electric field resulted in much larger orientation parameters than could be obtained by aligning at a fixed temperature in the mesophase.