Instabilities in electric fields of nematic liquid crytals with positive dielectric anisotropy: domains,loop domains and reorientation

Instabilities in electric fields are reported for p,p′-di-n-butyl azoxybenzene (DIBAB), a new room temperature nematic with a positive dielectric anisotropy. The threshold voltage is found to be almost independent of the frequency up to 100 kHz. However, unlike in nematics with negative dielectric anisotropy usually no normal domains are observed. The instabilities take the form of a reorientation, while loop domains are observed as a transiet effect. These loop domains indicate that the instability is not purely dielectric in origin.     

Transverse dipole association and negative dielectric anisotropy of nematic liquid crystals

Measurements are reported of the dielectric properties of three laterally substituted dicyano compounds with large molecular dipole moments perpendicular to the long molecular axis. Results are given for the materials in solution, in the isotropic liquid state, and for one compound in the nematic state. There is evidence of a high degree of local ferroelectric order in all phases, and dipole correlation factors in excess of 3 were obtained. The significance of these observations for macroscopic ferroelectrically ordered and biaxial liquid-crystalline phases is considered.     

Design and synthesis of nematic liquid crystals with negative dielectric anisotropy

Axially fluorinated cyclohexane derivatives are a promising new type of liquid crystals with negative dielectric anisotropy. Up to now, the technical application of these materials in LCDs was impeded by their strong tendency to form smectic B mesophases. We present a convenient synthetic pathway leading to new homologues carrying alkenyl side chains which induce nematic phases.     

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.     

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.     

NMR spectra of meta-difluorobenzene in nematic solvents: molecular geometry and anisotropy of the indirect fluorine-fluorine coupling

Nuclear magnetic resonance spectra of 1,3-difluorobenzene in thermotropic and lyotropic nematic solvents have been measured and interpreted. The geometrical data calculated from the dipolar couplings are compared with those obtained from microwave spectra and from a recent electron diffraction experiment. Evidence has been found of a small anisotropy in the indirect fluorine-fluorine coupling.     

Performance optimization in electric field gradient focusing

Electric field gradient focusing (EFGF) is a technique used to simultaneously separate and concentrate biomacromolecules, such as proteins, based on the opposing forces of an electric field gradient and a hydrodynamic flow. Recently, we reported EFGF devices fabricated completely from copolymers functionalized with poly(ethylene glycol), which display excellent resistance to protein adsorption. However, the previous devices did not provide the predicted linear electric field gradient and stable current. To improve performance, Tris–HCl buffer that was previously doped in the hydrogel was replaced with a phosphate buffer containing a salt (i.e., potassium chloride, KCl) with high mobility ions. The new devices exhibited stable current, good reproducibility, and a linear electric field distribution in agreement with the shaped gradient region design due to improved ion transport in the hydrogel. The field gradient was calculated based on theory to be approximately 5.76 V/cm² for R-phycoerythrin when the applied voltage was 500 V. The effect of EFGF separation channel dimensions was also investigated; a narrower focused band was achieved in a smaller diameter channel. The relationship between the bandwidth and channel diameter is consistent with theory. Three model proteins were resolved in an EFGF channel of this design. The improved device demonstrated 14,000-fold concentration of a protein sample (from 2 ng/mL to 27 μg/mL).     

A new type of electrohydrodynamic instability in nematic liquid crystals with positive dielectric anisotropy

Abstract

A new type of electrohydrodynamic instability originally reported in nematic liquid crystal mixtures with positive dielectric anisotropy and as moderately thick samples is further studied. The ability of homogeneously aligned nematics with positive dielectric anisotropy, in the presence of a magnetic field, to exhibit Williams domains as a threshold effect is numerically investigated. The variation of the threshold voltage for domain formation and dielectric alignment with dielectric anisotropy is calculated theoretically and compared with the experimental results as moderately thick and thin samples.     

Fast spatially localised electrooptical mode in vertically aligned nematic LCs with negative dielectric anisotropy

ABSTRACT

Electro-optical switching and the liquid crystal (LC) director distribution are studied in spatially periodic electric field for vertically aligned LC with negative dielectric anisotropy. Two electro-optical switching modes characterised by different switching times are observed. These modes are well distinguished optically by choosing proper geometry for the polarisers axes orientation. One of these modes is significantly faster as compared to the other. The fast switching is explained in terms of localised near-to-surface director reorientation. The 3D-numerical simulation shows very good agreement with the experiment: it points out the existence of the disclination lines and field-stabilised walls responsible for the localised director field switching and its relaxation. Possibilities of enhancing the fast mode for high-speed light modulators are discussed.     

A new type of electrohydrodynamic instability in nematic liquid crystals with positive dielectric anisotropy

A new type of electrohydrodynamic instability originally reported in nematic liquid crystal mixtures with positive dielectric anisotropy and as moderately thick samples is further studied. The ability of homogeneously aligned nematics with positive dielectric anisotropy, in the presence of a magnetic field, to exhibit Williams domains as a threshold effect is numerically investigated. The variation of the threshold voltage for domain formation and dielectric alignment with dielectric anisotropy is calculated theoretically and compared with the experimental results as moderately thick and thin samples.     

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.     

Electrical orientation behaviour of a nematic liquid crystal with negative dielectric anisotropy above the microelectrodes

Factors for the electrical orientation on interdigitated microelectrodes deposited on a substrate, a microdielectrometric sensor (or a sensor), were discussed experimentally using a model nematic liquid crystal with negative dielectric anisotropy. The electrical orientation behaviour of the nematic liquid crystal was investigated using microdielectrometry and polarised optical microscopy. The experimental results were discussed in relation to electric-field line, surface topology, anchoring of the liquid crystalline molecules to the microelectrodes, and transversal rotation of a rode-like molecule of the liquid crystal with negative dielectric anisotropy.     

An analysis of the influence of vibrations on electric field gradient tensors in solid hydrates

A method is described for calculating the influence of vibrational motion on the electric field gradient tensors at deuterons in solid hydrates using librational tensors. The method is applied to model calculations to illustrate the effect of normal modes which involve a mixture of standard wagging, twisting and rocking modes.     

Mean value of the electric field gradient in a molecule

The relationship between the mean value of the second derivative of the electrostatic potential of the electronic shell of a molecule on its axis and the first derivative of the energy of the molecule with respect to the internuclear distance has been established.Scientific-Research Institute of Physics, Leningrad State University. Translated from Zhurnal Strukturnoi Khimii, Vol. 32, No. 2, pp. 47–50, March–April, 1991.     

Bipolar electrode focusing: tuning the electric field gradient

Bipolar electrode (BPE) focusing is a developing technique for enrichment and separation of charged analytes in a microfluidic channel. The technique employs a bipolar electrode that initiates faradaic processes that subsequently lead to formation of an ion depletion zone. The electric field gradient resulting from this depletion zone focuses ions on the basis of their individual electrophoretic mobilities. The nature of the gradient is of primary importance to the performance of the technique. Here, we report dynamic measurements of the electric field gradient showing that it is stable over time and that its axial position in the microchannel is directly correlated to the location of an enriched tracer band. The position of the gradient can be tuned with pressure-driven flow. We also show that a steeper electric field gradient decreases the breadth of the enriched tracer band and therefore enhances the enrichment process. The slope of the gradient can be tuned by altering the buffer concentration: higher concentrations result in a steeper gradient. Coating the channel with the neutral block co-polymer Pluronic also results in enhanced enrichment.     

Programed elution and peak profiles in electric field gradient focusing

Electric field gradient focusing (EFGF) methods have received increased attention in recent years, with potential applications demonstrated by several research groups. In order to move EFGF from the research stage to routine use in application areas, a more detailed understanding of practical aspects of device performance is required. Useful theoretical models for EFGF are available but have not been verified through systematic checks under a variety of conditions. In this paper, we compare modeled and experimental results for an EFGF device with the goal of optimizing the time sequence of voltages applied to the device for maximum resolution of analytes with close electrophoretic mobilities. Measured peak profiles depend strongly on the sequence of voltages applied to the device. We investigate the characteristic behavior of the elution profile under various voltage programs. Rapid voltage drops lead to fast elution of closely spaced protein peaks with narrow widths, whereas a carefully designed voltage program can be used to increase the separation between analytes and achieve higher resolution. Simulated and experimental results demonstrate that the behavior of analyte diffusion at an electric field singularity associated with the transition from the EFGF device to elution capillary can be used to separate analyte peaks which may not be resolved within the EFGF device itself, thereby increasing the achievable resolution of the EFGF technique.     

Numerical evaluation of matrix elements of the electric field and electric field gradient operators in a slater basis set

A method is proposed for evaluating all matrix elements of the electric field and electric field gradient operators in a Slater basis set fo systems with an arbitrary number of nuclei and geometry. These integrals are evaluated using a numerical quadrature after having modified the integrand to remove all infinities. The integration ranges are broken to avoid integrating through cusps in the integrand. With reasonable grids these procedures are adequate for numerical evaluation with an accuracy of five to six decimal places (in a.u.). Particular cases are briefly discussed.     

Many-body calculation of the electric field gradient in the hydrogen molecule

The electric field gradient in the hydrogen molecule has been calculated by diagrammatic many-body perturbation theory (MBPT ) in Gaussian basis sets. The procedure through third order in electron correlation gives a value for the field gradient of 0.34041 a.u., which is 0.8% greater than the accurate value. The result is discussed in terms of the completeness of the basis sets and the convergence of the perturbation expansion.     

Influence of transport properties in electric field gradient focusing

Miniaturized devices for electric field gradient focusing (EFGF) were developed that consist of a cylindrical separation channel surrounded by an acrylic-based polymer hydrogel. The ionic transport properties of the hydrogel enable the manipulation of the electric field inside the separation channel. A changing cross-section design was used in which the hydrogel is shaped such that an electric field gradient is established in the separation channel. One of the challenges with this type of EFGF device has been that experimental resolution between protein analytes is lower than theoretically predicted. In order to investigate this phenomenon, a mathematical transport model was developed using FEMLAB. Model results and experimental observations showed that the reduced performance was caused by concentration gradients formed in the EFGF channel, and that these concentration gradients were the result of an imbalance in cation transport between the open separation channel and the hydrogel. Removing acidic impurities from the monomers that form the hydrogel reduced this tendency and improved the resolution. These transport-induced concentration gradients can be used to establish electric field gradients that may be useful for sample pre-concentration. Both the results of simulation and experiments demonstrate how transport-induced concentration gradients lead to the establishment of electric field gradients.