Intrinsic characteristic times in the drift-diffusion problem

The redistribution of impurities in a sample in the shape of a slab filled with an isotropic liquid submitted to an external field is considered. The case in which the impurities are ions dissolved in the liquid, and the external field is an electric field is also investigated. It is shown that the intrinsic time connected with the presence of the electric field is proportional to the drift time. The constant of proportionality is of the order of the ratio between the thermal agitation energy and the electrostatic energy. A closed solution for the time evolution of the density of particles is obtained. The importance of the effect on real situations is investigated. The possible application of these results to nematic samples submitted to an external electric field is discussed.     

Quantum bits with polyacetylene

The dynamics of a polyacetylene single chain as a system for possible physical implementations of quantum bits is determined. This novel proposition is studied by varying intensity and duration of application of an electric field as well as the intensity, number, and position in the polymer chain of impurity molecules. The behavior of soliton pairs, whose associated energy levels form the quantum bit, is analyzed. The chain is modeled by a modified Pariser-Parr-Pople Hamiltonian extended to include the effects of an external electric field and the parameters of the impurity molecules. The effect of the variation of the field and impurities on the separation of the energy levels associated with soliton pairs is analyzed by numerical integration of the equations of motion. Two different approaches for controlling the separation of levels are presented, and their features compared. First, the use of changes in the electric field to control the distance (and ultimately coupling) between two solitons moving freely on the chain or captured by the potential generated by the impurity molecules. Second, the change in the intensity of the impurities alone, with no application of an external field. We have found that the effect of the use of the field on the separation of levels is much smaller than the one obtained by changes in the parameters of the impurity molecules, which eventually led us to achieve quantum bit behavior in a polyacetylene chain. The influence of the field and impurity parameters in the energy levels is determined, as well as their role in the coupling of the two solitons on the chain. Critical values for distance between solitons, intensity of field, and impurities that determine whether a pair of solitons can work as a quantum bit are obtained.     

Permeability control in electro‐sensitive microcapsules with immobilized ferroelectric liquid crystalline segments

Nylon‐polystyrene microcapsules with immobilized ferroelectric liquid crystalline segments were prepared, and permeability control of an encapsulated core material was investigated under an external electric field. A ferroelectric liquid crystal monomer possessing both mesogenicity and chirality responded effectively to the external electrical field. Permeation of the material (oxprenolol) contained in the inner aqueous core of the microcapsules was enhanced under a weak electric field (2 V). Furthermore, the permeability of oxprenolol did not depend on the external electric field in the absence of the ferroelectric liquid crystal segments. To clarify the controlled‐release mechanism of the core material, the light transmittance of the polymer membranes was quantitatively evaluated under an external electric field using a handmade polarized light transmittance apparatus. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1749–1757, 2008     

Solution of the mixed Dirichlet–Neumann problem for molecular orientation in liquid crystals

The orientation of the nematic director field under the action of an external time‐dependent field is theoretically investigated as a mixed Dirichlet–Neumann boundary‐value problem. This mathematical problem represents the situation in which a nematic liquid crystal sample is limited by two inhomogeneous flat surfaces, separated by a distance d, on which the anchoring is weak. By considering the one‐constant approximation and a parabolic approximation for the surface energy, the initial conditions and boundary‐value problem for the profile of the tilt angle can be analytically solved even in the case in which the surfaces are not identical, which represents the more general situation. The results are valid for small deviations from the homeotropic orientation and for θ?Θ?1, where θ is the actual tilt angle and Θ characterizes the easy direction imposed by the surface, and can be relevant to investigation of the molecular orientation in a nematic cell submitted to a small external voltage.     

Surface viscosity and reorientation process in an asymmetric nematic cell

The influence of surface viscosity and anchoring energy on the reorientation process of a nematic liquid crystal cell is theoretically investigated. The cell is a slab of thickness, d, whose limiting surfaces are characterised by different anchoring strengths and present easy directions parallel to the bounding surfaces, changing with time due to some external action. The exact space-time profile of the director angle is obtained by means of integral transform techniques and a Green function approach. From this formalism, the time dependence of the optical path difference is exactly determined and its behaviour is analysed in connection with the presence of surface viscosity and different anchoring energies. The problem is also exactly solved in the presence of a constant electric field. It is shown that the compatibility problem between the time derivative of the director field on the surface and in the bulk can be avoided.     

Scaling analysis of a charged jet formed upon electrospinning of a viscoelastic liquid

The dependences of the parameters of a jet and the intensity of the current generated in it during electrospinning of a viscoelastic liquid on the conductivity, viscosity, surface tension, and other characteristics of the liquid, as well as the strength of an external electric field and the liquid flow rate, are considered. The process is described in terms of the Maxwell liquid model, which is characterized by a single relaxation time and convective-type nonlinearity. Two regimes corresponding to high and low liquid flow rates are identified. The jet diameter is shown to decrease with a rise in the liquid conductivity coefficient and electric field strength. Moreover, at high flow rates, the current intensity almost linearly grows with the field strength.     

外电场作用下三元环状水分子团簇的特性研究

The influence of an external field on the ground states energy and dipole moment of the cyclic water trimer is investigated. Employing the Hartree-Fork method with basis sets 3-21G. The field-induced H-F force including the internal and external forces and clusters equilibrium structures under balance of this force are analyzed. The external field is varied in the 0.001~0.01 a.u. range. It is shown that the magnitude of the external electric field has important effects on these characteristics of the cyclic water trimer. The energy was found to decrease and the dipole moment to increase with the increasing external field. The change of the electronic population of every atom leads the electrostatic field to vary accordingly.     

Dependence of the anchoring energy on the applied voltage in a nematic cell

A nematic liquid crystal cell in the shape of a slab of thickness d and containing ionic impurities is considered in the presence of a dc voltage. A complete theoretical model to determine the electric field distribution across the sample is used to explain the experimental dependence of the effective anchoring energy of the cell on the applied voltage, in the limit of high voltage.     

Electrooptic properties of polymer dispersed liquid crystals

An investigation of the electrooptic properties of polymer dispersed liquid crystals (PDLC) is presented. These materials are light modulating systems. They show a reversible optical response from an opaque state to a highly transmitting state under the action of an appropriate electric field which aligns the liquid crystal director. The switching voltage required to establish such an electric field has been monitored as a function of (i) the starting materials used for the preparation of the PDLCs, (ii) the ageing (curing time) of the PDLC cells. Other physical properties, such as the electrical resistivity and the dielectric constant of the materials, have been measured. The correlations between these properties have been studied. The PDLC switching voltage appears to be strongly correlated with the resistivity. Our data suggest that ionic impurities play a dominant role with respect to the electrooptic response of PDLC films.     

Electrooptic properties of polymer dispersed liquid crystals

An investigation of the electrooptic properties of polymer dispersed liquid crystals (PDLC) is presented. These materials are light modulating systems. They show a reversible optical response from an opaque state to a highly transmitting state under the action of an appropriate electric field which aligns the liquid crystal director. The switching voltage required to establish such an electric field has been monitored as a function of (i) the starting materials used for the preparation of the PDLCs, (ii) the ageing (curing time) of the PDLC cells. Other physical properties, such as the electrical resistivity and the dielectric constant of the materials, have been measured. The correlations between these properties have been studied. The PDLC switching voltage appears to be strongly correlated with the resistivity. Our data suggest that ionic impurities play a dominant role with respect to the electrooptic response of PDLC films.     

On the Control of Magnetic Anisotropy through an External Electric Field

The effect of an external electric field on the magnetic anisotropy of a single‐molecule magnet has been investigated, for the first time, with the help of DFT. The application of an electric field can alter the magnetic anisotropy from “easy‐plane” to “easy‐axis” type. Excitation analysis performed through time‐dependent DFT predicts that the external electric field facilitates metal to π‐acceptor ligand charge transfer, leading to uniaxial magnetic anisotropy and concomitant spin Hall effect in a single molecule.     

Effect of External Electric Field on H‐Bonding and π‐Stacking Interactions in Guanine Aggregates

The structure and electronic properties of guanine oligomers and π stacks of guanine quartets (G‐quartets) with circulene are investigated under an external field through first‐principles calculations. An electric field induces nonplanarity in the guanine aggregates and also leads to an increase in the H‐bond distances. The calculations reveal that the binding energy of the circulenes with G‐quartets increases on application of an electric field along the stacking direction. The HOMO–LUMO gap decreases substantially under the influence of an external field. The contribution of a simple dipole–dipole interaction to the stability of the stacked system is also analyzed. The electric field along the perpendicular axis increases the dipole moments of the guanine dimer, trimer, and quartet. Such an increase in the dipole moment facilitates stacking with circulenes. The stability of G‐quartet–circulene π stacks depends on the phase of the dipole moment (in‐phase or out‐of‐phase) induced by an external electric field. The stability of stacks of bowl‐shaped circulenes with G‐quartets depends on the direction of the applied field.     

Anchoring effects on the electrically controlled optical band gap in twisted photonic liquid crystals

We study a one-dimensional twisted photonic liquid crystal (TPLC), consisting of various nematic liquid crystal cells adopting a twisted configuration intercalated by isotropic dielectric layers, submitted to a dc electric field (E^dc ) aligned along the periodicity axis. We write the corresponding Euler–Lagrange equations describing the nematic layer configuration. By assuming arbitrary anchoring quasi-planar boundary conditions, we calculate the equilibrium textures for the nematic, parametrized by the two types of strength of its interaction (polar and azimuthal) with the plane walls. We write the electromagnetic equations in a 4?×?4 matrix representation and using the transfer matrix formalism, we obtain the transmittance and reflectance coefficients for normal incidence as functions of the external electric field and anchoring strengths. We have observed a remarkable dependence of the electric field on the transmission and reflection spectra in opening and closing band gaps.     

Electric field versus surface alignment in confined films of a diblock copolymer melt

The dynamics of alignment of microstructure in confined films of diblock copolymer melts in the presence of an external electric field was studied numerically. We consider in detail a symmetric diblock copolymer melt, exhibiting a lamellar morphology. The method used is a dynamic mean-field density functional method, derived from the generalized time-dependent Ginzburg-Landau theory. The time evolution of concentration variables and therefore the alignment kinetics of the morphologies are described by a set of stochastic equations of a diffusion form with Gaussian noise. We investigated the effect of an electric field on block copolymers under the assumption that the long-range dipolar interaction induced by the fluctuations of composition pattern is a dominant mechanism of electric-field-induced domain alignment. The interactions with bounding electrode surfaces were taken into account as short-range interactions resulting in an additional term in the free energy of the sample. This term contributes only in the vicinity of the surfaces. The surfaces and the electric field compete with each other and align the microstructure in perpendicular directions. Depending on the ratio between electric field and interfacial interactions, parallel or perpendicular lamellar orientations were observed. The time scale of the electric-field-induced alignment is much larger than the time scale of the surface-induced alignment and microphase separation.     

外部电场下水分子间静电相互作用的分子动力学研究

本文利用分子动力学模拟探讨了不同外电场下,液态水的分子间作用及分子排布的变化. 在不同外电场下,O…O原子间的径向分布函数差别很小,但是单个水分子的偶极矩的取向变化却很大. 当外电场为0时,单个水分子偶极取向的范围很宽（30-150度）. 与此同时,本文给出了局域诱导电场随着位置的变化关系图. 当外加电场增强时,局域的诱导电场强度也随之增加. 由于电场下偶极矩有序性的增加,局域诱导的静电相互作用能显著增加. 计算结果表明,相对介电常数随着电场强度的增加而呈现指数衰减的变化形式. 这一变化趋势可以用来理解不同电化学环境下,静电相互作用和局域诱导电场的变化.     

Electrorheological effect in hybrid fluids with liquid crystalline additives

Conventional electrorheological (ER) fluids consist of electrically polarizable particles dispersed in an inert insulating liquid. They are characterized by a development of a yield stress upon application of an external electric field. They resemble Bingham fluids with yield stress value depending on electric field. A viscosity increase in the presence of an electric field has been also found in homogeneous solutions of liquid crystalline polymers with no yield stress observed. In this study these two types of fluids and combined dispersions of the solid particles in the liquid crystalline matrix were investigated. A lyotropic liquid crystalline polymer—poly(n‐hexyl isocyanate) (PHIC)—dissolved in xylene was chosen as the active matrix. The dispersed solid phase was comprised of two kinds of polymers: pyrolyzed polyacrylonitryle (PAN) showing electron conductivity, and PAN doped with two salts (KSCN, NaSCN), resulting in ionic conductivity. The rheological measurements under an electric field were performed. The pristine xylene solution of PHIC was characterized first as well as the 15% m/m dispersions of PAN powders in silicone oil. Then the dispersions in the liquid crystalline matrix were investigated showing a strong ER effect whose magnitude was considerably enhanced in comparison to both ER active components measured separately. Copyright © 2006 John Wiley & Sons, Ltd.     

LOCAL ELECTRIC FIELD EFFECT ON THE CHLOROPHYLL FLUORESCENCE

Abstract. Polarized absorption and fluorescence of chlorophyll a and bacteriochlorophyll solutions in nematic liquid crystal mixtures have been studied in the presence of an external electric field.
The electric field caused a reorientation of the pigment molecules as concluded from changes in the polarized absorption. However, no correlation was found between the change in the polarized absorption and the change in the polarized fluorescence as a function of the field strength. The field influence was much stronger than expected only from the molecular orientation. The fluorescence polarized parallel to the direction of the liquid crystal was found to increase, whereas the similarly polarized absorption decreases. As a whole, the fluorescence yield significantly increased. It is suggested that the additional electric field is reinforced by a local field, most probably due to a protonation of the liquid crystal molecules.
Charged solvent molecules are reoriented in an external electric field which changes the local electric field acting on chlorophylls. Similar changes in CHI fluorescence yield due to local electric field can be created in vivo by the shift of charged molecules present in surrounding pigments. Such effects can be at least partially responsible for slow induction of fluorescence phenomena.     

Molecular Dynamics Simulation: Influence of External Electric Field on Bubble Interface in Air Flotation Process

Molecular dynamics(MD) simulation was performed to investigate the influence of external electric field on the vapour-liquid interface of the bubble during the process of toluene separation by air flotation. The physicochemical properties of vapour-liquid interface, surface tension, probability of a hydrogen bonding near the vapour-liquid interface and the viscosity of liquid phase caused by external electric field were analyzed. The results show that the angle between the water molecule dipole moment and the normal z axis in the vapour phase changes smaller when the external electric field is applied. The surface tension and the probability of hydrogen bonding near the vapour-liquid interface increase with the increase of electric field strength. And the viscosity also increases under an external electric field. The results confirm that the external electric field has a positive effect on the performance of bubbles in air flotation, which may provide useful guidance for the combination of electric field and air flotation technology.     

Interaction of electromagnetic waves in planar waveguide with a nematic layer

This work is a theoretical study of energy exchange between two coupled TE-wave modes on director diffraction grating in a planar waveguide containing a layer of nematic liquid crystal. The diffraction grating is produced by an external electric field in the nematic layer with spatial periodic anchoring energy between director and waveguide surface. The intensity of a signal mode at the output of the nematic layer has been calculated in dependence of anchoring energy amplitude and modulation period, the size of nematic layer and electrical field value. The cases of co-propagating and oppositely propagating modes have been analysed. The analytical expressions that describe the maximum values of signal mode intensity have been derived. The maximum intensity value output from the nematic has been shown to depend monotonously on the anchoring energy parameters in the case of oppositely propagating wave modes and non-monotonously in the case of co-propagating wave modes. In both cases, the maximum value of signal mode intensity grows with the increase in electric field.