Simulations of polyelectrolyte dynamics in an externally applied electric field in confined geometry

We consider the dynamics of charged polymers in free solution in a slit geometry under the influence of an electrical field, applied at an angle to the plane parallel walls of the confinement. The simulations are carried out using the Brownian dynamics method with explicit counterions and implicit hydrodynamics. The hydrodynamic interactions between all the particles and the plane parallel walls are taken into account using a diffusion matrix which depends on slit geometry and the actual polyelectrolyte-solute conformations. We observe a selective transport of the charged polymers, as a function of the degree of polymerization and slit height.     

Properties of galactocerebroside layers transferred to glassy carbon electrodes: effect of an applied electric field

Galactocerebroside films deposited onto glassy carbon electrodes have been previously studied through the electrochemical response of a redox couple present in solution. Those experiments indicated that the film is inhomogeneous and that there are lipid-free places. In this work, we present experimental results indicating that those bare regions are formed when the electrode is introduced in an aqueous solution, and that the size and/or amount of uncovered domains increase when negative potentials are applied to the film. The experimental techniques employed for these findings are epifluorescence microscopy and ellipsometry.     

Force acting on a dielectric particle in a concentration gradient by ionic concentration polarization under an externally applied DC electric field

There is a concentration-polarization (CP) force acting on a particle submerged in an electrolyte solution with a concentration (conductivity) gradient under an externally applied DC electric field. This force originates from the two mechanisms: (i) gradient of electrohydrodynamic pressure around the particle developed by the Coulombic force acting on induced free charges by the concentration polarization, and (ii) dielectric force due to nonuniform electric field induced by the conductivity gradient. A perturbation analysis is performed for the electric field, the concentration field, and the hydrodynamic field, under the assumptions of creeping flow and small concentration gradient. The leading order component of this force acting on a dielectric spherical particle is obtained by integrating the Maxwell and the hydrodynamic stress tensors. The analytical results are validated by comparing the surface pressure and the skin friction to those of a numerical analysis. The CP force is proportional to square of the applied electric field, effective for electrically neutral particles, and always directs towards the region of higher ionic concentration. The magnitude of the CP force is compared to that of the electrophoretic and the conventional dielectrophoretic forces.     

Electrostatic interactions in the adhesion of an ion-penetrable and ion-impenetrable bacterial strain to glass

Deposition to glass of Streptococcus salivarius HB-C12 and Staphylococcus epidermidis 3399 in a parallel plate flow chamber has been studied as a function of ionic strength. Electrophoretic mobility measurements revealed that S. epidermidis 3399 possesses a thick ion-penetrable layer, probably associated with its encapsulation, while S. salivarius HB-C12 has an ion-impenetrable surface. Streaming potential measurements indicated that also the glass surface was covered with a relatively thin, ion-penetrable layer. Theoretical initial deposition rates of both strains to glass were obtained by numerically solving the convective-diffusion equation, while accounting for the ion-penetrability of the interacting surfaces. Experimentally, the initial deposition rate of the ion-penetrable strain S. epidermidis 3399 was found to be higher and less dependent on ionic strength than of the ion-impenetrable S. salivarius HB-C12, in accordance with theoretical expectations. Agreement between theoretical and experimental deposition rates could be obtained when glass was considered ion-penetrable when interacting with the ion-penetrable organism S. epidermidis 3399, while glass behaved as an ion-impenetrable surface when interacting with the ion-impenetrable S. salivarius HB-C12. Probably, interaction with an ion-impenetrable strain drives the diffuse double layer charges into the limited volume of the thin ion-penetrable layer on the glass, readily filling it up and making it appear ion-impenetrable. During interaction of glass with another ion-penetrable surface, as of S. epidermidis 3399, diffuse double layer charges move into both ion-penetrable surfaces, resulting in a much lower mobile charge density in the ion-penetrable layer on the glass which consequently continues to behave as ion-penetrable.     

Reaction fronts in an ionic autocatalytic system with an applied electric field

The effects of applying an electric field to an ionic autocatalytic reaction with a cubic rate law are discussed. The constant field strength approximation is made and the resulting equations for the model examined by first considering the corresponding travelling wave equations. These show ranges of field strength over which travelling waves do not exist, these ranges being dependent on whether D< 1 or D< 1, where D is the ratio of diffusion coefficients of autocatalyst and substrate. Numerical simulations of the full system are obtained and these show that, when travelling waves exist, these are formed as the long time behaviour of the system. When travelling waves do not exist, complete electrophoretic separation of the reacting ionic species results, forming separate fronts in autocatalyst and substrate, their direction of propagation depending on D. Comparisons with a related problem with a quadratic rate law are made and the implications for experiments based on the iodate–arsenous acid reaction assessed.     

The impact of ultraviolet light on bacterial adhesion to glass and metal oxide-coated surface

Biofouling of glass and quartz surfaces can be reduced when the surface is coated with photocatalytically active metal oxides, such as TiO2 (anatase form) or SnO2. We measured the attachment of eight strains of bacteria to these two metal oxides (TiO2 and SnO2), and to an uncoated glass (control; designated Si-m) before and after exposure to UV light at wavelengths of 254 nm (UVC) or 340 nm UV (UVA). TiO2-coated surfaces were photocatalytically active at both 254 and 340 nm as evidenced by a decrease in the water contact angle of the surface from 59 degrees +/-2 to <5 degrees. The water contact angle of the SnO2 surface was reduced only at 254 nm, while contact angle of the Si-m glass surface was not altered by light of either wavelength. Bacterial adhesion decreased by 10-50% to photocatalyzed glass surfaces. In all cases, bacteria exposed to the UV light were completely killed due to a combination of exposure to UV light and the photocatalytic activity of the glass surfaces. These results show that UV light irradiation of TiO2-coated surfaces can be an effective method of reducing bacterial adhesion.     

Potential energy surfaces for a mixed-valence dimer in an applied electric field

Summary A mixed-valence dimer with an applied external field aligned along the internuclear axis is studied using a two-site small-polaron model. Potential energy surfaces are calculated in the adiabatic (Born-Oppenheimer) approximation. It is shown that two nuclear coordinates (one totally symmetric and the other antisymmetric) are coupled to the electronic motion, whereas only the antisymmetric coordinate is coupled in the absence of an electric field. For a strongly localized (valence trapped) system, the displacement along the totally symmetric coordinate is directly proportional to the applied field strength. For delocalized (valence-averaged) systems, there is significant displacement along the antisymmetric coordinate, an effect which also vanishes in the absence of an applied field. Contributions to the linewidth are estimated.On sabbatical leave at Aarhus University through March 1994.     

The effect of an electric field on the vibrational frequency of CN

Ab initio calculations are used to compute the change in the vibrational frequency of CN with an applied electric field aligned parallel to the bond axis. The effect of the field on the CN⁻ vibrational frequency is of the same magnitude as for CN, but in the opposite direction. These results are compared to previous results for CO. © 1997 John Wiley & Sons, Inc.     

Molecular dynamics simulation of an electric field driven dipolar molecular rotor attached to a quartz glass surface

Molecular dynamics simulations of the response of a dipolar azimuthal 3-chloroprop-1-ynyl rotor mounted on the surface of quartz glass to a rotating electric field were performed. The rotor motion was classified as synchronous, asynchronous, random, or hindered, based on the value of the average lag of the rotor behind the field and a comparison of the intrinsic rotational barrier V(b) with kT. A phase diagram of rotor behavior was deduced at 10, 300, and 500 K as a function of field strength and frequency. A simple model for the rotor motion was developed, containing the driving force, the temperature, the height of the torsional barrier, and the friction constant of the rotor. Defining E(bo) to be the electric field strength necessary to get rotational response from the rotor ("breakoff field") and mu to be the rotor dipole moment component in the plane of rotation, we find that E(bo) is frequency independent when 2 microE(bo) is less than either V(b) or kT (the driving force needs to overcome the more important of the two, the intrinsic barrier or random thermal motion). At higher frequencies, E(bo) is a quadratic function of the frequency and the driving force fights friction, which is dictated by intramolecular vibrational redistribution (IVR) from the pumped rotational mode to all others. Fitting the simple model to simulation data, we derived a friction constant of 0.26 ps eV x (nu - 0.5)/THz between 500 and 1000 GHz.     

The externally contracted CI method applied to N2

An approximate multireference CI method is presented. By grouping together configurations with the same internal parts and freezing their relative weights by the use of perturbation theory, the number of variational parameters is drastically reduced. The loss of correlation energy is shown to be usually less than 2%, and the timing is less than one ordinary CI iteration. Examples from calculations on some states of the nitrogen atom and nitrogen molecule are given. The basis set convergence for the lowest excitation energy in the atom is very slow. Less than 50% of the correlation effect is obtained at the s, p, d limit. After the inclusion of ? functions this value is improved to 83%. The dissociation energies of the molecule also show slow basis set convergence with errors of 0.5 eV even after addition of ? functions. The bond distances are, howeever, accurately reproduced with errors of less than 0.005 Å for all the states. A qualitative discussion of predissociation in the a ¹Π_g and B ³Π_gstates caused by spin–orbit interaction with the ⁵Σ_g⁺ state, is finally presented. Rapidly oscillating lifetimes between the different vibrational states are predicted.     

Antiferroelectric siloxane liquid crystal dimers with large molecular tilt under an applied electric field

The scope of the present study is the response of a series of antiferroelectric dimeric or bi-mesogenic siloxanes to an applied electric field with focus on their pretransitional behaviour and the field-induced antiferro-ferroelectric (AF-F) transition. Most of these compounds possess a molecular tilt close to 45° and spontaneous polarization in the field-induced ferroelectric (F) phase in the range of 250-300 nC cm^-2. In the dimers with a spacer length exceeding five carbons, a transformation from first to second order of the field-induced AF-F transition is found with temperature. Several different indications for this transformation are identified and their characteristics are discussed in the framework of the existing theoretical models. A large field-induced in-plane deviation of the sample optic axis was observed in the pretransitional region of several of the siloxane dimers and is likely due to the flexibility of the linking chains. The potential of the antiferroelectric bimesogenic siloxanes for displaying high contrast images and grey scale capability is shortly discussed. The large molecular tilt close to 45° in combination with the field-induced AF-F transition of second order seems to be the most attractive features of these materials.     

Antiferroelectric siloxane liquid crystal dimers with large molecular tilt under an applied electric field

The scope of the present study is the response of a series of antiferroelectric dimeric or bi‐mesogenic siloxanes to an applied electric field with focus on their pretransitional behaviour and the field‐induced antiferro–ferroelectric (AF–F) transition. Most of these compounds possess a molecular tilt close to 45° and spontaneous polarization in the field‐induced ferroelectric (F) phase in the range of 250–300 nC cm^?2. In the dimers with a spacer length exceeding five carbons, a transformation from first to second order of the field‐induced AF–F transition is found with temperature. Several different indications for this transformation are identified and their characteristics are discussed in the framework of the existing theoretical models. A large field‐induced in‐plane deviation of the sample optic axis was observed in the pretransitional region of several of the siloxane dimers and is likely due to the flexibility of the linking chains. The potential of the antiferroelectric bimesogenic siloxanes for displaying high contrast images and grey scale capability is shortly discussed. The large molecular tilt close to 45° in combination with the field‐induced AF–F transition of second order seems to be the most attractive features of these materials.     

Effect of an electric field on an interfacial profile

We consider the interface between two dielectric liquids in an electric field which is parallel to the interface. Using the Ginsburg-Landau model for binary fluids we add a term for the electric field energy and show that near the critical point the effect of the electric field is to change both the interface width and the critical temperature.     

Protective effect of sorbitol on enzymes exposed to microsecond pulsed electric field

Lysozyme (alpha-helix dominant structure) and pepsin (beta-sheet dominant structure) were exposed to microsecond pulsed electric field (PEF) at 3.5x10(6) V/m. The response of enzymes to the stress of PEF was investigated in this study. Unfolding of enzyme structures and disruption of secondary and three-dimensional structures occurred when the exposed PEF dosage exceeds a critical value, which caused the decrease in activity. In this work, sorbitol was found to be effective to stabilize the conformations and activities of enzymes against electric field. The protective effect increased with the increase of concentration of sorbitol.     

A molecular orbital study of the effect of an external electric field on methanol

We report the results of INDO-MO calculations to investigate the influence of an external electric field on methanol. We predict that during a field-ionisation experiment, ionisation will occur from a quite different molecular geometry than the field-free olie.     

Effect of the applied electric field on new cholesterics with extremely short pitch

New lactic acid derivatives have been prepared and studied and found that they form cholesteric phase with the helix pitch length in interval 120–200 nm within a broad temperature range. Due to the positive dielectric anisotropy and the short pitch, the applied electric field causes reversible optical changes in planar cell, in which the applied electric field reorients the long molecular axis. Presence of short pitch and possibility to effectively affect electro-optical properties are promising from the point of view of specific applications.     

Effect of an electric field applied during the injection procedure on the pretilt angle of a nematic liquid crystal

We examined the effect of an electric field applied during the injection procedure on the polar pretilt angle of a nematic liquid crystal (LC). The pretilt angle of the sample injected at 25°C gradually increased as the electric field was increased. On the other hand, the pretilt angle of a sample injected at 90°C (which is above the nematic-isotropic phase transition temperature of LC) showed a sudden increase in the presence of the electric field and also increased with a greater electric field. We think the alignment layer might be swollen with LC molecules, and the rotation of the immersed LC molecules by the electric field induces a deformation of the alignment layer. These results imply LC and the alignment layer were coupled, and their cooperation had an influence on determining the bulk pretilt angle.     

Photophysics and dynamics of surface plasmon polaritons-mediated energy transfer in the presence of an applied electric field

The possibility to transfer energy between molecular excitons across a metal film up to 150 nm thick represents a very attractive solution to control and improve the performances of thin optoeletronic devices. This process involves the presence of coupled surface plasmon polaritons (SPPs) at the two dielectric-metal interfaces, capable of mediating the interactions between donor and acceptor, located on opposite sides of the metal film. In this Article, the photophysics and the dynamics of an efficient SPP-mediated energy transfer between a suitable dye and a conjugated polymer is characterized by means of steady-state and time-resolved photoluminescence techniques. The process is studied in model multilayer structures (donor/metal/acceptor) as well as in electrically pumped heterostructures (donor/metal cathode/acceptor/anode), to verify the effects of applied electric fields on the efficiency and the dynamics of SPP-mediated energy transfer. A striking enhancement of the overall luminescence was recorded in a particular range of applied bias, suggesting the presence of cooperative effects between optical and electrical stimulations.     

Solvation dynamics and electric field relaxation in an imidazolium-PF6 ionic liquid: from room temperature to the glass transition

Time-resolved phosphorescence spectra and anisotropy of quinoxaline were measured in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-HFP), in its supercooled state near the glass-transition temperature. The solvation dynamics results are compared with the rotational motion of the probe and with the dielectric behavior of the neat ionic liquid. The dynamics in the viscous state are highly dispersive and show a super-Arrhenius temperature dependence, as typical for glass-forming materials. Combined with room-temperature results, solvation dynamics is observed to follow the structural relaxation times in terms of eta/T for more than 10 decades, from subnanoseconds at room temperature to seconds near the glass-transition temperature T(g). The dielectric modulus relaxation follows this trend only for temperatures T > 1.2T(g) and departs significantly from eta/T in the 1.1T(g) > T > T(g) range. This deviation is reminiscent of the enhanced translational diffusion or fractional Stokes-Einstein behavior observed in many fragile supercooled liquids. Because the electric field relaxation in BMIM-HFP includes dc conductivity, this correlation function involves translational motion and thus displays the effect of enhanced diffusivity. A microscopic model is required for rationalizing the decoupling of solvation dynamics from the longitudinal time scales and the limitation of this effect to the viscous regime with T < 1.2T(g).