Charges Of Different Signs Research Articles

Mathematical modeling of inhomogeneous electric field impact on a liquid crystal layer

AbstractThe authors analyze the Fréedericksz transition phenomenon of reorientation of molecules in the plane liquid crystal layer in inhomogeneous electric field generated by parallel plate capacitor using the Oseen–Frank mathematical model. The capacitor is a set of short metal plates periodically located on both sides of the extended liquid crystal layer, connected in series in two circuits. When electric charges of different signs appear on opposite plates of the capacitor, the electric field in the periodicity cell turns out to be significantly inhomogeneous both in length and in thickness of the layer. Governing equations of the model represent Euler's variational equations in the problem on minimization of the potential energy of liquid crystal. Numerical solution of the equations uses the authorial variational difference scheme. The Laplace equation for electric potential in the exterior of the liquid crystal layer is solved using the method of lines. The electric field and the patterns of orientation angles of molecules inside the layer are determined by iterating with numerical solution of Poisson's equation using the fast Fourier transform at each iteration step. The algorithm is implemented by means of CUDA technology for computing systems with graphics accelerators. The scale of the mesh to ensure the computation accuracy is selected from calculation of Tsvetkov's order parameter in a rectangular cell pattern. Verification of the algorithm and program is implemented by way of comparing the computational results and the exact solution of the problem on the homogenous electric field with the constant initial orientation angle of liquid crystal molecules. The computations accomplished yield the patterns of the electric potentials and orientation angles of molecules depending on the position of the capacitor plates at uniform and s‐shaped initial distribution of the orientation angles through the thickness of the liquid crystal layer.

A Laboratory Investigation of the Probable Mechanisms of the Action of an Artificial Thunderstorm Cell on Model Aircraft Radomes

The results of experimental laboratory investigations of possible mechanisms of the impact of lightning and thunderclouds on aircraft radomes and equipment inside them are presented. An artificial thunderstorm cell of negative polarity and model aircraft radomes with lightning diverter strips have been used. Experiments have shown that the discharge processes in a radome model significantly depend on the magnitude of the charge that accumulates on the inner and outer surfaces of the radome shell. It is established that the accumulation of large-magnitude charges of different signs on the outer and/or inner surface of the radome (up to hundreds of µC/m2) shell leads to a multivariance of the mechanisms of development of discharge processes inside the radome model, along its surface, and in the space near it. Significant influence of the “reverse” discharge from the antenna model under the radome on the types of current impulses recorded on the antennas under impact of the artificial thunderstorm cell is established. Peculiarities of the discharge formation in the radome model when using solid and segmented diverter strips for its protection are revealed. Parameters of the current impulses registered on the diverter strips and the antennas have been determined. Based on the conducted research, the possible mechanisms of the impact of thunderclouds and lightning discharges on radio-transparent aircraft radomes and the equipment inside them are considered.

Toroidal Vortices of Energy in Tightly Focused Second-Order Cylindrical Vector Beams

In this paper, we simulate the focusing of a cylindrical vector beam (CVB) of second order, using the Richards–Wolf formula. Many papers have been published on focusing CVB, but they did not report on forming of the toroidal vortices of energy (TVE) near the focus. TVE are fluxes of light energy in longitudinal planes along closed paths around some critical points at which the flux of energy is zero. In the 3D case, such longitudinal energy fluxes form a toroidal surface, and the critical points around which the energy rotates form a circle lying in the transverse plane. TVE are formed in pairs with different directions of rotation (similar to optical vortices with topological charges of different signs). We show that when light with a wavelength of 532 nm is focused by a lens with numerical aperture NA = 0.95, toroidal vortices periodically appear at a distance of about 0.45 μm (0.85 λ) from the axis (with a period along the z-axis of 0.8 μm (1.5 λ)). The vortices arise in pairs: the vortex nearest to the focal plane is twisted clockwise, and the next vortex is twisted counterclockwise. These vortices are accompanied by saddle points. At higher distances from the z-axis, this pattern of toroidal vortices is repeated, and at a distance of about 0.7 μm (1.3 λ), a region in which toroidal vortices are repeated along the z-axis is observed. When the beam is focused and limited by a narrow annular aperture, these toroidal vortices are not observed.

Molecular Recognition of Zwitterions with Artificial Receptors.

Many biomolecules exist as internal ion pairs or zwitterions within a biologically relevant pH range. Despite their importance, the molecular recognition of this type of systems is specially challenging due to their strong solvation in aqueous media, and their trend to form folded or self-assembled structures by pairing of charges of different sign. In this Minireview, we will discuss the molecular recognition of zwitterions using non-natural, synthetic receptors. This contribution does not intend to make a full in-depth revision of the existing research in the field, but a personal overview with selected representative examples from the recent literature.

Unraveling the Multistimuli Responses of a Complex Dynamic System of Pseudopeptidic Macrocycles

Dynamic combinatorial libraries (DCLs) are excellent benchmark models to study the stimuli-responsiveness of chemical networks. However, increasingly complex systems are difficult to analyze with simple data analysis methods, because many variables and connections must be considered for their full understanding. Here we propose the use of multivariate data analysis methods to bisect the evolution of a complex synthetic dynamic library of pseudopeptidic macrocycles, containing side chains with charges of different sign. Several stimuli (ionic strength, pH and the presence of a biogenic polyamine) were applied to the same dynamic chemical mixture, and the adaptation of the whole system was characterized by HPLC and analyzed with principal component analysis (PCA) and multivariate curve resolution-alternating least squares (MCR-ALS) methods. Both multivariate data analysis chemometric approaches are an excellent combination to extract both qualitative and semi-quantitative information about the adaptive process of the library upon the action of each stimulus. The resolution of the system with these chemometric tools proved to be especially useful when two inter-connected stimuli were combined in the same dynamic system. Our results demonstrate the utility of these two approaches for the analysis of complex dynamic chemical systems and open the way toward the application of these powerful tools in the emergent field of systems chemistry.

Fractal Model of a Compact Intracloud Discharge. I. Features of the Structure and Evolution

We propose a new model of a compact intracloud discharge considered as the result of interaction between two (or more) bipolar streamer structures formed in a strong large-scale electric field of a thundercloud. The model assumes two stages of the compact discharge development. At the preliminary stage, two or more bipolar streamer structures appear successively in the thundercloud in the region of a strong electric field (at the boundaries between the regions of the main positive and the main negative electric charges or between the main positive charge region and the top negative screening layer). The time of development of such structures is determined by the characteristics of the conducting channels that form them and can reach tens of milliseconds. Spatiotemporal synchronization of the bipolar streamer structures is provided by the altitude modulation of the electric field, which, in particular, can originate from a large-scale turbulence of the cloud medium or the stream instability. It is shown that a single bipolar streamer structure accumulates significant electric charges of different signs at its ends as it develops. The start of the main stage of a compact intracloud discharge corresponds to the occurrence of the conducting channel (breakdown of the gap) between the mature streamer structures. The electric charge accumulated at the adjacent ends of the structures at this stage is neutralized over a time much shorter than the duration of the preliminary stage. The parameters of the current pulse are in good agreement with the estimates of the current of a compact intracloud discharge which were obtained in the transmission-line approximation.

Relation between mechanical dynamic processes and the accompanying electric fields

The dependence of the electric field in the plane of motion of a nylon string on the string velocity is experimentally studied. The shape and number of the charges that accompany the motion of solid bodies, which have various geometric parameters, in air up to transonic velocities are determined. The formation and shape of electric charges of different signs in an initially neutral dielectric rod are investigated during the motion of a deformation pulse of the same sign along the rod.

Features of the Physicochemical processes in a Metal– Polymer System

An examination is made of the mechanism of charge mozaic formation on the surface of a polycrystalline metal particle. It is shown that two particles in contact with each other, owing to the presence of a potential barrier between them, acquire electric charges of different sign. This effect must be taken into account when assessing the adhesion, adsorption, tribotechnical, and other service characteristics of metal-polymer systems.

Measurement of triboelectric charging of moving micro particles by means of an inductive cylindrical probe

We present a method based on induced currents in a cylindrical probe which allows analysis of the micro-particle charging processes in an aerosol. The micro particles were triboelectrically charged by passing through a dielectric tube coaxially mounted into the probe. The cylindrical probe enabled the quantification of particle charging without prior calibration of the probe. An analytic model was developed for the description of the measured induced currents and implemented into a computer simulation program. The combination of model simulations and an appropriate experimental setup revealed comprehensive data for the determination of the particles' electric charge against time of flight through the tube. In methodological proof of principle experiments, the formations of particle clouds with charges of different signs were observed using magnetite micro particles.

Investigation of triboelectric charging in dry powder inhalers using electrical low pressure impactor (ELPI™)

Electrostatics and triboelectrification phenomena in dry powder inhalers (DPI) are not well understood, but as shown in this study they may play an important role. Using model formulations of albuterol in lactose, the extent of triboelectrification in the operation of DPI was investigated using an electrical low pressure impactor (ELPI™). An experimental apparatus was developed, the performance of the ELPI™ was evaluated for consistency and reproducibility, and compared to a conventional inertial impactor. Using a statistical experimental design the effects of lactose type, drug load, capsule fill, capsule material, and inhaler were assessed. DPI formulations appear to be subject to strong triboelectric effects. Charge separation can occur between different size fractions, i.e. different fractions can carry charges of different sign. In particular, lactose type, inhaler, and capsule material have a strong effect on the magnitude and polarity of the charge developed during DPI operation. The study suggests that the polarity of the aerosol can be controlled by choice of lactose type, capsule material, and inhaler, which could be exploited for targeting different lung physiologies.

Protons in collision with hydrogen atoms: Influence of unitarity and multiple scattering

Three-body integral equations when applied to collisions of protons with hydrogen atoms yield the amplitudes for direct scattering and electron exchange which automatically satisfy two-body and (if solved exactly also) three-body unitarity. As a consequence, differential cross sections (DCS) for both reactions are calculated on equal footing, and the total exchange cross section (TCS) results from the corresponding DCS. This is to be contrasted with standard models used in atomic physics which either are of the (Distorted-Wave) Born type (to be applied at higher energies), or solve the Schrodinger equation by expanding the wave function in a basis (close-coupling method which has problems especially for rearrangement processes). Hence, at higher energies in general separate models have to be developed to describe the DCS for either direct scattering or electron exchange, and frequently also for the TCS. For instance, the most sophisticated traditional models, which provide a very good reproduction of the TCS data, are the continuum distorted wave [1] and the boundary corrected first Born model [2]. Both, however, fail to describe differential cross sections which represent a much more stringent test. On the other hand, three-body integral equations suffer from the principal defect that their kernels are not compact when particles with charges of different sign are involved, as it happens in applications to atomic reactions (references can be traced from [3]). The consequence is that naive application of standard solution methods of integral equations theory would not be justified. Additional, practical difficulties arise from the complicated singularity structure of the off-shell twoparticle Coulomb T-matrix which is the basic dynamical ingredient. As is well known, the latter develops nasty singularities in the on-shell limit and, in case of attraction, has in addition an infinite number of poles.

Substituent Effects and Threshold Energies for the Unimolecular Elimination of HCl (DCl) and HF (DF) from Chemically Activated CFCl2CH3 and CFCl2CD3

Combination of CFCl2 and methyl-d0 and -d3 radicals form CFCl2CH3-d0 and -d3 with 100 and 101 kcal/mol of internal energy, respectively. An upper limit for the rate constant ratio of disproportionation to combination, kd/kc, for Cl transfer is 0.07 ± 0.03 for collision of two CFCl2 radicals and 0.015 ± 0.005 for CH3 and CFCl2 radicals. The chemically activated CFCl2CH3 undergoes 1,2-dehydrochlorination and 1,2-dehydrofluorination with rate constants of 3.9 × 109 and 4.9 × 107 s-1, respectively. For CFCl2CD3 the rate constants are 8.7 × 108 s-1 for loss of DCl and 1.1 × 107 s-1 for DF. The kinetic isotope effect is 4.4 ± 0.9 for HCl/DCl and appears to be identical for HF/DF. Threshold energies are 54 kcal/mol for loss of HCl and 68 kcal/mol for HF; the E0's for the deuterated channels are 1.4 kcal/mol higher. Comparison of these threshold energies with other haloethanes suggests that for HF and HCl elimination the transition states are developing charges of different signs on the carbon containing the depa...

Nucleation in molecular and dipolar fluids: Interaction site model

We consider the effect of molecular anisotropy and dipole moment on the rate of homogeneous and heterogeneous nucleation of molecular and dipolar fluids. Density functional theory is applied to an interaction site model of molecules composed of two tangent or fused hard spheres interacting through Lennard-Jones and coulombic potentials. Interfacial properties are conditioned by entropy vs energy competition. For nucleation on a charged hard sphere, the calculated rates depend strongly on the size and charge of the central seed. Preferred surface orientations of the dipolar molecules are responsible for nucleation rates differing by several orders of magnitude for charges of different sign.

Influence of negatively and positively charged scattering centers on electron mobility in semiconductor inversion layers: A Monte Carlo study

The effects of the presence of charged centers of different sign on the electron mobility in n-channel metal-oxide-semiconductor transistors are studied by a Monte Carlo simulation. By solving the Poisson equation for the potential fluctuations, an expression for the Coulomb-scattering rate when there are charged centers of different sign is provided. In addition, it is shown that, when charges of different sign exist in the structure, local band-bending fluctuations are greater, thus resulting in lower electron mobility. In contrast, since in this case the potential mean value is lower, the influence of the charged centers on the threshold voltage is lesser. The electron mobility in compensated substrates is also analyzed.

Measuring electrostatic, van der Waals, and hydration forces in electrolyte solutions with an atomic force microscope

In atomic force microscopy, the tip experiences electrostatic, van der Waals, and hydration forces when imaging in electrolyte solution above a charged surface. To study the electrostatic interaction force vs distance, curves were recorded at different salt concentrations and pH values. This was done with tips bearing surface charges of different sign and magnitude (silicon nitride, Al(2)O(3), glass, and diamond) on negatively charged surfaces (mica and glass). In addition to the van der Waals attraction, neutral and negatively charged tips experienced a repulsive force. This repulsive force depended on the salt concentration. It decayed exponentially with distance having a decay length similar to the Debye length. Typical forces were about 0.1 nN strong. With positively charged tips, purely attractive forces were observed. Comparing these results with calculations showed the electrostatic origin of this force.In the presence of high concentrations (> 3 M) of divalent cations, where the electrostatic force can be completely ignored, another repulsive force was observed with silicon nitride tips on mica. This force decayed roughly exponentially with a decay length of 3 nm and was approximately 0.07-nN strong. This repulsion is attributed to the hydration force.