Surface Charge Density Distribution Research Articles

Interaction between charged nanoparticles and vesicles: coarse-grained molecular dynamics simulations.

An enhanced understanding of the interactions between charged nanoparticles (CNPs) and a curved vesicle membrane may have important implications for the design of nanocarrier agents and drug delivery systems. In this work, coarse-grained molecular dynamics (CGMD) simulations of the CNPs with vesicles were performed to evaluate the effects of hydrophobicity, surface charge density and distribution on the curved vesicle membrane. The simulations reveal that there exist four distinct modes (insertion, repulsion, adhesion, and penetration) in the CNP-vesicle interaction. In contrast to previous studies on a planar membrane, the interactions of CNPs and a curved vesicle membrane show some novel properties. CNPs with low surface charge density (or neutral ones) can penetrate into the interior of the vesicle membrane more easily because of the increased membrane tension. The asymmetry between two leaflets of the membrane induces different interaction strengths of the negatively CNPs with the outer and inner leaflets. After penetration, the negatively CNPs prefer to stay close to the inner leaflet inside the vesicle where CNPs have stronger interactions with their surroundings. In the present work, we analyze the detailed mechanism of CNP's spontaneous penetration into vesicles, which is rarely mentioned in previous simulations. Moreover, we found that the negatively CNPs with the same surface charge density but different distribution result in different modes: the homogeneous mode is more likely to adsorb on the vesicle surface while the inhomogeneous mode tends to be more penetrable. In addition, the flip-flop phenomenon of the lipid membrane and the exchanging of water in or out of the vesicle were observed during penetration. Our results demonstrate that the electrostatic effect plays an essential role in the interaction between CNPs and vesicles. These findings suggest a way of controlling the CNP-vesicle interaction by coupling the hydrophobic properties, surface charge density and distribution of CNPs to enhance the probability of CNP's penetration into vesicles.

Electric field and charge density in the plane of a quasi-equilibrium asymmetric 2D p–n junction with zero current

The problem of the distribution of potential and surface charge density in a thin layer containing a quasi-two-dimensional p–n junction is reduced to the solution of an integral equation. Numerical solution of such an equation is obtained for an asymmetric 2D p–n junction in the strong degeneracy and quasi-equilibrium conditions. The boundary conditions on the lines of intersection of the Fermi level and the thresholds of size-quantization subbands are used. The dependence of the width of the depletion region on the impurity concentration and bias voltage is analyzed.

A surface charge simulation method based on advanced numerical integration

Numerical models for computing low-frequency electromagnetic fields can contain spatial 2D finite elements, which are numerically most demanding due to problem of singularity. In this paper, an advanced time-harmonic quasistatic surface charge simulation method for computation of scalar electric potential and electric field intensity distribution is presented. Subparametric spatial 2D finite elements with an arbitrary number of nodes for description of surface charge density distribution are developed. The problem of singularity that occurs in the double 2D integration over these elements is solved using an originally developed advanced numerical integration based on 2D Gaussian quadrature. Self and mutual coefficients of spatial 2D finite element nodes are numerically computed and included in the system of linear equations for surface charge density distribution computation. The accuracy of the computer program, based on the presented model, is shown in the chosen numerical example with known analytical solution. Numerical model and advanced integration presented herein could be easily extended to non-homogeneous regions and multilayer problems using the image method.

Surface Streamer-to-Leader Transition Under Positive Impulse Voltage

The residual surface charge density distribution under 12.5 - 16.0 kV application impulse voltage is measured. From these charge distributions, the potential distributions are calculated. The characteristics of the potential gradient in streamer and leader channel and the mechanism of streamerto-leader transition under positive impulse voltage is analyzed. When the charge density is as high as 210-320 pC/mm 2 , the streamer channel will transit into a leader channel.

Does like attract like?

We present numerical simulations that allow us to compute the electrostatic force for several static configurations (i.e., metal-metal or metal-metal coated with an insulating layer) of two spherical macroscopic bodies. A key finding, relevant for potential applications in industry and future research, is that the proximity of the bodies induces in all configurations a repulsion-to-attraction (RTA) transition. It turns out that the RTA transition and the surface charge density distribution are controlled by the coating layer permittivity and the gap distance between the bodies giving credence to the interpretation that the transition mechanism is universal considered.

Investigation of a single dielectric barrier discharge in submillimeter air gaps: Uniform field

The characteristics of a single discharge in 0.01–0.2-cm air gaps are investigated under atmospheric pressure between the metal electrodes and in the presence of a polymer barrier on one of the electrodes. The experimentally determined surface charge density distributions are satisfactorily described by a 2D Gaussian function with the proposed parameters. In the uniform field, the breakdown voltage weakly depends on the presence of a dielectric barrier. At the same time, the change from the positive to negative polarity of the metal electrode leads to a manifold increase in the discharge current and in the surface charge. The breakdown occurs under considerable overvoltage; the estimates indicate the streamer mechanism of the breakdown.

Modeling of Water Sorption and Swelling in Polymer Electrolyte Membranes

Energy conversion in polymer electrolyte fuel cells depends on the ability of the polymer electrolyte membrane (PEM) to transport protons from anode to cathode. The PEM, in turn, requires sufficient hydration for efficient proton conduction. The water content is the main thermodynamic variable to characterize the state of a PEM. It determines structure formation, proton conductivity and mechanical response of the membrane. As a result, a consistent description of water sorption and swelling under relevant conditions of PEFC operation is essential for the understanding of transport properties, performance, and degradation phenomena in PEM.We present a theoretical model of water sorption and swelling in PEMs [1]. The model employs conditions of thermal, electrochemical and mechanical equilibrium of water in the porous network. The single pore model relates the charge density, σ0, at the pore walls to a microscopic swelling parameter, η. The single pore model is expanded to an ensemble model using a statistical distribution function of the surface charge density, n(σ0). The volumetric expansion of the PEM can be expressed as a convolution integral, [equation attached as a picture, equation 1]normalized to the dry volume of the PEM, V 0. Furthermore, we obtain microscopic statistical distributions of pore size and elastic pressure in pores.We performed an extensive parametric study. Water sorption and swelling behaviour of the PEM are sensitive to variations in ion exchange capacity, IEC, and elastic modulus, G. They are rather insensitive to variation in external gas pressure.Freger [2] proposed that the deformation of pore walls is isotropic in the plane perpendicular to the stress caused by the expanding water domains. We have evaluated different deformation scenarios corresponding to anisotropic expansion of pore walls or uniaxial stretching of ionomer bundles. The anisotropic deformation scenario shows the best agreement with experimental sorption data [3, 4], as shown in Figure 1. The best result is obtained with a microscopic elastic modulus of 380 MPa. The statistical distributions of pore size and elastic pressure at pore walls are shown in Figure 2a and b, respectively.The model can be applied to the analysis of water sorption in a PEM that is undergoing degradation (Figure 3). Figure 3a shows water sorption isotherms after 0, 80 and 280 days. The data were extracted from a study of Gebel et al.[5]. The total water content decreases because of the decrease in IEC and increase in elastic modulus of the membrane. Surface charge density distributions corresponding to the water sorption isotherms are shown in Figure 3b. The model-based analysis suggests that the total number of pores decreases while the statistical distribution is shifting to pores with larger surface charge density.

Alumina Membrane with Hour‐Glass Shaped Nanochannels: Tunable Ionic Current Rectification Device Modulated by Ions Gradient

A new alumina membrane with hour‐glass shaped nanochannels is reported using the double‐side anodization method and the subsequently in situ pore opening procedure, which is applied to develop the tunable ionic current rectification devices that were modulated by ions gradient. By regulating the pH gradient, tunable ionic current rectification properties which are mainly dependent on the asymmetric surface charge density or polarity distribution on the inner walls of the nanochannels can be obtained. The enhanced ionic current rectification properties were presented due to the synergistic effect of the voltage driven ion flow and diffusion driven ion flow with the application of pH and electrolyte concentration gradients. Therefore, such specific alumina nanochannels would be considered as a promising candidate for building bioinspired artificial ion channel systems.

Multiscale Multiphysics and Multidomain Models I: Basic Theory.

This work extends our earlier two-domain formulation of a differential geometry based multiscale paradigm into a multidomain theory, which endows us the ability to simultaneously accommodate multiphysical descriptions of aqueous chemical, physical and biological systems, such as fuel cells, solar cells, nanofluidics, ion channels, viruses, RNA polymerases, molecular motors and large macromolecular complexes. The essential idea is to make use of the differential geometry theory of surfaces as a natural means to geometrically separate the macroscopic domain of solvent from the microscopic domain of solute, and dynamically couple continuum and discrete descriptions. Our main strategy is to construct energy functionals to put on an equal footing of multiphysics, including polar (i.e., electrostatic) solvation, nonpolar solvation, chemical potential, quantum mechanics, fluid mechanics, molecular mechanics, coarse grained dynamics and elastic dynamics. The variational principle is applied to the energy functionals to derive desirable governing equations, such as multidomain Laplace-Beltrami (LB) equations for macromolecular morphologies, multidomain Poisson-Boltzmann (PB) equation or Poisson equation for electrostatic potential, generalized Nernst-Planck (NP) equations for the dynamics of charged solvent species, generalized Navier-Stokes (NS) equation for fluid dynamics, generalized Newton's equations for molecular dynamics (MD) or coarse-grained dynamics and equation of motion for elastic dynamics. Unlike the classical PB equation, our PB equation is an integral-differential equation due to solvent-solute interactions. To illustrate the proposed formalism, we have explicitly constructed three models, a multidomain solvation model, a multidomain charge transport model and a multidomain chemo-electro-fluid-MD-elastic model. Each solute domain is equipped with distinct surface tension, pressure, dielectric function, and charge density distribution. In addition to long-range Coulombic interactions, various non-electrostatic solvent-solute interactions are considered in the present modeling. We demonstrate the consistency between the non-equilibrium charge transport model and the equilibrium solvation model by showing the systematical reduction of the former to the latter at equilibrium. This paper also offers a brief review of the field.

A Novel Isolation Method for Half-Bridge Power ICs

A novel integrated solution for two high voltage power n-channel MOSFET transistors (NMOSTs) MH and ML and two level-shifting high-voltage NMOSTs M1 and M2 are proposed to save chip area and reduce process complexity. The optimized variation lateral doping technique is used for designing the four NMOSTs. The four NMOSTs are isolated by PN junction isolation technique to integrate together. The isolation areas between ML and M1 and M1 and M2 are needed to sustain high voltage in X-direction. Hence, the effective surface charge density distribution in the isolation areas is designed almost the same as the NMOST. The layout area of the two level-shifting high-voltage NMOSTs of the proposed solution is only 6.61% of that conventional one. The simulation results from TCAD MEDICI and DAVINCI show that the expectant functions are well achieved.

Two-dimensional residual charge density distribution measurement of surface leader

The charge density distribution of the surface leader has never been measured before. Because the surface leader usually covers a long distance, and the surface potential caused by leader discharge is usually very high, this creates difficulties in measuring the potential distribution of the surface leader. In this paper, with a feedback type electrostatic probe based on a field-nullify technique, a charge density distribution scanning system is developed. A two-layer structure pipe is designed to lower the surface potential after discharge. In this way, the surface potential distribution caused by the residual charge of the leader discharge under the application voltage as high as to 40 kV can be measured. The surface charge density distribution including the leader and streamer is perfectly measured, which is in good agreement with the photograph. • A surface potential scanning system with a feedback type electrostatic probe is developed. • Surface charge decay in 3 h is approximately 10%. • Measured charge density pattern agrees well with high speed photograph. • A knee point on main discharge channel is related to streamer-to-leader transition.

Residual Charge Distribution of Surface Leader Discharge Under Positive Impulse Voltage

The performance of high-voltage apparatus is seriously restricted by the surface discharge on dielectric materials. A surface charge density distribution scanning system is developed with a dc feedback-type electrostatic probe based on the field-nullifying technique. The residual charge density distribution after the occurrence of a positive leader discharge is measured. Using the 2-D Fourier transformation in the spatial domain and Tikhonov's regularization method, the surface potential distribution and electric field distribution are arrived at. The leader stem and radial lament streamer zone located at the head of the leader can clearly be distinguished from the distributions. There is an obvious knee point that divides the main channel of the discharge into a leader channel and a streamer channel. The knee point is at about 9 kV, which corresponds to about 320 pC/mm2.

Influence of the Substrate and Tip Shape on the Characterization of Thin Films by Electrostatic Force Microscopy

Electrostatic force microscopy has been shown to be a useful tool to determine the dielectric constant of nanoscaled thin films that play a key role in many electrical, optical, and biological phenomena. Previous approaches have made use of simple analytical models to analyze the experimental data for these materials. Here, we show that the electrostatic force shows a completely different behavior when the shape of the tip and sample are taken into account. We present a complete study of the interaction between the whole tip and the layers below the thin film. We demonstrate that physical magnitudes such as the surface charge density distribution and the size of the materials have a strong influence on the EFM signal. The EFM sensitivity to the substrate below the thin film decreases with the substrate thickness and saturates for thicknesses above two times the length of the tip, when it is close to that of an infinite medium.

Effects of Cs adsorption on the field emission characteristics of closed single-walled carbon nanotubes

Abstract

The self-assembly of a macroion with anisotropic surface charge density distribution

A macroion, having anisotropic surface charge density distribution, shows unique self-assembly behaviour in polar solvents. Regular "blackberry"-like assemblies form in methanol-water mixtures due to counter-ion mediated attraction and the strong contribution of hydrogen bonding. However, rod-like assemblies form in acetone-water mixtures as the charge inhomogeneity effect overcomes the non-directional hydrogen bonding.

Residual charge density distribution measurement of surface leader with feedback electrostatic probe

A charge density distribution scanning system is fabricated using an electrostatic probe, which utilizes DC voltage feedback principle to nullify the electric field between the probe and charged insulator surface. A two-layer structure pipe is designed to lower the surface potential after discharge. In this way, the surface potential distribution caused by the residual charge of leader discharge under the impulse application voltage up to 40 kV can be measured. In this Letter, the residual surface charge density distribution under 23.5 kV application voltage is given, and the numerical characteristics of residual charge in the leader channel and streamer channel are analyzed.

DEMONSTRACINIAI BANDYMAI IR LOGINIŲ SAMPROTAVIMŲ SCHEMOS VIDURINĖS MOKYKLOS ELEKTROSTATIKOS KURSE

The article discloses the importance of demonstration tasks in physics teaching in sec-ondary schools. The place of the demonstration tasks and the possibilities of applying them during physics teaching are analyzed. Four demonstration tasks of electrostatics phenomena in the secondary school are presented and analyzed. They are: Two types of electrical charge; Distribution of surface-charge density; Electric field of electrified sphere; Electric field of two electrically charged plates. Logical reasoning schemes to these physics demo tasks are developed. The proposed reasoning schemes designed to improve students' thinking, explaining the observed demonstrations of physical phenomena which help students to: un-derstand the essence of the demo task, determine cause - effect relationships and dependen-cies, compare the conditions and findings, summarize the results and do conclusions. Such using of demonstration tasks is useful to both of teacher and pupil: teacher manages the con-tent of teaching and a learning of pupils, pupils - are focused to self-activities, encouraged to think, analyze, summarize and do conclusions. Key words: demonstration task, electrostatics phenomena, logical reasoning schemes, phys-ics teaching,

Spatial Distribution of Ion Current Around HVDC Bundle Conductors

The ion flow from the bundle conductors of HVDC transmission lines is different from that of a single conductor. This paper presents systematic research on the spatial distribution of ion current due to corona around bundle conductors. A corona cage capable of fitting different bundle conductors is designed for the experiments. Extensive measurements are carried out in different voltages and polarities for four kinds of bundle conductors. To determine the roughness factor of the wires, the experiment with a single conductor is also included. The numerical methods, based on computation along the electric flux lines or based on triangle meshes, are used to calculate the ion current distribution around bundle conductors. Charge density distributions on different bundles are compared. To improve the accuracy of the mesh-based method, a new scheme of surface charge density distribution on the bundle is proposed. The characteristics of the ion current distribution and the total ion current of the bundle are also analyzed in this paper.

Numerical simulation of probing the electric double layer by scanning electrochemical potential microscopy

Abstract Computational modeling of probing the electric double layer (EDL) by scanning electrochemical potential microscopy (SECPM) was carried out in order to evaluate the applicability of this method. The modeling is based on a continuum approach for the electric double layer in dilute solution using the modified Poisson–Boltzmann equation. This model takes into account the finite ion size and prevents steric effects near the charged surface. The approach of the SECPM probe towards a charged electrode, immersed in electrolyte solution, is simulated obtaining the potential profile in the direction normal to the electrode. The effects of the shape and the size of the metallic protrusion at the apex of the probe were studied. A clear dependence of the probe potential on the apex geometry was observed, and correlated to the apex surface charge density distribution. The overlap of the EDLs located at the probe and the electrode is studied by setting different initial open circuit potentials, i.e., different charging, to the probe. We have found that the obtained potential profiles are consequences of the EDL overlap, characterized by an ionic distribution in the gap separating the probe and electrode. Depending on the strength of both EDLs and their polarity, the Debye screening effect severely influences the probe potential.

Research on the piezoelectric torsional effect of a rectangular quartz disc and a novel drilling dynamometer

The theoretical foundation of a new-type drilling dynamometer (i.e. the torsional effect of a rectangular piezoelectric quartz disc), the structure design and calibration of the drilling dynamometer are investigated in this paper. By using the theory of anisotropic elasticity and the Maxwell electromagnetism, the torsion stress and the distribution of surface charge densities of a rectangular quartz disc are calculated. According to the theoretical analyses of the bound charge densities, the detection electrodes are effectively disposed on the surface of the piezoelectric disc. The experimental results show that the torsional effect exists in the rectangular quartz disc and the bound charges are linear with the torque applied. Based on the torsional effect, a new type of drilling dynamometer is designed, in which only four quartz discs are used. The axial force measuring cell consists of two X0°-Cut quartz discs, and by the partitioned electrode method only two Y0°-Cut quartz discs can simultaneously sense the radial force and torque. After static calibrations, the torque sensor has full reached the dynamometer standard stipulated by CIRP-STCC. Undoubtedly the drilling dynamometer will provide a new method for measuring the drilling force and monitoring the drilling process.