Arrangement Of Charges Research Articles

Numerical Model of Cesium Ion Beams in Field Emission Electric Propulsion Thrusters

A numerical model of the ion beam in field emission electric propulsion thrusters is presented. The objective of the study is to develop a computational tool capable of predicting the effects of thruster configuration parameters (geometry, voltage distribution between the electrodes) on thruster performance (beam divergence, ion exhaust velocity). A two-dimensional, steady model has been adopted. A simple emission model has been assumed, neglecting the detailed, microscopic description of the physical emission mechanism. The electric field is evaluated by a finite element scheme; boundary conditions are provided by an auxiliary technique based on the placement of fictitious charges in proximity to the electrode surfaces. The beam is simulated through a discrete number of charged particles; the effects of the associated space charge density distribution on the electric field is considered. The results of the simulations of typical thruster configurations are presented and discussed. The need to take space charge effects into account to attain a sufficiently accurate solution is indicated: the beam charge density modifies the electric potential distribution generated by the electrodes voltage difference, causing a decrease in the local electric field at emitter tip and an increase in beam divergence. Neutralization at a finite distance must be taken into account to fully appreciate the effects of different voltage distributions between the electrodes: a higher accelerator electrode voltage determines a lower ion exhaust velocity and a larger beam divergence. The capability of an additional, decelerating neutral electrode to focus the ion beam is also investigated, showing that only little improvement in beam containment is attained.

Molecular dynamics Monte Carlo simulations of grain boundary electron transport in n-silicon

Electron transport in Si low-angle bicrystals is analyzed by a novel Monte Carlo molecular dynamics simulation scheme. The effect of discrete charges at the grain boundaries is studied and compared to results from one-dimensional treatments. The average grain boundary charge density strongly influences transport, and a field-dependent threshold effect is predicted. Details of the internal charge arrangement are shown to be quite important at low fields and/or high grain boundary charge densities. Substantial increases in current conduction are predicted at lower temperatures over the thermionic emission model. Finally, analyses of interacting grain boundaries indicate site-correlation effects and a strong dependence of conductivity on the separation distance.

Discrimination between native and intentionally misfolded conformations of proteins: ES/IS, a new method for calculating conformational free energy that uses both dynamics simulations with an explicit solvent and an implicit solvent continuum model

A new method for calculating the total conformational free energy of proteins in water solvent is presented. The method consists of a relatively brief simulation by molecular dynamics with explicit solvent (ES) molecules to produce a set of microstates of the macroscopic conformation. Conformational energy and entropy are obtained from the simulation, the latter in the quasi-harmonic approximation by analysis of the covariance matrix. The implicit solvent (IS) dielectric continuum model is used to calculate the average solvation free energy as the sum of the free energies of creating the solute-size hydrophobic cavity, of the van der Waals solute-solvent interactions, and of the polarization of water solvent by the solute's charges. The reliability of the solvation free energy depends on a number of factors: the details of arrangement of the protein's charges, especially those near the surface; the definition of the molecular surface; and the method chosen for solving the Poisson equation. Molecular dynamics simulation in explicit solvent relaxes the protein's conformation and allows polar surface groups to assume conformations compatible with interaction with solvent, while averaging of internal energy and solvation free energy tend to enhance the precision. Two recently developed methods--SIMS, for calculation of a smooth invariant molecular surface, and FAMBE, for solution of the Poisson equation via a fast adaptive multigrid boundary element--have been employed. The SIMS and FAMBE programs scale linearly with the number of atoms. SIMS is superior to Connolly's MS (molecular surface) program: it is faster, more accurate, and more stable, and it smooths singularities of the molecular surface. Solvation free energies calculated with these two programs do not depend on molecular position or orientation and are stable along a molecular dynamics trajectory. We have applied this method to calculate the conformational free energy of native and intentionally misfolded globular conformations of proteins (the EMBL set of deliberately misfolded proteins) and have obtained good discrimination in favor of the native conformations in all instances.

Simulations of charge transport across a grain boundary in n-silicon bicrystals

Electron transport in Si low-angle bicrystals is analyzed by a novel Monte Carlo–molecular dynamics simulation scheme. The effect of discrete charges at the grain boundaries are studied and compared to results from one-dimensional treatments. The average grain boundary charge density strongly influences transport, and a field-dependent threshold effect is predicted. Details of the internal charge arrangement are shown to be relatively important at low fields and/or high grain boundary charge densities.

A phenomenological model of the rotation dynamics of carbon nanotube gears with laser electric fields

A phenomenological model for the rotational dynamics of a single laser-powered molecular motor is discussed and tested through molecular dynamics simulations. The motor is used to power carbon nanotube-based gears. For a given laser power density and arrangement of free charges in the body of the gear we have defined an intrinsic frequency of the gear oscillatory rotations. The nanotube rotations are not of an oscillatory nature if the laser field frequency is of the same order of magnitude as the intrinsic frequency of the tube and there is an additional phase match between the two. For the laser-powered gear motor dynamics, the rotational angular momentum of the driven gear tend to stabilize the rotational dynamics of the system, and unidirectional rotations for the entire duration of the simulations are observed.

Charged residues of the rotor protein FliG essential for torque generation in the flagellar motor of Escherichia coli

The FliG protein of Escherichia coli is essential for assembly and function of the flagellar motor. Certain mutations in FliG give a non-motile, or Mot −, phenotype, in which flagella are assembled but do not rotate. Mutations with this property are clustered in a C-terminal segment of FliG that is stable when expressed alone, and thus probably constitutes an independently folded domain. Previously, we suggested that this domain forms the rotor portion of the active site for torque generation in the motor. In this work, we have used a mutational approach to identify the amino acid residues in the C-terminal domain of FliG that are most important for motor function. Site-directed mutagenesis was used to replace each of the conserved residues in this domain with alanine, and the effects on motor function were measured. Because charged residues have often been suggested to have important roles in torque generation, conserved charged residues were changed individually and in all pairwise combinations. The results show that three charged residues of FliG, Arg279, Asp286 and Asp287, are directly involved in torque generation. Mutations in these residues cause motility defects that suggest that they function jointly, in an active site whose most important property is a specific arrangement of charges. Two other charged residues, Lys262 and Arg295, may also be involved in torque generation, but are less critical than Arg279, Asp286 or Asp287. Uncharged residues of the FliG motility domain do not appear to have direct roles in torque generation, although some are needed for the stability of the protein or for normal clockwise/counter-clockwise switching. The Mot − mutations of fliG isolated previously by random mutagenesis do not alter the putative active-site residues, but render the proteins abnormally susceptible to proteolysis, suggesting significantly altered conformations or reduced stabilities.

Structural Aspects of Interfacial Adsorption: A CRYSTALLOGRAPHIC AND SITE-DIRECTED MUTAGENESIS STUDY OF THE PHOSPHOLIPASE A2 FROM THE VENOM OF AGKISTRODON PISCIVORUS PISCIVORUS

Recent genetic and structural studies have shed considerable light on the mechanism by which secretory phospholipases A2 interact with substrate aggregates. Electrostatic forces play an essential role in optimizing interfacial catalysis. Efficient and productive adsorption of the Class I bovine pancreatic phospholipase A2 to anionic interfaces is dependent upon the presence of two nonconserved lysine residues at sequence positions 56 and 116, implying that critical components of the adsorption surface differ among enzyme species (Dua, R., Wu, S.-K., and Cho, W. (1995) J. Biol. Chem. 270, 263-268). In an effort to further characterize the protein residues involved in interfacial catalysis, we have determined the high resolution (1.7 A) x-ray structure of the Class II Asp-49 phospholipase A2 from the venom of Agkistrodon piscivorus piscivorus. Correlation of the three-dimensional coordinates with kinetic data derived from site-directed mutations near the amino terminus (E6R, K7E, K10E, K11E, and K16E) and the active site (K54E and K69Y) defines much of the interface topography. Lysine residues at sequence positions 7 and 10 mediate the adsorption of A. p. piscivorus phospholipase A2 to anionic interfaces but play little role in the enzyme's interaction with electrically neutral surfaces or in substrate binding. Compared to the native enzyme, the mutant proteins K7E and K10E demonstrate comparable (20-fold) decreases in affinity and catalysis on polymerized mixed liposomes of 1-hexadecanoyl-2-(1-pyrenedecanoyl)-sn-glycero-3-phosphocholine and 1,2-bis[12-(lipoyloxy)dodecanoyl]-sn-glycero-3-phosphoglycerol, while the double mutant, K7E/K10E, shows a more dramatic 500-fold decrease in catalysis and interfacial adsorption. The calculated contributions of Lys-7 and Lys-10 to the free energy of binding of A. p. piscivorus phospholipase A2 to anionic liposomes (-1.8 kcal/mol at 25 degrees C per lysine) are additive (i.e. -3.7 kcal/mol) and together represent nearly half of the total binding energy. Although both lysine side chains lie exposed at the edge of the proposed interfacial adsorption surface, they are geographically remote from the corresponding interfacial determinants for the bovine enzyme. Our results confirm that interfacial adsorption is largely driven by electrostatic forces and demonstrate that the arrangement of the critical charges (e.g. lysines) is species-specific. This variability in the topography of the adsorption surface suggests a corresponding flexibility in the orientation of the active enzyme at the substrate interface.

RECENT DEVELOPMENTS IN GUN PROPELLANTS AND GUN PROPULSION RESEARCH AT S.N.P.E.

This paper describes recent developments in gun propellants and gun propulsion research from the energetic material point of view, performed under French MOD sponsorship and directed toward providing technical solutions to new weapon system requirements. The three main types of solid propellants on which research is conducted at SNPE are: conventional nitrocellulose-based and multibased propellants, composite gun propellants based on inert binder and composite gun propellants based on energetic binder. Recent results obtained for each kind are shown and discussed. Common granulation propellants cannot offer required performance improvement. Modular charges, or charges in two pieces for large caliber guns, need higher loading density to get good ignition, to minimize pressure waves, and to reach required performance levels. Therefore, SNPE performed some studies on new partially-cut multi-perforated propellant sticks ("kerfed" sticks). Some results are presented showing their advantages in better ignition of a modular charge and vulnerability reduction. For future 45 mm medium caliber guns, consolidated charges obtained by agglomeration of propellant grains with an energetic binder, provide up to a 40 % increase in volumetric loading. This configuration of grains allows caseless ammunition manufacturing and also charge arrangement for telescoped projectiles. The paper also describes recent improvements in theoretical dimensioning tools developed by SNPE, which help in diagnosis, research orientation, and firing behavior prediction. The thermodynamics code BAGHEERA has been adopted by NATO, the chemical kinetic code BALOO takes into account new propellant types and gun barrel wear; the interior ballistic code family MOBIDIC has been selected at the French national level for future research. The last includes a version with internal boundaries and is able to take into account discontinuities.

Mobility and quantum lifetime in a GaAs/AlGaAs heterostructure: tuning of the remote-charge correlations

We demonstrate that different spatial correlations among remote impurity charges in the GaAs/AlGaAs quantum well lead to different values of (i) the transport-relaxation time, (ii) the single-particle relaxation time, and that (iii) the different width of the integer quantum Hall plateauxs could be achieved. These findings supply qualitatively new information about the important contribution of the non-random arrangement of remote charges to the transport properties of semiconductor heterostructures.

Spatial correlations of In-donor charges in CdTe layers

We present experimental evidence that indium donors in CdTe localize electrons in a spatially correlated manner. We have studied the Hall mobility, μH, as a function of the electron concentration, nH, at T = 77 K. Three methods leading to changes of nH have been used (i) high pressure freeze-out of electrons onto localized states of In-donors, (ii) the persistent photoconductivity effect, and (iii) a subsequent annealing of the sample. Each method causes a related modification in a degree of spatial correlations in the arrangement of donor charges. As a consequence, different values of μH correspond to the same value of nH.

Polyelectrolyte electrostatics: Salt dependence, entropic, and enthalpic contributions to free energy in the nonlinear Poisson–Boltzmann model

AbstractUsing the nonlinear Poisson–Boltzmann (PB) equation, expressions for the nonspecific salt dependence of the free energy, and the entropic and enthalpic contributions to the electrostatic free energy of a polyelectrolyte are derived. The results are generalized for any number and valence of mobile ions, and now permit a more rigorous treatment of the salt dependence of polyelectrolyte–ligand binding using the PB equation. The salt dependence of the electrostatic free energy depends on an “osmotic pressure like” term identified previously [K. Sharp and B. Honig (1990), Journal of Physical Chemistry, Vol. 94, pp. 7684–7692], which in turn depends on the integrated excess/deficit of all mobile ion species around the polyelectrolyte. For the simple salt case (two ions only) this expression is equivalent to that derived previously [H. Eisenberg (1976), Biological Macromolecules and Polyelectrolytes in Solution, Clarendon. Oxford] in terms of Donnan, or preferential interaction coefficients. Moreover, since all ions enter into the PB expression for the salt dependence of the free energy in an identical way, it is equally applicable to polyelectrolytes with charge of one sign (e.g., nucleic acids), with oppositely charged groups (such as proteins), and even polyelectrolytes that have no net charge, for which the Donnan coefficient is ill‐defined. The distribution of counterions around three simple polyelectrolyte geometries—spherical, cylindrical, and planar—was examined. In each case the excess number of counterions associated with the polyelectrolyte is not constant, but increases with decreasing salt. Although a limiting value is reached at low salt for a cylinder, in agreement with the limiting law condensation models, for the spherical geometry the number of associated counterions decreases to zero, while for the planar case it increases to 100%. For the cylindrical case Donnan coefficients agree closely with those computed previously by Monte Carlo simulations [M. C. Olmstead, C. F. Anderson, and M. T. Record (1991) Biopolymers, Vol. 31, pp. 1593–1604]. An all atom B‐DNA model shows about 5% less accumulation of counterions than a cylinder, due to the helical charge arrangement and groove structure. The entropic and enthalpic contributions to the salt‐dependent free energy of a cylinder were examined in the PB model. In addition to the well known ion cratic (redistribution) entropy, there are significant contributions from electrostatic enthalpy, and from dielectric entropy (water re‐orientation due to the electrostatic field from polyelectrolyte charges and salt ions). The latter entropy contribution has not previously been taken into account in salt‐dependent polyelectrolyte phenomena, but at higher salt concentrations (> 0.01M), this can be as large as the cratic entropy contribution. © 1995 John Wiley & Sons, Inc.

Utopia dielectrica

The dielectric constant of a material is a macroscopic property that measures the reduction of the electrostatic forces between charged plates separated by the material, compared to a vacuum as intermediate material. It is next encountered as a scaling parameter in Coulomb's law for interacting charges, not only in the force, but also in the energy. In deriving the theory for dielectrics, the macroscopic nature is essential: Only then is the basic assumption that the dielectric material is homogeneous and isotropic a valid one. The appearance of the dielectric constant as a simple scaling factor in Coulomb's law has tempted many computational chemists to forget about the macroscopic nature of the dielectric and to apply the screened Coulomb's law between charges, supposedly in a low-dielectric medium such as proteins, in microscopic force fields. Optimization of force fields even led to distance-dependent “dielectric constants.” Another use of the dielectric constant appears in the dielectric continuum reaction field approaches for the computations of solvation energies and solvent effects. The solute is embedded in a cavity surrounded by the dielectric. Specific interactions between solvent molecules and solute are thus neglected. The cavity size and dielectric constants of interior and exterior are optimized for the model. The aim of this article is to show, by means of calculations on interacting point charges embedded in cavities surrounded by dielectrics and microscopic models of “low-dielectric” materials by explicit polarizabilities, that as far as the dielectric “constant” is concerned anything can happen, depending on the nature of the charges, the distance to the cavity boundary, the spatial arrangement of charges, and polarizabilities. Thus, a warning is issued to injudicious use of dielectric models in microscopic calculations. © 1995 John Wiley & Sons, Inc.

Multicentric charges for the accurate representation of electrostatic interactions in force-field calculations for small molecules

Abstract A strategy to define multicentric electrostatic charges is presented. It is based on the placement of potential-derived charges at the atomic nuclei, but also at other clearly defined positions. In general, extra charges are located near the nuclei, and often along chemical bonds, which facilitate their implementation in force fields. The use of multicentric charges notably improves the ability of Coulombic molecular electrostatic potentials ( MEPs) to reproduce quantum mechanical values. The strategy seems especially useful for the classical study of condensed phases and in general for the electrostatic representation of any small, rigid molecule.

Self-consistent calculations of edge channels in laterally confined two-dimensional electron systems.

We calculate the distribution of incompressible and compressible regions at an electrostatically defined edge of a two-dimensional electron system (2DES) in a strong perpendicular magnetic field B for two models, one with a strictly 2D arrangement of gate and charges, and one with a realistic 3D structure. We ensure electrochemical equilibrium by self-consistent calculation of the electrostatic potential from the electron density and the boundary conditions defined by the metallic gates, and of the electron density in this potential, using the Thomas-Fermi approximation. Both models yield qualitatively the same results. In the limit of zero temperature and strong magnetic field, and at distances from the edge which are much larger than the effective Bohr radius, our 2D model yields quantitatively the same result for the density profile in the 2DES as recent work by Chklovskii, Shklovskii, and Glazman, who considered a purely electrostatic model, without requiring electrochemical equilibrium for B=0. We demonstrate explicitly how the incompressible regions are destroyed with increasing temperature.

Electrostatic droplet generation: Mechanism of polymer droplet formation

AbstractThe mechanism of alginate droplet formation and experimental parameters for producing very small polymer microbeads (less than 100 μm dia.) using an electrostatic droplet generator studied showed that the microbead size was a function of needle diameter, charge arrangement (electrode geometry and spacing) and strength of electric field. Perfectly spherical and uniform polymer beads, 170 μm dia., for example, were obtained at a potential difference of 6 kV with a 26‐gauge needle and an electrode distance of 2.5 cm. Increasing the electric field, and thus the surface charge in the vicinity of the needle, by increasing the applied potential, resulted in needle oscillation, giving a bimodal bead size distribution with a large fraction (30–40%) of microbeads with a mean diameter of 50 μm. The process of alginate droplet formation under the influence of electrostatic forces assessed with an image analysis/video system revealed distinct stages. After a voltage was applied, the liquid meniscus at the needle tip was distorted from a spherical shape into an inverted cone‐like shape. Consequently, alginate solution flowed into this cone at an increasing rate causing formation of a neck‐like filament. When this filament broke away, producing small droplets, the meniscus relaxed back to a spherical shape until flow of the polymer caused the process to start again. A large‐scale multineedle device with a processing capacity of 0.7 L/h was also designed and produced uniform 400 ± 150 μm microbeads.

Fluorescence lifetimes of substituted indoles in solution and in free jets: evidence for intramolecular charge-transfer quenching

Fluorescence lifetime measurements for a variety of substituted indoles were taken in cyclohexane and in supersonic gas expansions. When the two data sets were compared, a strong trend became evident. Solvent polarizability effects can be seen as stabilizing intramolecular charge-transfer quenching processes in the substituted indoles that are stimulated by the correct placement of charge accepting/releasing substituents

An approach to improve blasting efficiency in an iron ore mine

ABSTRACT This paper presents an approach to improve overall blasting efficiency in an iron ore mine. The problems existing in the mines were oversize fragmentation, severe backbreak, frequent secondary drilling and blasting operations, sloping bench faces and irregular bench profiles. Thorough analysis of the blasting practices and results was done in the search of key parameters affecting blasting efficiency. The parameters were burden, stemming length, delay period between the rows, placement of booster charge, booster ratio and firing pattern. Optimum blast geometry was evolved by making proper adjustments of the above parameters during trial blasts.

Coulomb versus Madelung Gap: Ordering in a System of Point Charges

Spatial correlations of impurity charges in a mixed valence regime are studied with the use of Monte Carlo simulations. The influence of various kinds of disorder on the one-particle density of states is determined. A continuous transitions from a soft Coulomb gapa (density of states vanishing only at the chemical potential) to a hard gap (with a finite range of energies with vanishing density of states) is found, driven by decreasing amount of built-in disorder in the system. The hard Coulomb gap resembles the Madelung gap, found in crystalline arrangements of charges. The similarity reflects the fact that both the Coulomb and Madelung gaps are manifestations of the same phenomenon, resulting from ordering of the positions of point charges, the only difference being the range of correlations. PACS numbers: 71.55.-i, 72.20.Fr

The arrangement of point charges with tetrahedral and octahedral symmetry on the surface of a sphere with minimum Coulombic potential energy

Up to 100 point charges have been distributed on the surface of a sphere such that the configurations display T and O symmetry as well as being a minimum, global or local, with respect to the Coulombic potential.

Electric potential gradient changes during explosive activity at Sakurajima volcano, Japan

We report electric potential gradient measurements carried out at Sakurajima volcano in Japan during: (1) explosions which generated ash plumes, (2) steam explosions which produced plumes of condensing gases, and (3) periods of ashfall and plume-induced acid rainfall. Sequential positive and negative deviations occurred during explosions which generated ash plumes. However, no deflections from background were found during steam explosions. During periods of ashfall negative electric potential gradients were observed, while positive potential gradients occurred during fallout of plume-induced acid rain from the same eruption. These results suggest that a dipole arrangement of charge develops within plumes such that positive charges dominate in the volcanic gas-rich top and negative charges in the following ash-rich part of the plume. The charge polarity may be reversed for other volcanoes (Hatakeyama and Uchikawa 1952). We suggest that charge is generated by fracto-emission (Donaldson et al. 1988) processes probably during magma fragmentation within the vent, rather than by frictional effects within the plume.