Point-like Model Research Articles

Overcharging and Free Energy Barriers for Equally Charged Surfaces Immersed in Salt Solutions.

The stability of dispersions containing charged particles may obviously be regulated by salt. In some systems, the effective charge, as measured by the potential some small distance away from the particles, can have a sign opposite to the bare surface charge. If charge reversal takes place, there is typically a salt concentration regime within which colloidal stability increases with added salt. These experimental findings on dispersions have been corroborated by atomic force microscopy investigations, where an attraction is found at short separations. This attraction is stronger than expected from standard DLVO theory, and there has been considerable debate concerning its origin. In this work, we use simple coarse-grained models of these systems, where the bare surfaces carry a uniform charge density, and ion-specific adsorption is absent. Our hypothesis is that these experimental observations can be explained by such a simplistic pure Coulomb based model. Our approach entails grand canonical Metropolis Monte Carlo (MC) simulations as well as correlation-corrected Poisson-Boltzmann (cPB) calculations. In the former case, all ions have a common size, while the cPB utilizes a point-like model. We devote significant attention on apparent surface charge densities and interactions between large flat model surfaces immersed in either a 2:1 salt or a 3:1 salt. In contrast to most of the previous theoretical efforts in this area, we mainly focus on the weak long-ranged repulsion and its connection to an effective surface charge. We find a charge reversal and a concomitant development of a free energy barrier for both salts. The experimentally observed nonmonotonic dependence of colloidal stability on the salt concentration is reproduced using MC simulations as well as cPB calculations. A strong attraction is observed at short range for all investigated cases. We argue that in our model, all non-DLVO aspects can be traced to ion–ion correlations.

Solar sailing near the Sun: A comparison study between point-like and spherical-like Sun models

A study on the behavior of flat and spherical solar sails near the Sun is performed for two Sun models, a point-like model and a spherical-like model. The study is carried on by taking into account two prominent effects near the Sun, Poynting-Robertson effect for high velocities and variation in solar disk intensity. Limb darkening effect has been applied as a model for the variation in solar disk intensity over the wavelength λ∈[302.5,997.5]nm. Acceleration equations for absorption and specular reflection has been developed analytically for spherical sails and flat sails at normal orientation for the spherical model with limb darkening and Poynting-Robertson effect. As a result of the spherical Sun model, in general, escape trajectories show a large decrease in solar sail speed and overall distance in comparison with the point model. Furthermore, for heliocentric bound orbits, large degradation in radial distance over the period of the solar sail mission is found, accompanied with trajectory oscillation. Between the two solar sail configurations, at close encounter to the Sun, the optimal choice is dependent on the possible values of reflectivity and absorptivity.

Influence of the nuclear charge distribution and electron correlation effects on magnetic shieldings and spin-rotation tensors of linear molecules.

The nuclear charge distribution effects (NChDE) on two response properties, the NMR magnetic shielding (σ) and the nuclear spin-rotation (SR) constants (M), are analyzed. We do it employing point-like and Gaussian-like models for describing the nuclear charge density of three linear molecules: HBr, HI and HAt. According to our results, both properties are sensitive to the NChDE. We show that the NChDE are almost completely relativistic, i.e., they nearly vanish in the non-relativistic limit of both properties. We calculated the NChDE on σ and M, and analyzed the differences between them in terms of a relativistic relation between these two properties. Using that relation we found that the electronic core mechanisms are the main ones for the NChDE on the shielding of nuclei of both, molecules and free atoms. The NChDE are smaller on SR constants than on shieldings. Nevertheless, within the relativistic polarization propagator formalism at the RPA level of approach they are very important for SR constants of nuclei in heavy-atom-containing compounds. Astatine in HAt has the largest influence: MAt = −9.95 kHz for a point-like model and −50.10 kHz for a Gaussian-like model. Correlation effects must be included and we do it using different DFT schemes. The PBE0 functional gives results that are closest to experiments for Br and I, though the LDA gives the closest for hydrogen. The value of the SR constant of At is reduced among 350 kHz and 500 kHz from its RPA value, when different and usual functionals are applied. Given that the NChDE on M and σ are mostly relativistic in their origin, these effects are also dependent on electron correlation. They have also a nonvanishing dependence with the Gaunt electron–electron interactions.

Spin motion determination of the Envisat satellite through laser ranging measurements from a single pass measured by a single station

The Satellite Laser Ranging (SLR) technology is used to accurately determine the position of space objects equipped with so-called retro-reflectors or retro-reflector arrays (RRA). This type of measurement allows to measure the range to the spacecraft with high precision, which leads to determination of very accurate orbits for these targets. Non-active spacecraft, which are not attitude controlled any longer, tend to start to spin or tumble under influence of the external and internal torques and forces.If the return signal is measured for a non-spherical non-active rotating object, the signal in the range residuals with respect to the reference orbit is more complex. For rotating objects the return signal shows an oscillating pattern or patterns caused by the RRA moving around the satellite’s centre of mass. This behaviour is projected onto the radial component measured by the SLR.In our work, we demonstrate how the SLR ranging technique from one sensor to a satellite equipped with a RRA can be used to precisely determine its spin motion during one passage. Multiple SLR measurements of one target over time allow to accurately monitor spin motion changes which can be further used for attitude predictions. We show our solutions of the spin motion determined for the non-active ESA satellite Envisat obtained from measurements acquired during years 2013–2015 by the Zimmerwald SLR station, Switzerland. All the necessary parameters are defined for our own so-called point-like model which describes the motion of a point in space around the satellite centre of mass.

Polarizability expressions for predicting resonances in plasmonic and Mie scatterers

Polarizability expressions are commonly used in optics and photonics to model light scattering by small particles. Models based on Taylor series of the scattering coefficients of the particles fail to predict the morphologic resonances hosted by dielectric particles. Here we propose to use the factorization of the special functions appearing in the expression of the Mie scattering coefficients to derive pointlike models. These models can be applied to reproduce both Mie resonances of dielectric particles and plasmonic resonances of metallic particles. They provide simple but robust tools to predict accurately the electric and magnetic Mie resonances in dielectric particles.

A three-compartment conductance-based model of the rat olfactory receptor neuron

Reduced compartmental models of single neurons, i.e. models with few Hodgkin-Huxley (HH) type compartments, provide an interesting balance between highly detailed, morphologically realistic models, and abstract, point-like models [1]. Simoes-de-Souza and Roque [2] constructed a four-compartment model of the vertebrate olfactory receptor neuron (ORN), which was used in a large-scale simulation of the olfactory system [3]. Their model contains a biochemical compartment, which allows the study of the role of molecular pathways in odor representation. However, the use of this compartment slows down computer simulations of models with many receptor cells and makes the analysis of network behavior more complicated – because of the large number of variables and parameters. In this work, we present a simpler ORN model which does not include biochemical pathways and contains only three HH type compartments. Our model is different from [2] not only because it has one compartment less but also because its passive and active parameters have new values adjusted by a combination of “hand-fitting” with automatic fitting procedures so as to reproduce well experimental results for the rat [5]. The model was constructed in GENESIS [4]. Its three compartments kept the same names used in [2]: soma, dendrite and dendritic knob. The latter two are passive compartments while soma has four voltage-gated ionic currents: high-voltage-activated calcium current, fast voltage- and calcium-dependent potassium current, slow calcium-dependent potassium current, and delayed rectifier potassium current. These ionic currents were modeled according to the standard HH formalism with kinetic parameters adjusted as mentioned above. Input stimuli were constant current steps injected directly in dendritic knob to simulate experimental results [5]. Some simulation results are shown in Figure ​Figure1.1. Our reduced three-compartment model may allow faster and more efficient large-scale network simulations of the rat olfactory system. Figure 1 Some simulation results. A. Spike train generated by a 10 pA step current (to be compared with Figure 2B of [5]); B. Action potential (to be compared with the inset of Figure 2A of [5]).

Molecular dynamics simulation of vascular network formation

Endothelial cells are responsible for the formation of the capillary blood vessel network. Wedescribe a system of endothelial cells by means of two-dimensional molecular dynamicssimulations of point-like particles. Cell motion is governed by the gradient of theconcentration of a chemical substance that they produce (chemotaxis). The typical time ofdegradation of the chemical substance introduces a characteristic length in thesystem. We show that point-like model cells form network resembling structurestuned by this characteristic length, before collapsing altogether. Successively,we improve the non-realistic point-like model cells by introducing an isotropicrepulsive force between them and a velocity dependent force mimicking the observedpeculiarity of endothelial cells of preserving the direction of their motion (persistence).This more realistic model does not show a clear network formation. We ascribethis partial fault in reproducing the experiments to the static geometry of ourmodel cells that, in reality, change their shapes by elongating toward neighboringcells.

In-medium properties ofΘ+as a KπN structure in relativistic mean field theory

The properties of nuclear matter are discussed with the relativistic mean-field theory (RMF).Then, we use two models in studying the in-medium properties of $\Theta^+$: one is the point-like $\Theta^*$ in the usual RMF and the other is a K$\pi$N structure for the pentaquark. It is found that the in-medium properties of $\Theta^+$ are dramatically modified by its internal structure. The effective mass of $\Theta^+$ in medium is, at normal nuclear density, about 1030 MeV in the point-like model, while it is about 1120 MeV in the model of K$\pi$N pentaquark. The nuclear potential depth of $\Theta^+$ in the K$\pi$N model is approximately -37.5 MeV, much shallower than -90 MeV in the usual point-like RMF model.

Some Features of Spatial Neutron Kinetics for Multiplying Systems

The paper considers some physical aspects of the neutron space kinetics of critical and source-driven subcritical systems. The possibility of introducing some indicators to qualify the spatial nature of neutronic transients is investigated. It is shown theoretically and then proved by numerical examples that the separation of the eigenvalues of the mathematical operator defining the problem can be taken as a good indicator of the importance of space effects in time-dependent conditions. To obtain good physical insight into the phenomena, paradigmatically simple configurations are considered, and whenever possible, a fully analytical approach is used. The presented results evidence the limits of applicability of classic simplified models for transient analyses, such as point kinetics. In a second part, the paper considers the open problem of the choice of the weighting function to be used either for the generation of the kinetic parameters of pointlike models or for the exploitation of quasi-static procedures, analyzing comparatively the effect of different options on the results of transient calculations.

Nonlinear effects in stopping of partially ionized swift heavy ions

We present stopping power calculations for swift heavy ions in a hydrogen gas target. The point-like model is reviewed and compared to a new theory which includes the projectile elastic form factor. It is shown that, even for highly ionized ions, the projectile form factor yields a substantial modification over the standard stopping results.

Off-shell mean-field electromagnetic T-matrix of finite-size spheres and fuzzy scatterers

We introduce a method for computing simple energy-conserving model T-matrices intended for use in multiple-scattering formalisms. Our method is to make separable approximations to the physical scattering potentials. Such a `mean-field' approximation permits a ready analytic evaluation of the T-matrix for a variety of scatterers, even `fuzzy' scatterers (e.g. fractal aggregates). In the small-size limit, our method reduces to the point-like resonant T-matrix introduced by Nieuwenhuizen et al in 1992 thereby demonstrating the intrinsic mean-field nature of their approximation. We discuss causality in both models and show that the separable potential model possesses a wider range of applicability than the point-like model does.

Hidden variables, wholeness, and the two-hole experiment

A very simple proof is given of the fact that no theory based on a pointlike model of particles subject to local processes can reproduce the phenomenon of quantum interference. It is argued that the `wholeness' character, typical of the pilot-wave theory of de Broglie and Bohm, is actually a common feature of all subquantum theories.Riassunto. Si fornisce una semplice dimostrazione del fatto che nessuna teoria basata su un modello di particelle puntiformi soggette a processi locali può riprodurre i fenomeni di interferenza quantistica. Si discute inoltre come le caratteristiche di `wholeness', tipiche della teoria di onda pilota, rappresentino in realtà un tratto comune a tutte le teorie subquantistiche.

The statistics of multiple gravitational lensing and the application of a single lensing approximation

The statistics of multiple gravitational lensing in a locally inhomogeneous universe is taken into account, based on the expansion of the mean amplification into the summation of various lenses and the pointlike models for both background sources and lensing objects. By comparing the deviation of the effects of various lenses from mean amplification, the redshift ranges over which different lenses dominate are investigated. In particular, the accuracy of a single lensing approximation, which has been widely used in previous statistical lensing, is demonstrated. As the application of a single lensing approximation, the lowest correction to the Mattig's relation due to the inhomogeneities in universe and the lowest effect on the number counts of background sources due to amplification bias are reconsidered. It is emphasized the fact that the effect of statistical lensing, even in a single lensing approximation, is of significance for modern observational cosmology.

A fast image simulation technique for high resolution electron microscopy with multicomponent atomic species

A new method has been developed for simulating high resolution electron microscope images of weak phase objects via a digital television frame store system and fast Fourier transforms of a graphical representation of structures containing several atomic species. Here masks are constructed which consist of circular disk regions whose areas are proportional to the scattering power of the different atom types. These masks represent the object transmission function. The method extends the previous work on image computations of monatomic species objects using small point-like model representations of atom positions. Several examples will be given to verify the relative scattering power from masks corresponding to different atomic species such as Y, Ba, Cu, and O, as well as the limitations of this method for representing objects. Image simulations, which are in agreement with experiments, will also be presented for superconducting oxide materials of the form YBa2Cu3O7 using the circular disk method.