Electrostatic Image Forces Research Articles

Adhesion forces of radioactive particles measured by the Aerodynamic Method–Validation with Atomic Force Microscopy and comparison with adhesion models

Median adhesion forces of tritiated tungsten micro-particles deposited on a glass substrate were successfully determined using an aerodynamic method (AM) which is presented in this paper. The original aerodynamic device built for these experiments has been carefully characterized in terms of friction velocities allowing to quantify aerodynamic torque exerted on the particles and to deduce median adhesion forces thanks to a force balance approach. Using the same particle/surface systems (non-radioactive tungsten particles in contact with a glass substrate), distribution of adhesion forces were obtained using AFM for comparison with the AM. The results show a good agreement between the two techniques which allowed to validate the AM. Furthermore, a precise description of the root-mean square roughness (rms) distribution of the glass substrate made it possible to compare the experimental results with different analytical adhesion force models. Integrating the rms roughness distribution of the substrate into the model of Rabinovich et al. showed the best agreement with the present experiments capturing most of the adhesion forces of 10µm to 18µm diameter tungsten particles. Moreover, the method developed in this work made it possible to show that the electrostatic image force arising from the self-charging of tritiated tungsten particles has a negligible contribution in the adhesion of the particles for the studied configuration.

Charged Colloids at the Metal–Electrolyte Interface

We discuss the peculiarities of the structure of the interface between a metal and a stable colloidal dispersion of charged nanoparticles in an electrolyte. It is demonstrated that a quasi-2D ionic structure of elevated density arises in its vicinity due to the effect of electrostatic image forces. The stabilized colloidal particles, being electroneutral and spatially distributed objects in the bulk of the electrolyte and approaching the interface, are attracted to it. In their turn, the counterions forming their coat partially retract into the 2D-layer, which results in an acquisition by the colloidal particle of the effective charge eZ*≫e and which, together with its mirror image, creates the electric dipole. The formed dipoles, possessing the moments directed perpendicularly to the interface, form the gas with repulsion between particles. The intensity of this repulsion, evidently, depends on the value of the effective charge eZ* acquired by the nanoparticle having lost a number of counterions. It can be related to the value of the excess osmotic pressure Posm measured in the experiment. On the other hand, this effective charge can be connected by means of the simple geometric consideration with the structural charge eZ of the nanoparticle core being in the bulk of the electrolyte.

Electrostatic image force energy for charges in three-layer structures: exact formulas and their approximations

The problem of image force energy W(Z) in three-layer plane structures, where Z is the coordinate perpendicular to the layers, has been reconsidered. In the classical electrostatic limit, where the dielectric permittivities ɛ i of all structure components (i = 1, 2, 3) are constants, the exact general dependences W(Z) were obtained for each layer and any ɛ i -combination in terms of the Lerch transcendent function. For certain combinations of ɛ i , an ion adsorption minimum was found to arise in one of the covers far from the interlayer. Some other combinations of ɛ i can lead to the appearance of a potential barrier, which does not permit a free charge existing in the cover to approach the interlayer, although it will be attracted to the interlayer in the close vicinity of the latter. For symmetric structures (ɛ 1 = ɛ 3), the asymptotic behavior of was shown to be rather than , as it takes place in the two-layer case. Simple approximate analytical formulas that describe W(Z) and possess high accuracy for arbitrary relationships among the ɛ i -constants were proposed.

Orientation of adsorbed polar molecules (dipoles) in external electrostatic field

A model is proposed in the framework of classical electrostatics to describe the behavior of an adsorbed polar molecule near the plane interface between two insulators under the action of an external electrostatic field. The molecule is considered as a permanent point dipole that polarizes the interface and interacts with it through electrostatic image forces. The latter and the applied field try to reorient the dipole in a competitive manner. The system behavior turns out to be rather complicated: it may show a bistable character with a hysteresis (a switch). Such a switch can serve as an element in a memory network made of adsorbed molecules.

Bound State of an Electron in a MOS Structure Due to the Spin–Orbit Interaction

It is shown that two electrons situated in a quantum well near a metallic electrode are attracted to each other due to the Bychkov–Rashba spin–orbit interaction (SOI) and electrostatic image forces. For quite accessible values of the characteristic parameters of the system, the effective attraction due to the SOI dominates over the Coulomb repulsion, and the formation of a bielectron becomes possible. The theory of the effect is especially simple and clear in the case of a quantum wire. The binding energy of the electron pair significantly increases under the application of a gate voltage of appropriate polarity.

Direct measurement of the electrostatic image force of a levitated charged nanoparticle close to a surface

Short-range forces have important real-world relevance across a range of settings in the nano world, from colloids and possibly for protein folding to nano-mechanical devices, but also for detection of weak long-range forces, such as gravity, at short distances and of candidates to solve the problem of dark energy. Short-range forces, such as Casimir-Polder or van der Waals are in general difficult to calculate as a consequence of their non-additive nature, and challenging to measure due to their small magnitude - especially for charged particles where dispersion forces are normally many orders of magnitude smaller than electrostatic image forces. Therefore short-range forces have represented a continuing theoretical and experimental challenge over the last half-century. Here we report on experiments with a single glass nanoparticle levitated in close proximity to a neutral silicon surface in vacuum, which allow for direct measurement of short-range forces in a new distance and sensitivity regime - outperforming existing force microscopies.

Bielectron Formed in a 2D System by the Spin–Orbit Interaction and Image Forces

It has been shown that two electrons situated in a quantum well near a metallic electrode attract each other owing to the Bychkov–Rashba spin–orbit interaction (SOI) and by electrostatic image forces. It has been shown within a simple model that the SOI-induced effective attraction is stronger than the Coulomb repulsion and the formation of a bielectron becomes possible at quite accessible values of the characteristic parameters of the system.

Electron states localized above the ellipsoidal surface of a nanoparticle in a homogeneous magnetic field

We have performed a theoretical study of the effect of a uniform magnetic field on electron states localized near the surface of an ellipsoidal germanium nanoparticle by electrostatic image forces in a vacuum. We study the effects of resonance interaction of optical radiation with such local electron states both in the presence, and in the absence, of a uniform magnetic field. The results obtained may be used to develop new optical and magneto-optical techniques for diagnosis of nanostructures.

Bipolaron Mechanism of High-Temperature Superconductivity of Ammonia Systems

In the present paper, we propose an explanation of the ambiguity of the results of experiments on the study of high-temperature superconductivity of ammonia systems. At the heart of the theoretical interpretation of the experiments, we put the bipolaron model. In this study, we have shown mathematically that the barrier of repulsion between polarons can be effectively reduced if the polarons are in the macroscopic dielectric layers, or capillaries. We constructed the theory of polaron states in the macroscopic dielectric layers. We specify the conditions under which the polarons are hold in the layer between dielectrics. It was found that the electrostatic image forces lead to the appearance of additional forces of attraction between polarons. These forces are conditioned by oscillations of polarons around the position of their fixation. Derivations are given of the upper and lower limits on the width of the gap in which the polaron oscillations are not suppressed. In this case take place disappearance Coulomb repulsion of the polarons. A long-range resonant interaction of two oscillators resulting in the appearance of effective attraction between polarons is discussed. This leads to the formation of diamagnetic singlet bipolarons due to quantum exchange interactions and the effects of electron-electron correlations. For glass capillaries (quasi-one-dimensional bipolaron) and for gap between glass plates (quasi-two-dimensional bipolaron) we give quantitative estimates of the gap width and the critical temperature at which there is a barrier-free formation of the bipolaron in ammonia. Numerical estimates are obtained for a case of the bipolaron in ammonia. We got a quantitative evaluation, which indicate that the barrier-free formation of singlet bipolaron in ammonia begins at temperatures below 80K. As the experiment showed the electrical resistance of ammonia systems decreases abruptly by 10-12 orders of magnitude in this temperature range. At the same time, experiments have shown that for the bulk superconductivity superconducting phase is only ~ 0.01%.

Local Electron States in Ellipsoidal Semiconductor Nanosystems in Homogeneous Magnetic Field: Theory

The influence of a homogeneous magnetic field on the electron states localized over the surface of an ellipsoidal semiconductor (dielectric) nanoparticle by the electrostatic image forces is studied theoretically. The effects of the resonant interaction of light with such local electron states in presence and in absence of a homogeneous magnetic field are investigated.

Local electron states in ellipsoidal nanosystems in homogeneous magnetic field

The influence of a homogeneous magnetic field on the electron states localized over the surface of an ellipsoidal dielectric particle by the electrostatic image forces is studied theoretically. The effects of the resonant interaction of light with such local electron states in presence and in absence of a homogeneous magnetic field are investigated.

Electrostatic Charge Effects on Pharmaceutical Aerosol Deposition in Human Nasal–Laryngeal Airways

Electrostatic charging occurs in most aerosol generation processes and can significantly influence subsequent particle deposition rates and patterns in the respiratory tract through the image and space forces. The behavior of inhaled aerosols with charge is expected to be most affected in the upper airways, where particles come in close proximity to the narrow turbinate surface, and before charge dissipation occurs as a result of high humidity. The objective of this study was to quantitatively evaluate the deposition of charged aerosols in an MRI-based nasal–laryngeal airway model. Particle sizes of 5 nm–30 µm and charge levels ranging from neutralized to ten times the saturation limit were considered. A well-validated low Reynolds number (LRN) k–ω turbulence model and a discrete Lagrangian tracking approach that accounted for electrostatic image force were employed to simulate the nasal airflow and aerosol dynamics. For ultrafine aerosols, electrostatic charge was observed to exert a discernible but insignificant effect. In contrast, remarkably enhanced depositions were observed for micrometer particles with charge, which could be one order of magnitude larger than no-charge depositions. The deposition hot spots shifted towards the anterior part of the upper airway as the charge level increased. Results of this study have important implications for evaluating nasal drug delivery devices and for assessing doses received from pollutants, which often carry a certain level of electric charges.

Local Electron States in Ellipsoidal Nanosystems in Magnetic Field: Theory

The influence of a homogeneous magnetic field on the electron states localized over the surface of an ellipsoidal semiconductor (dielectric) nanoparticle by the electrostatic image forces is studied theoretically. The effects of the resonant interaction of light with such local electron states in presence and in absence of a homogeneous magnetic field are investigated.

A note on image potentials at dielectric interfaces and their possible relevance in organic electronics

This note recalls the existence of electrostatic image potentials and forces, which may be large, at dielectric–dielectric interfaces, and discusses possible consequences in organic electronics (OLEDs and photovoltaic cells).

The estimation of size and position of contaminating particle adhering to the insulating spacer surface in gas-insulated systems

The presence of metallic particles has been recognized as a dangerous threat in gas-insulated substation (GIS). Such particles are initially free and move toward higher electric field regions such as triple junction i.e., spacer-electrode-gas interface. However, once these particles reach the spacer surface, they adhere to the spacer easily due to electrostatic image forces. From insulation point of view, the triple junction is the weakest point in GIS. The presence of such metallic particles on the spacer surface deteriorates the insulation strength. Thus, in order to improve the reliability of GIS, it is important to identify the size and the position of the particle adhering to the insulating spacer surface. One of the most promising methods to carry out such identification is by recognizing the partial discharges (PDs) provoked by such particles. This paper is aimed to discuss the particle size and position estimation by using the PD patterns and statistical analysis. The PD patterns were acquired using IEC 60270 method. Measurements were made to determine various PD signals caused by particle of different sizes at different locations on the spacer surface. The acquired PD patterns were characterized by a number of statistical parameters. The results show that the implemented technique could be used to distinguish between various particle sizes and positions at different SF6 pressures with a fairly high accuracy.

Image Charge Effects on the Formation of Pickering Emulsions.

Vigorous mixing of an aqueous particle dispersion with oil usually produces a particle-stabilized emulsion (a "Pickering emulsion"), the longevity of which depends on the particles' wetting properties. A known exception occurs when particles fail to adsorb to the oil-water interface created during mixing because of a strong repulsion between charges on the particle surface and similar charges on the oil-water interface; in this case, no Pickering emulsion is formed. Here, we present experimental evidence that the rarely considered electrostatic image force can cause a much bigger hindrance to particle adsorption and prevent the formation of Pickering emulsions even when the particle interaction with the interface charge is attractive. A simple theoretical estimate confirms the observed magnitude of this effect and points at an important limitation of Pickering emulsification, a technology with widespread industrial applications and increasing popularity in materials research and development.

Role of dipole image forces in molecular adsorption

Electrostatic image force energy W was calculated for a finite-size (extended) dipole located in vacuum near a plane surface of a condensed-matter substrate using the HF molecule over graphite as an example. The spatial dispersion of substrate static dielectric permittivity was taken into account, with the contributions of both free and bound charge carriers being considered in various realistic models. The dependences of W on the distance z between the dipole and the substrate were shown to be finite at all z’s, contrary to the classical point-dipole case. An existence of a crossover between the preferable normal and planar orientations of extended dipole with respect to the surface was found. The applicability of point-dipole approximation for the calculation of W(z) was discussed. Numerical quantum chemical calculations were carried out for the HF molecule near two graphite layers. The results obtained confirm the validity of non-local electrostatic approach beyond the region of Pauli repulsion (in the closest vicinity to the interface). On the other hand, at large distances z, the quantum chemical consideration becomes less reliable owing to the computational restrictions, whereas the electrostatics preserves its capabilities and demonstrates, in particular, the subtle orientation crossover.

Van der Waals interactions between silica spheres and metallic thin films created by e-beam evaporation

The long-range and adhesion forces between a molecularly smooth silica sphere and a relatively smooth thin metallic film have been measured by atomic force microscopy (AFM) operating in contact mode. The majority of the dispersion force measurements have been done with root mean square (RMS) roughness of more than 10nm. This paper investigates lower roughness to prove or disprove theory of the van der Waals force for irregularly distributed size of asperities. Metallic films of Cu, Al, and Ag were obtained by coating silicon wafers using an electron beam evaporation (e-beam) method. The e-beam deposition rates were within the range of 0.6–1.2Å/s at vacuum pressures between 2×10−6 and 3×10−6mbar. The developed coatings had peak-to-peak distance ranges of 80–120nm and 1–2μm. Long-range attractive forces were obtained from the extending portion of the force/distance curves measured between a silica particle attached to the AFM cantilever and the metallic film surface. Certain samples demonstrated an attractive force much larger than the theoretically calculated dispersion force, which we attribute to an electrostatic image force. For other samples, the extending curves were comparable with the theoretical dispersion forces, especially when the curves were shifted by a distance of 1.82RMS. Adhesion forces were measured from the retracting portion of the force/distance curves. For peak-to-peak distance near 90nm, films having an RMS roughness of 0.5–3nm displayed a force reduced by a factor of 3–40 from that predicted by theory. This effect of the surface roughness supports a DMT model for sample interaction at a single contact point after particle detachment. We also demonstrate the absence of any capillary forces under relative humidities up to 40%. It was explained by the thickness of wetting film smaller than the peaks height.

ADHESION OF FOOD POWDERS WITH NONELECTROSTATIC AND ELECTROSTATIC COATING

ABSTRACTThis study investigated the effects of powder resistivity, coating voltage, relative humidity (RH) and coating density on adhesion. Cocoa powder with a high resistivity (1.15 × 1013 Ωm) showed a stronger electrostatic adhesion than starch powder with a medium resistivity (2.56 × 1010 Ωm) and NaCl powder with a low resistivity (7.31 × 105 Ωm). The adhesion of starch and cocoa powders coated at 0, 40 and 95 kV increased with increasing voltage. The adhesion at 0 kV should be dominated by the van der Waals force, at 40 and 95 kV by the electrostatic image force and at high RH by capillary force. Theoretical calculations were in the correct range, but the assumptions in those calculations make them unreliable. For nonelectrostatic coating (0 kV), there was no significant change in adhesion when RH increased from 30 to 60%, while adhesion increased when RH increased from 60 to 80%. For electrostatic coating, the adhesion decreased when RH increased from 30 to 60%, but the adhesion at 80% RH was larger than the adhesion at 30 and 60% RH. For both nonelectrostatic and electrostatic coating, the adhesion force decreased as coating density increased to 1.0 mg/cm2, but there was no significant change from 1.0 mg/cm2 to 2.0 mg/cm2.PRACTICAL APPLICATIONSThe adhesion of seasoning powders on snack foods plays an important role in the product quality, but insufficient adhesion between food powders and the surface of snacks results in the seasonings falling off the snacks, leading to a poor distribution and waste of seasonings. This study suggests that the adhesion between food powders and targets can be effectively improved by controlling the properties of the powders, as well as optimizing coating conditions. For example, using food powders with a high resistivity and high voltage during electrostatic coating and increasing relative humidity will increase adhesion.

Mitigation of lunar dust adhesion by surface modification

Dust has been recognized as one of the greatest hazards in continued lunar exploration due to its tenacious adhesion to everything with which it comes into contact. Unfortunately, there is little known about the mechanisms of adhesion on widely varying surface types: van der Waals and electrostatic forces are the dominant forces under consideration here. Surface energy, roughness, mechanical properties and electronic properties are all known to contribute to the adhesion characteristics. An optimal solution to mitigate dust adhesion would be to identify the dominant components of the adhesive force and to reduce that force by surface modification. In this study, an ion beam process was used to modify (treat) the surfaces of three dramatically different materials spanning the range of conductor (black Kapton), semiconductor (silicon), and insulator (quartz). Adhesive forces between less than 25 μ m JSC-1 lunar simulants and these virgin/treated surfaces were measured in vacuum using a centrifugal force detachment method. We found that JSC-1 particles adhered less to treated silicon and quartz surfaces, correlated with a reduction in van der Waals force due to a reduced surface energy. The large reduction in adhesion for treated black Kapton is mainly due to the large decrease in the electrostatic (image) force that results from reduced contact charging. Materials in space and on the lunar surface will be directly exposed to high-energy ultraviolet radiation prior to being covered by dust, so the UV irradiation effects on surface adhesion were also examined. Both virgin and treated quartz surfaces are most affected by the UV-irradiation, showing dramatically increased adhesion.