Polyelectrolyte Adsorption

The problem of charged polymer chains (polyelectrolytes) as they adsorb on a planar surface is addressed theoretically. We review the basic mechanisms and theory underlying polyelectrolyte adsorption on a single surface in two situations: adsorption of a single charged chain, and adsorption from a bulk solution in $\theta$ solvent conditions. The behavior of flexible and semi-rigid chains is discussed separately and is expressed as function of the polymer and surface charges, ionic strength of the solution and polymer bulk concentration. We mainly review mean-field results and briefly comment about fluctuation effects. The phenomenon of polyelectrolyte adsorption on a planar surface as presented here is of relevance to the stabilization of colloidal suspensions. In this respect we also mention calculations of the inter-plate force between two planar surfaces in presence of polyelectrolyte. Finally, we comment on the problem of charge overcompensation and its implication to multi-layers formation of alternating positive and negative polyelectrolytes on planar surfaces and colloidal particles.Comment: 11 pages, 4 PS figures (Latex/RevTex), submitted to C.R. Acad. Sci (Paris

Nonequilibrium thermal boundary layer in a capillary discharge with an ablative wall

A thermal nonequilibrium region near wall in a capillary discharge is considered. The proposed model suggests that nonequilibrium thermal boundary layer thickness strongly depends on the capillary wall ablation rate. It is shown that the applicability of the thermal equilibrium condition, widely employed in capillary models, is limited to a case with a large ablation rate.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87756/2/114503_1.pd

Plasma flow and plasma–wall transition in Hall thruster channel

In this paper a model of the quasineutral plasma and the transition between the plasma and the dielectric wall in a Hall thruster channel is developed. The plasma is considered using a two-dimensional hydrodynamic approximation while the sheath in front of the dielectric surface is considered to be one dimensional and collisionless. The dielectric wall effect is taken into account by introducing an effective coefficient of the secondary electron emission (SEE), s. In order to develop a self-consistent model, the boundary parameters at the sheath edge (ion velocity and electric field) are obtained from the two-dimensional plasma bulk model. In the considered condition, i.e., ion temperature much smaller than that of electrons and significant ion acceleration in the axial direction, the presheath scale length becomes comparable to the channel width so that the plasma channel becomes an effective presheath. It is found that the radial ion velocity component at the plasma–sheath interface varies along the thruster channel from about 0.5Cs0.5Cs (Cs(Cs is the Bohm velocity) near the anode up to the Bohm velocity near the exit plane dependent on the SEE coefficient. In addition, the secondary electron emission significantly affects the electron temperature distribution along the channel. For instance in the case of s = 0.95,s=0.95, the electron temperature peaks at about 16 eV, while in the case of s = 0.8s=0.8 it peaks at about 30 eV. The predicted electron temperature is close to that measured experimentally. The model predictions of the dependence of the current–voltage characteristic of the E×BE×B discharge on the SEE coefficient are found to be consistent with experiment. © 2001 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70923/2/PHPAEN-8-12-5315-1.pd

Analyses of the anode region of a Hall Thruster cahnnel

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77093/1/AIAA-2002-4107-729.pd

Ionization and ablation phenomena in an ablative plasma accelerator

Several interrelated phenomena near the surface ablated into a discharge plasma, such as ablation and ionization in accelerated plasma are studied. Two characteristic ablation modes are identified, namely, ablation mode with a velocity at the Knudsen layer edge smaller than the local sound speed and a velocity at the Knudsen layer edge close to the sound speed. The existence of these two ablation modes is determined by the current density in the acceleration region. The nonequilibrium ionization region in the presence of strong electromagnetic plasma acceleration is studied. In the subsonic regime, the ionization region thickness is proportional to the ionization rate and inversely proportional to the magnetic field. Conditions for ionization equilibrium in the accelerating plasma are determined. The specific example of a micropulsed plasma thruster is considered. It is concluded that both the equilibrium and nonequilibrium ionization regimes occur in this device.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69809/2/JAPIAU-96-10-5420-1.pd

Modeling of a high-power thruster with anode layer

Among Hall thruster technologies, the thruster with anode layer (TAL) has much wider technical capabilities, especially in the high-power regime of operation. In this paper, various aspects of the plasma flow in a high-power thruster with anode layer are studied. Based on a 2D hydrodynamic model, the formation of a space-charge sheath near the acceleration channel wall and the sheath expansion in the acceleration channel are calculated. It is found that the high-voltage sheath near the channel wall expands significantly and the quasineutral plasma region is confined in the middle of the channel. For instance, in the case of a 3 kV discharge voltage, the sheath thickness is about 1 cm, which is a significant portion of the channel width (which is typically a few cm). In addition, a simplified quasi-1D model is developed to study the anode acceleration layer, which is confined by channel walls. It is found that near-wall sheath expansion leads to an increase in current density along the channel, and this in turn causes decrease of the acceleration region length. This is an important finding as it has implications for high-power TAL behavior, in which contact of the plasma with acceleration channel walls can be limited. © 2004 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69797/2/PHPAEN-11-4-1715-1.pd

Etching of random solids: hardening dynamics and self-organized fractality

When a finite volume of an etching solution comes in contact with a disordered solid, a complex dynamics of the solid-solution interface develops. Since only the weak parts are corroded, the solid surface hardens progressively. If the etchant is consumed in the chemical reaction, the corrosion dynamics slows down and stops spontaneously leaving a fractal solid surface, which reveals the latent percolation criticality hidden in any random system. Here we introduce and study, both analytically and numerically, a simple model for this phenomenon. In this way we obtain a detailed description of the process in terms of percolation theory. In particular we explain the mechanism of hardening of the surface and connect it to Gradient Percolation.Comment: Latex, aipproc, 6 pages, 3 figures, Proceedings of 6th Granada Seminar on Computational Physic

PERIODICAL PLASMA STRUCTURES CONTROLLED BY EXTERNAL MAGNETIC FIELD

The characteristics of 2D periodical structures in propulsion type magnetized plasma are studied in kinetic PIC MCC simulations.With increasing an angle of magnetic field the ridges (maxima) of electron and ion densities form in the plasma volume in cylindrical chamber. These ridges are shifted relative each other that results in the formation of 2D double-layers structure. Depending on Larmor radius and Debye length up to nineteen potential steps appear across the oblique magnetic field40-4

Vaporization of heated materials into discharge plasmas

The vaporization of condensed materials in contact with high-current discharge plasmas is considered. A kinetic numerical method named direct simulation Monte Carlo (DSMC) and analytical kinetic approaches based on the bimodal distribution function approximation are employed. The solution of the kinetic layer problem depends upon the velocity at the outer boundary of the kinetic layer which varies from very small, corresponding to the high-density plasma near the evaporated surface, up to the sound speed, corresponding to evaporation into vacuum. The heavy particles density and temperature at the kinetic and hydrodynamic layer interface were obtained by the analytical method while DSMC calculation makes it possible to obtain the evolution of the particle distribution function within the kinetic layer and the layer thickness. © 2001 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69973/2/JAPIAU-89-6-3095-1.pd

Characterization of carbon nanotubes produced by arc discharge: Effect of the background pressure

Single walled carbon nanotubes (SWNT) produced by the anodic arc discharge over a range of constant background pressures of helium (100–1000 Torr) were examined under a high-resolution transmission electron microscope, and a Raman spectrometer. It was found that the average SWNT diameter is about 2 nm and fairly independent of the background pressure. Analysis of the relative purity of SWNTs samples suggests that highest SWNT relative concentration can be obtained at background pressure of about 200–300 Torr. Measured anode ablation rate increases linearly with background pressure. The model of the anodic arc discharge was developed. It was found that the predicted anode ablation rate agrees well with experiment suggesting that electron temperature in the anodic arc is about 0.5 eV. © 2004 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69876/2/JAPIAU-95-5-2749-1.pd