Computer simulations of charged colloids in alternating electric fields

We briefly review recent theoretical and simulation studies of charged colloidal dispersions in alternating electric fields (AC fields). The response of single colloid to an external field can be characterized by a complex polarizability, which describes the dielectric properties of the colloid and its surrounding electrical double layer. We present computer simulation studies of single spherical colloid, using a coarse-grained mesoscale approach that accounts in full for hydrodynamic and electrostatic interactions as well as for thermal fluctuations. We investigate systematically a number of controlling parameters, such as the amplitude and frequency of the AC-fields. The results are in good agreement with recent theoretical predictions.     

Multiplicities of charged particles prior to fission

Electric monopole transitions between the 0 ₂ ⁺ , 0 ₁ ⁺ and 2 ₂ ⁺ , 2 ₁ ⁺ levels in⁷⁶Se, populated in the decay of⁷⁶Br, were investigated by means of conversion electrons and gammaray spectroscopy. TheK-electron intensity ratios ofE0 andE2 transitions connecting the 0 ₂ ⁺ level to the 0 ₁ ⁺ and 2 ₁ ⁺ levels and of theE0 andE2 components in the 2 ₂ ⁺ →2 ₁ ⁺ transition were determined. The ratioX(E0/E2) of theE0 toE2 reduced transition probabilities and, from the available lifetimes, theE0 strength parameters ρ(E0) were deduced for the two transitions. The results are compared with the predictions of current models.     

Rayleigh instability of charged droplets in the presence of counterions

The classical instability of a charged spherical droplet is reconsidered in the presence of counterions. An ensemble of such droplets is studied within a simplified cell model. Screening of the electric field by the counterions is found to increase the equilibrium droplet size. Furthermore, if the ions can enter the droplet, a first-order phase transition occurs upon increasing Bjerrum length, surface tension or droplet density, leading to a phase separation. Simple scaling properties of the free energy give the shape of the phase boundary and show the system to be scale-invariant there. Pearl-necklace structures of hydrophobic polyelectrolytes are discussed as an application. Received 30 August 2001     

Energetics of the fission process

The mass asymmetry of fragments from nuclear fission of heavy nuclei is reviewed. While mass asymmetry is a common and well-known phenomenon for low-energy fission of the lighter actinides, more recent experiments have demonstrated that, for the heaviest actinides, the mass distribution switches to a symmetric one. On the other hand, it has been discovered that, though for fissioning nuclei with mass numbersA225 the mass distribution is basically symmetric, an asymmetric component is clearly to be identified for nuclei down to the Pb-region. In the absence of a generally accepted dynamical theory of fission, the above experimental findings are discussed in terms of static energy considerations. Triggered from the outset by the structure of the potential energy surface at the saddlepoint, the energy balance at the scission point between the available energy (Q-value) of the reaction and the Coulomb and deformation energy of the nascent fragments is shown to steer the characteristics of the fragment mass distributions.     

Absorption of solar radiation by charged water droplets

We consider the absorption of solar radiation by charge accumulated on the surface of an otherwise neutral water droplet. We estimate the order of magnitude of the additional contribution from the surface charges to the photoabsorption cross-section. The approximate location and strength of major spectral lines is indicated. This spectral information requires determination of the quantum mechanical energy levels and wave functions of the absorbing system. An approximate calculation is carried out using a simple model for electrons on the surface of the droplet. The implications for atmospheric applications are considered.     

Polar light charged particles accompanying spontaneous fission

Simple model of emission of light charged polar particles accompanying fission of heavy nuclei is presented. We assume the emission from the whole surface of fragments after scission, taking into account the influence of the accompanying fragment. The initial energy and time of emission are treated phenomenologically. The motion of light particles and fission fragments are subsequently calculated in the framework of classical dynamics. We get a good agreement with experimental data if we assume a prompt emission. The possibility of the connection with giant resonances is discussed.     

Simulations of coulombic fission of charged inviscid drops

We present boundary-integral simulations of the evolution of critically charged droplets. For such droplets, small perturbations are unstable and eventually lead to the formation of a lemon-shaped drop with very sharp tips. For perfectly conducting drops, the tip forms a self-similar cone shape with a subtended angle identical to that of a Taylor cone, and quantities such as pressure and velocity diverge in time with power-law scaling. In contrast, when charge transport is described by a finite conductivity, we find that small progeny drops are formed at the tips, whose size decreases as the conductivity is increased. These small progeny drops are of nearly critical charge, and are precursors to the emission of a sustained flow of liquid from the tips as observed in experiments of isolated charged drops.     

On the theory of the noncoalescence effect for oppositely charged droplets

A theory is proposed and numerical simulation is conducted for oppositely charged mutually approaching droplets of an aqueous electrolytic solution in silicon oil. It is shown that at small distances between droplets, a conductive bridge leveling out the potentials of the droplets may form between them due to electrohydrodynamic instability of the equilibrium surface of one of the droplets. As a result, the droplets start to repel each other and may drift apart without coagulation. The proposed theory is confirmed by the effect of nonconfluent droplets observed in experiments [W.D. Ristenpart, J.C. Bird, A. Belmonte, et al., Nature 461, 377 (2009)].     

Symmetric and asymmetric fission of charged sodium clusters

A deformed-jellium model is used to calculate the fission barrier height of Na ₂₄ ²⁺ → Na ₂₁ ⁺ +Na ₃ ⁺ and Na ₄₂ ²⁺ → 2Na ₂₁ ⁺ within the Kohn—Sham Density Functional Theory (i.e., including shell effects). Although the shape of the barrier obviously depends on the parametrization of the fission path, we have found that the barrier maximum corresponds to a configuration in which the emerging fragments are already formed and rather well apart. The two examples chosen serve to illustrate the influence of the electronic shell effects.     

Millimeter-wave scattering from neutral and charged water droplets

We investigated 94 GHz millimeter-wave (MMW) scattering from neutral and charged water mist produced in the laboratory with an ultrasonic atomizer. Diffusion charging of the mist was accomplished with a negative ion generator (NIG). We observed increased forward- and backscattering of MMW from charged mist, as compared to MMW scattering from an uncharged mist. In order to interpret the experimental results, we developed a model based on classical electrodynamics theory of scattering from a dielectric sphere with diffusion-deposited mobile surface charge. In this approach, scattering and extinction cross-sections are calculated for a charged Rayleigh particle with effective dielectric constant consisting of the volume dielectric function of the neutral sphere and surface dielectric function due to the oscillation of the surface charge in the presence of applied electric field. For small droplets with radius smaller than 100 nm, this model predicts increased MMW scattering from charged mist, which is qualitatively consistent with the experimental observations. The objective of this work is to develop indirect remote sensing of radioactive gases via their charging action on atmospheric humid air.     

Computer simulations of the forest-fire model

We present results of simulations of the forest-fire model proposed by P. Bak et al. containing a tree growth rate p and fire spreading to nearest neighbors. The space-time structure of the fire and the scaling properties of the forest clusters show that the model cannot be critical in the limit p → 0. Instead we observe regular and quasideterministic spiral-shaped fire fronts. We present a deterministic forest-fire model that helps understand the origin of these spirals.     

Direct numerical simulations of electrophoresis of charged colloids

We propose a numerical method to simulate electrohydrodynamic phenomena in charged colloidal dispersions. This method enables us to compute the time evolutions of colloidal particles, ions, and host fluids simultaneously by solving Newton, advection-diffusion, and Navier-Stokes equations so that the electrohydrodynamic couplings can be fully taken into account. The electrophoretic mobilities of charged spherical particles are calculated in several situations. The comparisons with approximation theories show quantitative agreements for dilute dispersions without any empirical parameters; however, our simulation predicts notable deviations in the case of dense dispersions.     

Some dynamical aspects of the fission process

The properties of the nuclear mass parameter and its influence on the fission process are discussed. In particular, correlations between dynamical and statical paths to fission are considered. The least action trajectories idea is used for finding the dynamical path to fission. Ritz method for the action integral minimalization is applied. An example of the nucleus ²⁵²Fm is considered.     

Recent radiochemical studies of the fission process

This article summarizes the recent radiochemical investigations on mass, charge kinetic energy and fragment angular distributions in low energy fission of actinides.     

Computer simulations of GIAR-film spectra

The response of ideal GIAR-films to an incident broad band of synchrotron radiation has been calculated as function of external parameters as beam divergence, electron densities of substrate and backing as well as density of the nuclear scatterers. These calculations give a survey on the possible range within which the nuclear and the electronic reflectivity can be varied and will help to find layer systems which make useful output beams from existing sources.     

Electrostatic field and charge distribution in small charged dielectric droplets

The charge distribution in small dielectric droplets is calculated on the basis of continuum medium approximation. There are considered charged liquid spherical droplets of methanol in the range of nanometer sizes. The problem is solved by the following way. We find the free energy of some ion in dielectric droplet, which is a function of distribution of other ions in the droplet. The probability of location of the ion in some element of volume in the droplet is a function of its free energy in this element of volume. The same approach can be applied to other ions in the droplet. The obtained charge distribution differs considerably from the surface distribution. The curve of the charge distribution in the droplet as a function of radius has maximum near the surface. Relative concentration of charges in the vicinity of the center of the droplet does not equal to zero, and it is the higher, the less is the total charge of the droplet. According to the estimates the model is applicable if the droplet radius is larger than 10 nm.