Arkadi Maisels

Dynamics of the aerosol particle photocharging process

In the photocharging process, aerosol particles become electrically charged through interaction with high-energy photons, e.g., ultraviolet (UV) irradiation. Photon adsorption by particles leads to electron emission and, as a result, particles become positively charged. While maximum achievable charges have been described in previous studies of dependency on particle and irradiation parameters, the influence of photoemitted charges on the charging process was not taken into account. In this work it is shown that such charges interact with the particles, which heavily influences the entire process. This complex process (the charging of particles positively by photons and simultaneously negatively by ions) is described in this work by a set of differential equations. These differential equations are solved numerically and, with simplifying assumptions, analytically. Multicomponent polydisperse aerosol is considered. As was found by comparing the analytical and numerical solutions, analytical results coincid...

On the effect of charge recombination on the aerosol charge distribution in photocharging systems

Abstract An experimental study has been carried out to determine the aerosol charge distribution through photocharging process. Particle charge distribution was studied in dependency on particle number concentration and irradiation intensity. While highly positively charged aerosols were obtained for particle number concentrations below 5×10 11 m −3 , approximately symmetrical bipolar charge distributions were measured for number concentrations of about 2×10 13 m −3 . The theoretical description was made on basis of the previously developed model, which was extended by a term accounting for ion diffusion losses. Theoretical results reflected experimental observations. The necessity of taking into account diffusion discharging of particles by gas ions is determined by a dimensionless factor depending on initial parameters. Since the charge distribution through photocharging becomes a known function of particle and irradiation parameters, a photocharger may be used instead of a neutralizer to produce a defined charge distribution.

Synthesis of tailored composite nanoparticles in the gas phase

We report on a method to obtain tailored nanoparticle aggregates of two components in the gas phase. The method is based on the modification of the Brownian collision rate by charging the nanoparticles. Particles of different components are charged oppositely in order to obtain composite nanoparticle aggregates via preferential coagulation. The resulting composite aggregates are uncharged, which allows for their separation from both, charged unaggregated particles and charged aggregates of only one component. The mean size and standard deviation of each particle component can be adjusted by means of differential mobility analysis. Experimental results are presented for composites of PbS and Ag.

Anomalous thermal behavior of gold nanostructures

Gold nanostructures exhibit anomalous thermal behavior such as the shape transformation of nanorods, generation of nanoparticles by heating a mesh etc. at extremely low temperature. Here, we report a lower onset temperature of evaporation as compared with that of Ag. This appears to be inconsistent at first glance as the vapor pressure of bulk Au is lower as compared to that of Ag at a particular temperature. Analysis of our experimental results based on the thermodynamic model suggests that the surface melting is the most plausible mechanism for the unusual thermal behavior of Au.

Mixing selectivity in bicomponent, bipolar aggregation

Abstract The selectivity of aggregation in mixtures of two charged aerosols containing chemically dissimilar nanoparticles is studied by means of a newly developed direct simulation Monte Carlo method. This method allows to trace changes in complex multidimensional systems, in this case describing particle size, charge and aggregate composition. A new procedure was developed for estimating the effective collision diameter of an aggregate composed of primary particles of any size. Three model systems were studied: polydisperse aerosols with initially bipolar charge distribution, unipolarly charged polydisperse aerosols and quasi-monodisperse oppositely charged aerosols. The study is focused on the aggregate compositions dependence on the initial size and charge distribution. It was found that the use of bipolarly charged aerosols does not increase the selectivity of mixing whereas unipolarly, oppositely charged aerosols reach more rapidly a more homogeneous distribution of components within the aggregates. In the last case, the addition of one more elementary charge to the particles roughly doubles the fraction of bicomponent, 1 : 1 mixed nanoaggregates and accelerates the process.

A Study of Nanoparticle Aerosol Charging by Monte Carlo Simulations

A Direct Simulation Monte Carlo (DSMC) technique is applied for describing the dynamics of aerosol charging. The method is based on the transformation of known combination coefficients into charging probabilities. Changes in the particle charge distribution are computed as a stochastic game, calculating the time-step after each event. The simulations are validated by comparison with analytical solutions for unipolar aerosol diffusion charging and aerosol photocharging. The advantage of the DSMC method lies in the uncomplicated simulation of multi-dimensional systems that would result in very elaborate population balances. The DSMC method is used for simulation of the photocharging of moderately concentrated bicomponent polydisperse aerosols. By means of this method, the influence of the particle parameters (size, material) on the dynamics of the charge distribution in different size and material fractions has been studied. It is shown that charge separation between size or material fractions can be achieved for aerosol components with dissimilar work functions, while the total aerosol charge is zero.

Evolution of crystallinity of free gold agglomerates and shape transformation

We report the shape evolution of free gold agglomerates with different morphologies that transform to ellipsoidal and then to spherical shapes during the heating cycle. The shape transformation is associated with a structural transition from polycrystalline to single crystalline. The structural transition temperature is shown to be dependent on the final size of the particles and not on the initial morphologies of the agglomerates. It is also shown that the transition occurs well below the melting temperature which is in contrast with the melt-freeze process reported in the literature.