James R. Brock

On the theory of thermal forces acting on aerosol particles

Abstract Disagreement between experimental results and aerosol thermal force theories for regimes from continuum toward free-molecule is explained through application of first-order slip-flow boundary conditions. The influence of the convective flow on the thermal force is explored through a perturbation technique.

Simulation of Aerosol Dynamics: A Comparative Review of Mathematical Models

A comparative review of mathematical models of aerosol dynamics is presented. Three approaches are considered that are based on continuous, discrete (sectional), and parametrized (lognormal) representations of the aerosol size distribution. Simulations of coagulation and diffusion-limited condensation are performed with these modeling approaches for three case studies typical of clear, hazy, and urban atmospheric aerosol concentrations. The relative accuracies and computational costs of models based on these approaches are compared. The models based on a continuous size distribution provide an accurate solution for both coagulation and condensation. Sectional approaches simulate coagulation very well but require a fine size resolution to minimize numerical diffusion in the simulation of condensation. The parametrized model based on log-normal modal size distributions is computationally efficient but tends to overestimate the rate of coagulation and the peak aerosol concentration resulting from condensatio...

Metal nanoparticles generated by laser ablation

Abstract We study a new method for producing ultrafine metal particles (nanopartides) that employs Laser Ablation of Microparticles (LAM). Pulsed excimer laser radiation at 248 nm wavelength was used to ablate ~2 μm feedstock of silver, gold, andpermalloy (Ni 81 %:Fe 19% ) under both normal atmospheric conditions and in other gases and pressures. A model for nanoparticle formation by LAM is proposed that includes plasma breakdown and shock-wave propagation through the initial microparticle. Behind the shock a large fraction of the original microparticle mass is converted to nanoparticles that diffuse to silicon substrates and TEM grids for collection and analysis. Nanoparticle morphologies are spherical except for gold nanoparticles >100 nm that are generally cubes. Electron micrographs of the samples were analyzed by computer-aided image processing to determine the effect of irradiation conditions on the nanoparticle size distribution. The results showed that mean particle diameters were normally in the range from 10 to 100 nm and that the particle size distributions were generally log-normal, with dispersion (diameter/standard deviation) ranging from 0.2 to 0.5. For metallic microparticle feedstock, the mean size of the produced nanoparticles generally increased with increasing laser fluence and were smallest for fluences not too far above the breakdown threshold.

The thermal force on spherical sodium chloride aerosols

Abstract Theories of the thermal force acting on aerosols are reviewed. Experimental measurements of the thermal force on spherical sodium chloride aerosols in argon are reported. The experimental measurements in the slip flow region are compared with slip flow theories, including a second-order theory presented here.

Simulation of aerosol kinetics

Abstract Results are presented for numerical simulation of aerosol growth processes as they occur in smog chamber experiments and in the atmosphere. A general model for the evolution in time of the particle size distribution and techniques for its numerical solution are discussed. The evolution of the particle size distribution depends on, among other processes, the growth processes of coagulation, condensation, and nucleation. Some initial numerical results are presented for combinations of these processes operating for various model aerosols.

Large-scale production of nanocrystals by laser ablation of microparticles in a flowing aerosol

We experimentally demonstrate the production of nanoparticles by laser ablation of microparticles entrained at high density in a flowing aerosol. The currently measured production rate of 20 grams per hour could be scaled to industrially useful rates. We have characterized the size distribution of particles and found nearly monodisperse distributions where mean sizes were smaller and varied less with laser fluence than was observed for ablation of microparticles held on flat plates. Mean size was controlled from 4–20 nm by varying the type and pressure of carrier gas. We found Ag and CdSe nanoparticles were crystalline having few dislocations. Materials tested included metals (Ag, Au, and W), semiconductors (Si, CdSe, GaN, and ZnO), ceramics (WC, SiC, and YBa2Cu3O7), and a ferroelectric. Two types of collection processes are described that preserve the nonagglomerated nature of the particles, even at high mass densities.

Simulation of Condensation Aerosol Growth by Condensation and Evaporation

A new numerical method is reported for the solution of the condensation—evaporation equation, a first-order hyperbolic equation. The solution and properties of the nonlinear integrodifferential equation arising when the mass of the condensing vapor is conserved are discussed. For aerosol evolution in the conserved case it is shown that there develops an asymptotic regime analogous to the asymptotic behavior found for the coagulation process.

Evolution of atmospheric aerosol particle size distributions via Brownian coagulation numerical simulation

Abstract Current atmospheric observations tend to support the view that continental tropospheric aerosols, particularly urban aerosols, show multimodal mass distributions. One of the obvious mechanisms causing the multimodality is the mixing of different primary sources. Other modes involve dissimilar aerosol formation processes in the atmosphere. Fine aerosol particles are generated from secondary processes such as nucleation, condensation and chemical reaction, whereas coarse particles usually consist of dust, fly ash and mechanically generated aerosols. With the use of a newly developed computer code GROWTH in our laboratory, we report here the simulated results of Brownian coagulation dynamics involving multimodal mass density functions for long periods of time. In our model calculations we assume that the aerosol particles are well mixed in an atmospheric volume so that spatial variation in the distribution is negligible. Our accurate numerical simulation of the Brownian coagulation dynamics indicates that once formed, an atmospheric multimodal aerosol distribution in the range ∼ 0.1 to ∼ 100 μ m will maintain its identity for a very long period of time (at least hours) unless “atmospheric perturbations” such as meteorological instabilities, rain-washout and gravitational settling occur. It is our belief that understanding the complex domain of atmospheric aerosols requires systematic investigation of each process. This paper is a continuation of such an investigation.

Growth of ferromagnetic particles from cation reduction by borohydride ions

Abstract An experimental and theoretical study on the formation of ferromagnetic particles from cation reduction by borohydride is reported. The effects of various parameters on primary particle size, composition, saturation magnetization, and chain formation have been studied. These parameters include the metallic cation and its concentration, the anion concentration, the reaction temperature, and the use of additives in the reaction. A relationship has been demonstrated between the fractal dimension of aggregates and their saturation magnetization. In basic media, primary particle formation has been shown to occur by two different mechanisms depending on the metallic cation concentration. A model has been developed which gives results in agreement with experimental observations on the formation of primary particles at higher metallic cation concentrations in basic media; the model indicates that primary particle growth occurs primarily by condensation of atoms on clusters and that collisions between clusters are probably suppressed by electrical double-layer forces.

Unipolar charging of small aerosol particles

Abstract Calculations are presented of the unipolar charging rate of small aerosol particles in the transition and free molecule regimes of Knudsen number. Numerical results for the charging rate in the transition regime are obtained from a constant collision frequency model of the Boltzmann equation. These results confirm experimental observations that the unipolar charging rates for larger particle charges are only weakly pressure dependent even when the Knudsen number is relatively small. Also, the modification of the near free molecular unipolar charging rate by ion-neutral molecule collisions, is found to depend importantly and in a complex manner on particle size and total particle charge when the image force is included in the ion-particle interaction.