Change in size distribution of ultrafine aerosol particles undergoing Brownian coagulation

Determination of coagulation coefficients and aggregation kinetics for charged aerosols

A new method has been developed which allows determination of the coagulation coefficient of two oppositely charged particles experimentally. For this purpose, quasi-monodisperse particles of different sizes and morphology were used to study the influence of different parameters on the coagulation coefficient. A good agreement between experimental results and the classic Fuchs' theory was obtained when including a method accounting for particle nonsphericity. In experiments with polydisperse bipolarly charged aerosols, no principal differences to uncharged aerosols were found when a dimensionless representation was used. Changes in particle number concentration and geometric mean diameter can be described by simple empirical expressions.

Cluster–Cluster Aggregation Kinetics and Primary Particle Growth of Soot Nanoparticles in Flame by Light Scattering and Numerical Simulations

The agglomeration kinetics of growing soot generated in a diffusion atmospheric flame are here studied in situ by light scattering technique to infer cluster morphology and size (fractal dimension D(f) and radius of gyration R(g)). SEM analysis is used as a standard reference to obtain primary particle size D(P) at different residence times. The number N(P) of primary particles per aggregate and the number concentration n(A) of clusters are evaluated on the basis of the measured angular patterns of the scattered light intensity. The major finding is that the kinetics of the coagulation process that yields to the formation of chain-like aggregates by soot primary particles (size 10 to 40 nm) can be described with a constant coagulation kernel beta(c,exp)=2.37x10(-9) cm3/s (coagulation constant tau(c) approximately = 0.28 ms). This result is in nice accord with the Smoluchowski coagulation equation in the free molecular regime, and, vice versa, it is in contrast with previous studies conducted by invasive (ex situ) techniques, which claimed the evidence in flames of coagulation rates much larger than the kinetic theory predictions. Thereafter, a number of numerical simulations is implemented to compare with the experimental results on primary particle growth rate and on the process of aggregate reshaping that is observed by light scattering at later residence times. The restructuring process is conjectured to occur, for not well understood reasons, as a direct consequence of the atomic rearrangement in the solid phase carbon due to the prolonged residence time within the flame. Thus, on one side, it is shown that the numerical simulations of primary size history compare well with the values of primary size from SEM experiment with a growth rate constant of primary diameter about 1 nm/s. On the other side, the evolution of aggregate morphology is found to be predictable by the numerical simulations when the onset of a first-order "thermal" restructuring mechanism is assumed to occur in the flame at about 20 ms residence time leading to aggregates with an asymptotic fractal dimension D(f,infinity) approximately = 2.5.

Collision Kinetics and Electrostatic Dispersion of Airborne Submicrometer Fractal Agglomerates

Collision and electrostatic dispersion rates of airborne submicrometer TiO(2) agglomerates were measured and compared with the classical collision theory for spheres as well as with models accounting for the agglomerate structure in terms of the fractal dimension and electrostatic effects such as Coulomb and van der Waals interactions. According to the authors' knowledge, this is the first time that the agglomerate fractal dimension and electrostatic effects have been considered simultaneously in determining the collision frequency function of agglomerates. The observed enhancement in the collision frequency of agglomerates was found mainly to be a result of electrostatic particle interactions. Nonspherical particle shape has only a comparatively small influence on the collision probability, on the order of 10-20%. Electrostatic dispersion coefficients of agglomerates were found to be similar to those of spheres. Copyright 2001 Academic Press.

Simulations of aerosol aggregation including long-range interactions

Current understanding of solid aerosol particle aggregation is limited to simulation models based on diffusive and ballistic motion of the colliding particles. The role of the long-range van der Waals forces in aggregation phenomena, although important, has never been examined. In an effort to address this issue, a simulation model, based on molecular dynamics techniques, is developed. Using this model to simulate thermal collisions of single spheres with small aggregates of similar spheres, we examine the effects of retardation of the long-range van der Waals forces, particle transport, ambient temperature, and pressure on the collision rates and mass and structure distributions of the aggregated particles. The model calculations were performed at simulated temperatures of 293 and 1500 K and at simulated pressures of 760 and 3040 torr for glassy carbon primary particles in the free molecular regime with diameters of 6 nm, and in the transition regime with diameters of 30 nm. Inclusion of the long-range van der Waals forces resulted in aggregates with relatively open structures and few branches and collision rate constants that were larger than the corresponding hard sphere rate constants, whereas exclusion of the forces resulted in compact structures with more branches and smaller enhancements in the rate constants. The above effects were found to be more pronounced in the free molecular regime than in the transition regime, which is consistent with the observation that the initial conditions and the interparticle forces play a more significant role in particle transport in the free molecular regime than in the transition regime. The effect of retardation of the forces is an increase in the percentage of open aggregates and the collision rate constants over that of the corresponding nonretarded case. An increase in temperature resulted in a collapse of aggregate structure and a decrease in collision rate constants corresponding to the reduced geometrical cross sections. Again, the effects were found to be more pronounced in the free molecular regime than in the transition regime. No significant difference was observed in the structure of the aggregates or in the collision rate constants with a change in pressure, indicating that the pressure effect, if any, is hidden by the much stronger effect of the long-range van der Waals forces.

Coagulation in cell suspensions: extensions of the von Smoluchowski model

Recently, a long-range interactive force between erythrocytes has been proposed (Rowlands et al., 1981, 1982a,b) based on an apparent increase in the rate of aggregation of erythrocytes in an aqueous suspension over that predicted by one model of Brownian aggregation. Here, we examine the assumptions underlying this model and propose modifications compatible with the biological constraints on the model. The refined model is represented as a series of coupled differential equations representing the change in particle number density as a function of time. Numerical solution of these equations is consistent with the absence of an intercellular force.