Simulations of aerosol aggregation including long-range interactions

Current understanding of solid aerosol particle aggregation is limited to s imulation models based on diffusive and ballistic motion of the colliding p articles. The role of the long-range van der Waals forces in aggregation ph enomena, although important, has never been examined. In an effort to addre ss this issue, a simulation model, based on molecular dynamics techniques, is developed. Using this model to simulate thermal collisions of single:le spheres with small aggregates of similar spheres, we examine the effects of retardation of the long-range van der Waals forces, particle transport, am bient temperature, and pressure on the collision rates and mass and structu re distributions of the aggregated particles. The model calculations were p erformed at simulated temperatures of 293 and 1500 K and at simulated press ures of 760 and 3040 torr for glassy carbon primary particles in the free m olecular regime with diameters of 6 nm, and in the transition regime with d iameters of 30 nm. Inclusion of the long-range van der Waals forces resulte d in aggregates with relatively open structures and few branches and collis ion rate constants that were larger than the corresponding hard sphere rate constants:, whereas exclusion; of the forces resulted in compact structure s with more branches and smaller enhancements in the rate constants. The ab ove effects were found to be more pronounced in the free molecular regime t h;m in the transition regime, which is consistent with the observation that the initial conditions and the interparticle forces play a more significan t role in particle transport in the free molecular regime than in the trans ition regime. The effect of retardation of the forces is an increase in the percentage of open aggregates and the collision rate constants over that o f the corresponding nonretarded case. An increase in temperature resulted i n a collapse of aggregate structure and a decrease in collision rate consta nts corresponding to the reduced geometrical: cross sections. Again, the ef fects were found to be more pronounced in the free molecular regime th;than in the transition regime. No significant difference was observed in the st ructure 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. [S1063-651X(99 )11508-3].