Interaction between two different species of particles in a fluid stream is promoted by generating turbulent eddies in the fluid stream. The turbulent eddies are designed to be of such size and/or intensity that the two species of particles are entrained into the eddies to significantly different extents. Consequently, the different species of particles follow different trajectories, and the likelihood of collisions or interactions between the particles is increased. Optimum collision rates will occur for a system which maintains a Stokes Number (St) much less than 1 for one species, and or order 1 or greater for the other species. The invention has particular application in air pollution control, by promoting agglomeration of fine pollutant particles in air streams into larger particles to thereby facilitate their subsequent removal from the air streams.