Diffusion

To explain the diffusion mechanism of particle capture, we first need to do a quick review of the kinetic theory of gases. This theory explains that a gas is composed of a large number of molecules that are small in size when compared to the distances between them. These molecules behave like ridged spheres that travel in straight lines when they're not bumping into each other.

Small particles, with diameters in the range of 0.1 µm and below, tend to make random motions due to their interaction with the zigzagging gas molecules. As these small particles bump into gas molecules they too begin moving randomly about, bumping into other particles as well. This random motion is referred to as Brownian motion of gas molecules.

Diffusion is predominant with low gas velocities and smaller particles. The smaller a particle is and the slower the flow, the more time it will have to zigzag around, thereby giving it much better chance of hitting and sticking to a filter. Hence, much like impaction, probabilities can be developed for collisions due to this diffusion. As with impaction, as more and more particles collect, the probability of collision (efficiency) for other particles is enhanced, but with an associated increase in pressure drop.

Click here to view the Diffusion animation