Aerosol Particle Deposition Mechanisms


 

There is a general misconception about how air filters work, i.e., that they work like a microscopic sieve. Particles are simply trapped due to its large size relative to the sieve. This is not the case for most types of air filters. Instead, aerosols are captured via collision and attachment to fiber surface.

There are five mechanisms by which an aerosol particle can be deposited on a fiber:

  • Interception

  • Inertia Impaction

  • Diffusion

  • Sedimentation

  • Electrostatic Attraction

Each will be discussed below.

 



Interception

Interception occurs when particles do not depart from the streamlines. The inertia or Brownian motion of particles is negligible. Particles following streamlines arrive at the fibers and get "intercepted" on the fiber surface.


Inertia Impaction

This occurs when particles cannot adjust to the "sudden" change of streamlines near fibers, and, due to inertia, depart from the streamlines and impact on the fiber surface.


Diffusion

Diffusion occurs when smaller particles having Brownian motion hit the surface of the fibers.


Sedimentation

When the only significant force acting on a particle is the gravity, then this mode of deposition is called sedimentation, or gravitational settling.


Electrostatic Attraction

This is an important mechanism in filtration; however, the electrostatic charges are hard to quantify. Therefore, unless the quantity of the charges are known, this mechanism is often ignored.