Filtration plays a crucial role in various applications, from cleaning drinking water to air filtration. However, there are common misconceptions when it comes to what is filtration. While interception, impaction, and diffusion are key mechanisms, electrostatic interactions also contribute to particle trapping. Fiber filters, such as those found in masks, can create complex webs with irregular openings. The size of the filter openings alone does not determine the particles it can effectively trap. Understanding these filtration methods is essential for developing efficient filters.
Filters have received a lot of attention in the past couple of years. We were mainly familiar with them for cleaning drinking water, but, since COVID, we also now know them for their role as filters and masks for air filtration. You also have them in your cars and in your houses, such as furnace filters.
Despite their common use we tend to have misconceptions about filtration and how filters work.
We typically think of a filter as acting like a sieve, so a large particle hits a smaller hole, cannot go through the filter and gets trapped. This is indeed one way a filter works. Scientifically we refer to this as interception, since the filter material intercepts the particle. This leads to the common assumption that only objects larger than a “hole” are getting trapped by the filter. But there are other mechanisms by which a filter can stop or trap particles.
One such method is impaction. If a particle moves at a high velocity towards the filter, it might be smaller than the opening but still have a certain inertia such that the particle cannot follow the fluid (gas or water) flowing past the filter material (for example, fiber), impacts on the filter and gets trapped even though it is smaller than the opening. This additional mechanism is also referred to as impaction. The particle impacts on the filter.
Now particles in a fluid also diffuse so they move by themselves as they interact with the fluid molecules like in Brownian motion. Hence they do not necessarily follow a straight line but, especially the smallest, nanosized particles, tend to zigzag around and through this diffusion also touch the filter or fibers and get trapped. This mechanism is especially effective on smaller, nanosized materials and explains why these smallest particles are often trapped quite well by filters, especially thicker ones, despite their small size.
For larger particles typically interception and impaction are the dominant trapping mechanisms while for the smallest particles Brownian motion tends to be more important. Official masks and air filter tests are often designed to test particles around 100-200nm in size as they are the most challenging to trap.
Besides these mechanical processes, there are additional mechanisms which can contribute to filtration and trapping. Most notable are electrostatic interactions. Here smaller but charged particles are attracted by electrostatic interactions towards the filter material and stick. Hence many high efficiency filters and masks have changed layers to enhance the overall trapping by mechanical mechanisms as discussed before and through electrostatic interactions.
An added complication in all this is that many air filters are typically fiber filters. These fibers could be cellulose, glass fiber, or even metal fibers. As you can see in this electron micrograph picture showing the three layers of a mask, these can make for intricate webs of material with quite irregular openings.
In all cases of filtration, the size of the openings in the filter does not necessarily tell us the whole story about the size of the particles it can protect against.