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Filter is not a sieve
- 2019-08-22-

The sieve and sieve size determine the retention of particulate matter.
The filter principle has nothing to do with the mesh principle. Small particles hit a stationary object, and the gravitational force between the two surfaces sticks them together. If you look at the used filter material with an electron microscope, you will see that the captured particles are much smaller than the pores of the material. Think about the mask, it may be loose, but it does block the microbes that are invisible to the naked eye. The filter power can be 99%, 99.99%, 99.99999%, but less than 100. Only when it reaches a certain level, people think it is 100.
Even porous filter materials such as ceramics, nuclear pore membranes, the pore size seen with a microscope may be relatively uniform, but their pore size does not represent filtration accuracy. The force of sticking to the dust is still van der Waals force, and the reason for the dust hitting the filter medium is still inertia and diffusion. A material with a uniform pore size has a sieving effect on debris larger than the pore size, but it can also filter out dust that is much smaller than the pore size.
In papermaking and other occupations, people imagine a porous material into a number of capillaries, and use the corresponding method to measure the capillary equivalent "aperture" to evaluate the gas permeability of the material. In the experiment, the outer cover of the material is covered with a water film, and the underside of the material is contacted with air which can adjust the pressure to improve the pressure, and the bubble is bubbled at the beginning of the material. The pore size of the material is an imaginary value corresponding to the capillary, which is ten times larger than the particle to be filtered, and there is no direct correspondence between the pore size and the particle size of the captured particles. You can say that the filter paper with a smaller aperture is better than the larger filter, but you still don't know the specific filter power.
Liquid filtration is similar to the principle of gas filtration, which is not a sieve and does not reach 100%. Involving air filtration, it should be inferior. However, when you encounter words such as "precision" and "aperture", please don't expect to be meaningful.