The perfect depollution filter exists, but there is a heavy price to pay

From the “sieve” theory to aerodynamic capture, the process described always has to do with clouds and precipitation, i.e. with the system that nature has arranged to clean up its house: the atmosphere

We have seen how a motion of particles suspended in a fluid, eternal in its agitation, allows the extraction of part of them from the aerosol system and therefore contributes to the depollution of the air. A contact of the particles with external walls and with each other is sufficient in the coagulation. Then just be patient, because this particular subtraction of particles from the gas is time dependent. It is Brownian motion. But we are always, continuing with our metaphor, in the area of ​​the “ant”, i.e. particles small enough to undergo random kicks from the surrounding molecules that make them toss randomly. We now move to the other extreme, to the side of hot air balloonfrom the particles so large that the fluid (either a gas or a liquid) is felt as a continuum. Here we find a trivial effect that we call “sieve” and another a bit more cunning that we call “aerodynamic capture”. The sieve is a well-known kitchen tool that was indispensable when the miller gave back to the farmer, who gave the grain to be ground, flour and bran together. With the sieve the housewife separated one from the other; what is larger than the pitch of the sieve net, the bran, remains on top, while the flour passes between the meshes of the net. Thus there is an effect similar to the sieve also in our masks, of good use at the time of Covid: the larger particles of the fabric shirts do not pass through and remain blocked. But something else also happens which is called “aerodynamic capture”, and to explain it we need a little drawing: