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viscous flow VS molecular flow
viscous flow is how gasses work regularly, like water from a pipe. But when the mean free path grows long enough, due to dropping pressure, there aren't enough molecules around for the gas to act like water from a pipe, the gas is now in molecular flow.
Now let's relate this to our vacuum chambers: They start at atmospheric pressure, which is in viscous flow. We turn on a pump, and after a few minutes, the pressure has dropped enough to be in molecular flow. And under molecular flow, wind doesn't blow and pumps don't suck, the atoms are just too far apart!
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