A fragrant example
What this means is that as distance increases, flux falls precipitously!
We'll give an example. Let’s say you walk into a room wearing your latest favorite perfume / aftershave. The gradient between you and the rest of the room is steep, because no one else in the room is wearing your fragrance. A mosquito buzzing right next to your ear gets a noseful almost immediately – diffusion (flux) occurs very quickly over this short distance. However, diffusion to even a few centimeters away occurs much more slowly. And diffusion to the object of your desire, a person standing a meter or more away, occurs at a glacial pace.
But luckily, you don't have to wait around for diffusion to occur. What actually happens, the reason why a person all the way across the room can smell you within seconds, is that there are also air currents carrying the perfume, so that diffusion only needs to occur right at the "unstirred boundary layer" between the inside and outside of your nasal membranes.
Copyright University of Maryland, 2007
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