In the biological physics course I am teaching, I talked a lot about the fact that on lengths greater than around a tenth of millimetre, diffusion of molecules is too slow to supply the needs of living organisms. To get round this they had to evolve pumps, propellers and cargo transport infrastructures. The time to diffuse a distances increases as the square of the distance travelled and so diffusion is slow on all lengths large enough to be visible to the human eye.
Diffusion is also too slow to shift things around in our environment. This is why we need to stir milk into coffee.
When we stir milk into coffee we can see the flow as the two liquids are different colours, but I think we probably don’t appreciate how much flow of air on all lengths there is in our homes and offices. If you want correct this, go to the gallery here and look the top movie. This is the one imaging the flow around a burning match. I think it’s fascinating. The hot burning match creates convection and so a plume of rising hot air as you would expect, but the complexity of the flow created is pretty amazing. The movie is part of a gallery of fluid dynamics movies; the water drop hitting a bed of tiny glass beads is also beautiful to watch.
Chance & Necessity 