Why do fans collect dust?

[Wanderlust]

It seems counterintuitive.

[evilhanz]

From a lecture on viscosity at Berkeley: http://astron.berkeley.edu/~jrg/ay202/lectures/node15.html

It turns out, although it is not at all self-evident, that in all circumstances where it has been experimentally checked, the velocity of a fluid is exactly zero at the surface of a solid. You have noticed, no doubt, that the blade of a fan will collect a thin layer of dust-and that it is still there after the fan has been churning up the air. Why isn’t the dust blown off by the air? In spite of the fact that the fan blade is moving at high speed through the air, the speed of the air relative to the fan blade goes to zero right at the surface. So the very smallest dust particles are not disturbed.

That explains why dust stays on the fan, as to how the dust got there in the first place … (WAG) well, when was the last time you cleaned your fan blades? When you’re not using your fan, where is it likely to end up? probably in a dusty closet somewhere.

[johnson]

But how about a ceiling fan, where the dust collects on the bottom of the blades, on both the leading and trailing edge?

[tomndebb]

Why is there dust on fan blades?

[anya_marie]

on my ceiling fan, all the dust is on the top of the blade, and some more dust is clinging to the sides.