Why does dust stick to rotating fan propeller?

Question

Why does dust stick to rotating fan propeller?

Intuitively, most people (including I) think of the dust will not stick to rotating fan propellers.

EDIT 1:

Thank you for the great explanations. I am still waiting for the "better" one if any.

Answer 1

First taking only speed of the fan into the account. If the fan rotates slowly then the situation is obviously not very much different from if it weren't rotating at all. The centrifugal force on the dust particles is not big enough to throw them away off the fan.

Second, there's static electricity that has to be taken into the account. It's perfectly possible that some residual charge is generated on the fan (this depends very much on what is the fan made off) and as the dust particles are often charged the ones with correct polarity will get attracted to the fan. In this case they would stick even if fan were rotating very fast.

Now, you can test whether the second option is realized in your case by touching the fan to discharge it and it should "kick" you a little. Or if you don't like that you can bring some charged object close to the fan and see whether it is affected.

Answer 2

I'm guessing that a very thin layer of oil covers the blade, maybe because of proximity to the oiled bearings, making it a little sticky. The blade would pick up excess dust because it moves through more air than if it were stationary. According to this hypothesis, static electricity is not involved.

Answer 3

The reason is that you have a boundary layer on the surface of the blade of the fan. On the frame of the blade (the blade moves with some velocity, but at the frame of the blade the air moves) the boundary layer starts from the surface of the blade where the fluids velocity is zero and as you move away from the blade, the velocity increases up to the value of the velocity of the blade (you can call that the undisturbed velocity of the flow).

So if you have some fine dust, it actually doesn't feel much wind and it can't be blown away. Static electricity could be an other factor, but you can see that on metallic propellers also.

Answer 4

What about this hypothesis:

Dust sticks everywhere, but since the propeller cuts through a lot of air, it meets more dust particles. Thus, more dust sticks to the propeller than elsewhere.

Evidence

I (Mark) took photos my the fan my room to support Damien's hypothesis. The first photo is of the leading edge of the fan blade, which impacts a lot of air, and the second photo is of the trailing edge of the same fan blade. I've never cleaned this fan. The leading edge is covered in a thick, 3-5mm layer of dust, while the trailing edge is almost clean.

Answer 5

Wind doesn't actually touch the surface. You can see the same effect on a car: even if you move at speeds beyond 70mph, the dust doesn't get blown away.

If you look closely, there is a boundary layer between the matter of the fan and the air around the fan. When you get closer to the fan blades, the air starts to move with the fan (the blade pulls it along), so air very close to the blade doesn't move (much) relative to the blade itself.

Obviously, this is true when you add matter to the blade (like dust). In this case, the friction of the air is less than the adhesion of the dust to the blade, so the dust sticks to the surface.

On my fans, I find a lot of dust and short strands on the edge that cuts the air. Here, the air flow presses the strands to the blade (parts of it on either side of the edge). This way, the fan actively collects dust. Again, the force of the air pressing the strands against the blade (plus the friction between dust and blade) is much stronger than the centrifugal force which might pull them sideways. Since the strands cling to the surface, the blade isn't strong enough to cut them, so they stay where they are.

This relation is true for all fan speeds, so the dust always ever gets more.

A simple countermeasure is a coarse net on the side where the fan sucks the air in. Most dust strands will get caught in the net and you can easily wash them away every few weeks or collect them with a vacuum cleaner.

Answer 6

Here are some of my observations:

• If a fan is not used, the sticking of dust is at the same rate as other objects.

• If a fan is used, even the rotation is slow, the dust can still stick much faster than un-used fan.

• The dust not only stick at the propeller, but also the cover bar behind and in front of it. It can give a thick layer of dust at both place.

• There is (plastic ?) coating around the fan, so not metal is direct expose there.

• Dust accumulation is a bit faster near the kitchen. The fan surface also has many oil.

Hence, I think that the most important reason of high sticking rate should be the high throughput of the air. Dust will not move toward the fan itself.

For the reason of sticking, I think the it should be due to the electrostatic charge on the dust. It is similar to the dust removal in power plant. Also, dust can adhere at most common surface such as table, wall and plastic, so whether the fan has charge should not be important factor.

Both electrostatic charge and oil should increase the sticking rate. After the initial sticking, the rough surface should allow easier sticking for later dust, so I guess the accumulation rate speed up.

Answer 7

The short answer is that there's no wind near the blade. This is called no-slip condition in hydrodynamics of viscous fluids.

[Concession] It is actually more than that. There's minor van der waals sticking which contributes to this otherwise purely hydrodynamic phenomenon.

Answer 8

Dust sticks almost anywhere. Almost? Yes almost, because yet we don't have no super perfect nano-surfaces. The point is that every macroscopic surface is raspy and not perfectly smooth. Therefore very small dust can easily stick. This very small dust makes the surface even more raspy, making it easy for the heavier dust to arrive.

In the near future we might have fan propellors that have no easy visible dust sticking.

And of course it is the electro-magnetic force that lets the dust stick. Make yourself clear how very, very small pieces of dust look like and you understand how it can easily stick on a non-smooth surface by means of the Coulomb-Force. In the end: Most things you see are electrically charged, at least if you "zoom in" a lot. Looking from far away the charges are effectively neutralized. Of course a super perfect nano surface even looks very good is you "zoom in".

Answer 9

since it seems like participating in bounty questions is something like our group sport, I could not resist to also think about the question.

I think the answers given according to the vanishing velocity on the propeller are unlikely to explain the phenomenon. Dust particles, even though they are very thin, have a three dimensional shape. So even if there is no velocity at zero distance (which is the mentioned boundary condition), there will be a relative movement along the dust particle (Remember that turbulence takes place on every scale for the Navier-Stokes equations) which to my opinion acts as a pulling force on the particle.

To my mind, the problem relates to an electrostatic one. The propeller is very likely to have some charge due to his movement through air and the ongoing friction. The (might even be very small) charge induces a dipole moment on the dust particles nearby which I guess might be treated as dielectric balls.

This dipole moment will then attract the dust particles which as a result touch the surface. If now everything would be metal, there would be immediate (full) charge balance and the attractive force would vanish. But under the assumption of a dielectric medium, some dipole moment stays, so does the attractive force and the dust on the propeller.

Greetings

Edit: I just saw that the electrostatic argument already came up. Nevertheless, I hope my explanations are still usefull.

Answer 10

True, the boundary layer makes it so that the dust can't get blown by the fan itself. But it could not answer the question of why the dust is attracted to the blade in the first place. I think it's linked to the cavitation phenomenon. But instead causing boiling bubbles that can be ruptured by a rotating fan in liquid, it just attracts the dust particles. Maybe because air density is 1/1000's the density of water.