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After reading and understanding the reasons why dust stick to rotating fan propeller, I am interested to find out why the dust particles stick so well. Spraying powerful jets of water does not effectively remove the dirt. Some scrubbing is still required, whether it is metallic blades or plastic ones.

Why is that so?

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Question Overflow
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I have observed this as well, and experiment suggests it's because the dust is hydrophobic. If you splash a small amount of water gently onto the dusty surface you will see the water roll up to form beads that do not wet the surface. This is my rather crude attempt to illustrate what happens when you try and wet the dusty surface:

Dust

The brown splodges are supposed to be the dust. because the water/dust interface has a high contact angle the water droplets cannot get to the surface underneath to wet it. Hence it's difficult to wash the dust off the surface. You should find that if you use a detergent solution rather than just water the dust will wash off the surface.

Why the dust should be hydrophobic isn't clear. Apparently the rumour that dust is mostly skin cells isn't true, which is a shame because it would neatly explain the hydrophobicity. Skin is pretty hydrophobic, partly because of the sebum layer on it and partly because the dead keratinocytes have intrinsically hydrophobic cell walls. If anyone knows of definitive measurements of dust composition it would be interesting to see them (Googling finds many opinions about dust composition but little in the way of peer reviewed literature!).

John Rennie
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  • The elemental composition and sources of house dust and street dust – RedGrittyBrick Oct 17 '13 at 15:27
  • @RedGrittyBrick: Cool :-) Though it doesn't say any more than about 50% of house dust originates from soil. Still, at least half the dust isn't decaying skin :-) – John Rennie Oct 17 '13 at 15:30
  • Nice illustration. I was wondering if the formation of a dust layer rather than just dust alone makes the cleaning difficult. I find it quite easy to wipe away dust on the floor, but not dust that adhere to the fan blade. Could there be more reasons that the just hydrophobicity? – Question Overflow Oct 18 '13 at 07:56
  • The hydrophobicity would only explain the problems removing the dust with water. Difficulty in wiping off the dust would be due to unrelated causes. I would guess that dust on the fan blades is held there by electrostatic forces while dust on the floor is mainly there because the floor happened to be in the way while the dust was settling. To test this try breathing on the dust, or using an antistat gun, before wiping and see if that affects the ease of wiping. – John Rennie Oct 18 '13 at 07:59