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?
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:
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!).
