Top surface of a ceiling fan ( that hasn't been cleaned for a long time)
The bottom surface
The top surface seems to be way more dusty than the bottom. I reason that it doesn't have anything to do with gravity, as I've seen pretty much the same results on table fan propellors and exhaust fans, which has a horizontal axis of rotation. What else could be the reason?
That are many factors at play here. Let us consider the case when the fan is rotating.Since there is a suction of air towards the bottom in the top surface of the fan the dust particle which move along the air flow collides with the top surface of the fan.On the contrary the dust particles get pushed away from the bottom surface of fan.This makes the top part more dusty.
According to this idea there should not be any dust on the bottom surface at all.But this is not the case. Since the fan blade is slightly slanted some dust particles below the fan obliquely collides with it.
Now consider the case when the fan is at rest.Assuming that the composition of dust particles in the room is uniform and the motion is somewhat random,both the surfaces of the blades gather almost the same amount of dust.But amount of dust will be slightly more on the top because of gravity. This explains why the bottom surface also has a little amount of dust.
Hope this explanation is satisfactory
