Question

Why does dust get deposited only on one side of the blades of a fan?

Answer 1

Ceiling/table fan blades are uniformly thick, but cambered at 8° to 12° (ideally 10°) off the plane, unlike aerofoils. Rotating a fan blade causes the streamlines above and behind it to separate, creating low pressure on that side. However, the lower/front streamlines are aligned with the blade surface.

Dust particles stick to blades at rest and in motion due to intermolecular forces and static charges. The streamlines are completely detached, preventing them from being displaced from the top/behind surface. During motion, a tiny layer of air remains stationary relative to the blade, keeping dust on the lower/front surface.