system
Given the following post, respond as if you're commenting on Reddit:
human
Why does dust accumulate on fans?

One would think the fan would blow the dust off instead.
gpt
Static. As the fan blade moves through the air it picks up electrons from the fluid molecules. Since most household fans are made of plastic, thus insulators, they readily retain charge as opposed to metal fan blades which will dissipate the charge more readily through conduction. The dust, which is inevitably sucked in by the fan, will be electrically attracted to the charged fan blade and stick. The reason it doesn't blow off as you'd expect is due to the boundary layer effect and some regions of flow separation. Essentially, the shear layer is too weak to move the electrostaticly bonded particles.