Turn off your window fan for a moment and you see fine dust on the fan blades. It seems strange when the fan is running, there's air blowing over the blades hour after hour. Yet that wind doesn't carry the dust away.

When you look at dirt on fan blades you're looking at a manifestation of one of the most complex processes in nature: air flowing over a solid surface. Figuring out the details requires the fastest supercomputers in existence.  But the overall picture is something like this:

If you could get small enough to sit on a spinning fan blade, you'd feel a strong breeze-- it would be like riding in a convertible with the top down. But here's the strange and mysterious part: as you got still smaller and closer to the surface, you'd feel less and less breeze. Within a fraction of a millimeter from the fan blade surface, you'd feel no breeze at all!

Because of friction between air and fan blade, there's a very thin layer of air next to the surface that doesn't move over the blade. Any dust particles small enough to stay within that quiet surface layer never feel the breeze, so they stay put.

This fact has other practical implications. Blowing on a phonograph record won't get rid of the very smallest dust particles, especially the ones at the bottom of the grooves. And blowing on a camera lens will never get it really clean. In both cases you have to use a brush that touches the surface to get the fine dust off.

More like this

• Dirt and empty space
• The cleaning power of soft drinks
• The color of lint

Sources

P. Feynman, “The Feynman Lectures on Physics”, chap. II-41 (1963).

Khurana, “Numerical Simulations Reveal Fluid Flows Near Solid Boundaries, “Physics Today”, May 1988;

C.D. Ahrens, “Meteorology Today”, 2nd ed. (1985) (discussion of saltation, p. 260).

This episode is an adaptation of a broadcast featured in the book Why You Can Never Get to the End of the Rainbow and Other Moments of Science, published by Indiana University press, and edited by producer Don Glass.