|Why dust collects on fan blades
||[Apr. 4th, 2017|08:26 pm]
»The properly skeptical reader may have detected a peculiar assumption in our demonstration of viscosity: the fluid must stick to the walls of the [container] rather than simply slide along the walls. Vogel, Life in Moving Fluids, page 21.
Now fluid certainly does stick to itself. If one tiny portion of a fluid moves, it tends to carry other bits of fluid with it—the magnitude of that tendency is precisely what viscosity is about. Less obviously, fluids stick to solids quite as well as they stick to themselves. As nearly as we can tell from the very best measurements, the velocity of a fluid at the interface with the solid is always just the same as the solid. This last statement expresses something called the "no-slip condition"—fluids do not slip with respect to adjacent solids. …
[A] peculiarity of this no-slip condition is that the nature of the solid surface makes very little difference. If water is flowing over a solid without an air-water interface to complicate matters, the no-slip condition holds whether the solid is hydrophilic or hydrophobic, rough or smooth, greasy or clean. The nature of the solid surface matters only when we have a liquid-gas interface present as well—in short, where surface tension becomes a factor."
The no-slip condition has a number of important ramifications. In particular, it means that any time a fluid flows across a solid, a velocity gradient is present. … In practice, the no-slip condition explains (in part) why dust and grime accumulate on fan blades, why pipes (including blood vessels) encounter trouble from accumulation of deposits rather than from wearing thin, and why a bit of suspended rock is needed in water for the latter to become effectively errosive. … Alternatively, just consider why dishcloths and mops are so much more effective for cleaning than any mere rinse.«