Why can't I get a car window to fog up twice?

29 January 2012

Question

This question is for my 7 year old son Colin. When I breathe hot breath on a cold car window it steams up. When I draw on the window in the foggy part or I wipe off the fog I can't breathe on it again to make it foggy. When I take a hot shower the mirror gets foggy. When I wipe the mirror it gets foggy again almost instantly. Why can't I get a car window to fog twice and why does my bathroom mirror stay foggy?

Steven King and Colin King
Virginia, USA

Answer

Dave - What's going on is that your fingers are greasy. When you draw on the window, you can actually see the grease left on the window, assuming it's clean to start with. Grease is hydrophobic, which means it doesn't like water, whereas glass is actually hydrophilic which means it does like water. When you breathe on the glass, water vapour in your breath is condensing to form droplets of water. This creates the foggy bit which scatters the light to make it look foggy.

It's a lot easier for a droplet to start on something hydrophilic than something hydrophobic. So on the glass, you'll get lots and lots of droplets forming. You'll get billions and billions of billions of tiny little droplets forming. Whereas on the grease, because it's so much harder to form droplets, you'll just get a few big ones and then those will grow much more and so end up being much bigger. It's much easier to see through a few big droplets than through lots and lots of tiny droplets, so it looks transparent on the bit which is greasy and not on the bit which isn't.Chris - And that's why it looks like you can't re-fog it. There is some water there but just all in big blobs, not little blobs.Dave - Yes, and they can merge together into a sheet of water which is entirely transparent.

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