Steven King asked:
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
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 itís clean to start with. Grease is hydrophobic, which means it doesnít 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.
Itís 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 itís 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. Itís 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.
King, Steven asked the Naked Scientists: 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 What do you think? King, Steven , Sun, 29th Jan 2012
Hi Steven and Colin.
Steven King eh! Or are you really Stephen King looking for a little info for your latest book?
Dave and I discussed this yesterday and we are of the opinion that, because skin is naturally oily, when a finger is used to draw on a steamy window, a small amount of grease from the fingertip is imparted to the glass surface. This oily residue is more hydrophobic (water repellent) than the native mirror surface. So when the mirror is re-fogged at a later time point, droplets form less readily on the areas with an oily surface and those that do form will coalesce to form isolated, larger droplets. Compared with the adjacent glass, which is relatively less hydrophobic and is consequently covered in many tiny individual droplets - which collectively make it look cloudy - the pre-oiled area looks clear.
Actually, it is because of nucleation. When a gas condenses it need a place to start. Like snow flakes water must have something of a different state, usually a solid, to begin its transformation from a gas to a liquid. It is an absolute must to have a dry speck of dust on which to form.
The best way to test it will be to "write" on a window with a clean cloth; if it's down to nucleation this should prevent it... who's up for doing the experiment to find out the answer? chris, Tue, 31st Jan 2012