Science Questions

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

Sun, 29th Jan 2012

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Question

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

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 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.

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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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.

When something happens that works differently to something very similar, you have to put a magnifying glass on the differences.

I do not know the exact answer to your question, but here are several possibilities:

(1) The car window is cold. Perhaps your drawing and wiping warms it and stops the fogging.
(2) With the car window your hand is greasy and perhaps a little dirty; it is very clean and grease free during/after the shower.
(3) Is one lot of fogging heavier with liquid than the other? which gets wetter -- the mirror or the car window?

I can tell you a little trick about the bathroom mirror, if you live in a neighbourhood where the water is "soft" (lathers well). You make your finger very soapy, or actually draw on the mirror with a wet block of soap, and your drawing will not steam over again! Or you can wipe the whole mirror with soap so that you can see to brush your hair after the shower. damocles, Sun, 29th Jan 2012

Steven King eh! Or are you really Stephen King looking for a little info for your latest book?

Maybe not, after all, I am aquainted with one your fellow contrymen by the name of John Wayne, but I've never seen him riding a horse or totting a six shooter.

I wonder if there's ever been another Don_1? Nah, I doubt it.

Anyway, to your question, I think Damocles is probably right in saying that oils from your finger prevent your car window from becoming foggy when you breathe on it again, while the mirror in your showeroom is subjected to very much higher levels of condensation. Your breath holds far less moisture than the air in your showroom once the shower has been running for a while and if you have washed your hands when you wipe the mirror, much of the oils on your hand would have been washed away, so there is little or none to come off onto the glass. Don_1, Mon, 30th Jan 2012

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.

chris chris, Mon, 30th Jan 2012

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.

After a piece of glass is wiped, the dust goes with the  wiper. Try this on the next fogged up window you see. I  am sure you will see dirt on your finger. The dust is already wet. JimBob, Mon, 30th Jan 2012

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



While this looks like a good way to go, it will not definitively settle the issue between nucleation and hydrophobicity/surface layer effects. Why not? Because the "clean" cloth will have a surface residue of (at least) surfactant, which can easily be transferred to the glass. damocles, Tue, 31st Jan 2012


Not really. A badly built cloud chamber will prove that condensation will take place without nucleation.
More importantly, the dust provides a surface for water to condense on. A mirror already has a surface: it doesn't need dust. Bored chemist, Wed, 1st Feb 2012

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