Why does soap stop mirrors from fogging?

05 July 2009


I have recently observed that if you are in a shower and the glass is fogged up why is it that when you put a fingertip of soap on the glass the fog recedes. Why?


Chris - Well the reason that a window or a cold surface mirror goes misty and foggy is because when you have a hot shower, you have got lots of water molecules in the form of steam or water vapor drifting around in the room. These then lose some energy to the cold surface and this enables them to condense. So in other words, instead of being free molecules floating around, they begin to stick together and they form little tiny droplets on the surface because water has attraction between one molecule and another. So, it pulls in other molecules and it likes to form little droplets and those droplets tend to be little spheres because that's the best arrangement, so you maximize your surface area to volume relationship, so you have the least amount of water in contact with the air for the most volume. Now, what that means is that you've covered your cold surface in a tiny series of little lenses because the water droplets are of course behaving like lenses and this is why it distorts or deforms the light which is coming back off of their reflective surface and you can't see through it. If you put a blob of soap or a smear of soap up and down on a shiny surface like a mirror, you won't get that mist forming. You will get a clear reflective surface remaining, for the simple reason that the way soap works is that it gets between the water molecules and it stops those interactions of the water molecules being so sticky. It has what's called a 'surfactant effect' and what that means is that the water molecules, instead of forming lots of little droplets, will instead spread out to form a thin film which is continuous across the surface and although a film of water will refract or bends the light going through it very slightly, they will nonetheless be a straight path to the light in and out and therefore, you will see clearly. So, this is effectively how anti-misting surfaces work. They have a surface coating which is very attractive to water molecules and instead of forming little droplets, they're pulled into a flat sheet which doesn't bend the light in the same way as the little droplets would.Ben - So, does grease from your skin do the same thing if you write a message on a window?Kat - I was going to say, yes because the grease is obviously repellant to water. Oil and water don't mix. But it's a nice thing to do if you have a clean bathroom mirror and you write something on it with your finger then when your housemate or partner goes into the shower and it says, "Don't forget to put the lid back on the toothpaste you get" or something like that. You know, a lovely message like that.

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