How do clothes dry?

05 July 2009

Question

This seems to be a simple phenomenon but I have a question on it. How do clothes dry or even how does water in any place dry out without heating it up? It’s obviously that water boils and evaporates but how does water just vanish off your clothes when they’re wet?

Answer

Chris - It's a good question. Water has energy. So, in other words, at any given temperature, the water molecules are vibrating or moving around, proportional to the temperature of the water and when we give energy to water sufficient to raise the temperature to 100 degrees, what that means is that the molecules of water are vibrating or moving around sufficiently fast, that they can readily break the attraction that's holding them onto other water molecules because water is sticky and this enables them to escape and get out into the atmosphere as vapor.

Kat - But you don't dry your clothes at 100 degrees so what's going on here?

Chris - Absolutely, not. What you are doing though if you, say put them in a tumble dryer or hang them on the line is that you are putting some heat into the clothes or just because they're at ambient temperature. They're not absolute zero. The atoms and molecules therefore have some energy. Now, because the energy is not shared equally amongst all the atoms or molecules and anything, in other words, if I come up to you and I shake your hand, I can give you some energy. When the molecules are bashing into each other, sometimes some of them will end up transiently with a load of energy from lots of other molecules bashing into them and others will have much less. This means that occasionally, you've got the odd molecule there that has sufficient levels of energy that it can break the bonds holding onto other molecules and it can escape. The reason its slower to dry at less than 100 degrees or however hot you want to make is because obviously, it takes longer for those interactions to occur so that the odd molecule gets enough energy to escape and that's why the sea for instance, can evaporate water when sunlight falls on it and warms up the ocean without having to boil itself. It's just much slower. If you put a pot on the stove, you give lots more energy to lots more atoms and molecules all at once and as a result, more of them have more energy more of the time and therefore, they're able to evaporate, and that's the reason.

Ben - Do you reach any equilibrium between water in the clothes and humidity in the air? I assume that when there's more wind blowing then you've got lower humidity in the air because it's more of its moving parts.

Chris - Yes. I mean around the item that you're drying, the air that's in contact with the clothing will become slightly higher saturation of water. So, in order to maintain the gradient, in other words, water wants to move from an area where there is lots of water to an area where there is much less water. If you have full winds blowing, this is moving away any molecules of water that get off of the clothing and into the surrounding air very quickly and therefore, you maintain that gradient. So, the molecules want to move more readily away from the clothing.

Kat - So, this explains why tumble dryers are great because they're hot and they're sort of blowing air around and tumbling things about?

Chris - Absolutely!

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