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There's this question that has been bugging me most of my life: how is it that wet clothes left hanging to dry, get actually dry? If I get it right, the clothes are a mesh of fibers (we could assume synthetic fibers to make it simpler) that capture droplets of water in the mesh. When one hangs the cloth gravity makes some of the droplets fall from the mesh; still if there's a side that's exposed to a heat source it dries faster, so in addition to gravity there's also heat. However, the heat does not boil the droplets.

So... - what is actually going on there ? - could the drying be optimized, e.g. maximizing surfaces exposed, etc ?

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    $\begingroup$ Ever hear of evaporation? $\endgroup$
    – user1631
    Sep 19, 2011 at 18:17

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There are effectively two ways that liquid turns to gas:

Boiling: Heat liquid water till it undergoes a phase transition to a gas

Evaporation: Surface water is absorbed by the air since, usually, the air has a lower concentration of water than a damp spot on your clothing.

Heating the clothing accelerates this process as, at a molecular level, heated water has more kinetic energy and is more likely to be absorbed into the air than "stationary" molecules (or ones with less kinetic energy). The tumbling in the dryer makes the clothing dry evenly.

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  • $\begingroup$ Could you elaborate this part, "usually, the air has a lower concentration of water than a damp spot on your clothing", as it might be better to put it in terms of equilibrium concentrations or something like that. $\endgroup$
    – Alan Rominger
    Sep 19, 2011 at 21:46
  • $\begingroup$ You mean, in the first line, "two ways that liquid turns to gas"? $\endgroup$
    – xpda
    Sep 20, 2011 at 4:53
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    $\begingroup$ on a rainy day have you noticed that cloths take longer to dry!!!That's precisely because of higher concentration of water in air...So to make cloths dry faster, you need lower concentration of water in air so that water evaporates faster, which is achieved by using a hot air blower, a simple fan, sunlight.... $\endgroup$
    – Vineet Menon
    Sep 20, 2011 at 5:19
  • $\begingroup$ Water can evaporate because even though temperature is not high enough to boil and turn into gaseous form, T is just average velocity of water molecules. Which means some molecules have velocity which is high enough to leave clothes. Clothes get a bit dryer and also cooler, since now mean velocity is lower and so is temperature. $\endgroup$
    – Ivana
    Sep 20, 2011 at 8:11
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The warmer side dries faster because:

  • Dry air holds more water than humid air. Heating air lowers it's relative humidity.
  • Warmer water evaporates faster.

Even without the heat, water evaporates, so long as the air's relative humidity is less than 100%.

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