capillary-actioneveryday-life
The question is based on a TEDx video, where the speaker claims that folding a paper towel before using it creates interstitial forces which help dry your hands faster.
The question:
Does this effect actually occur? If it does, then why, and how significant is it?
In addition, would there be a significant difference between folding the towel once (i.e. in half), versus folding it twice?
I think it would evaporate quicker for anything we normally call a "towel". The evaporation of the plain puddle is limited by the surface area of the water. A towel provides many capillary pathways for the water to diffuse thru the fabric, eventually presenting a much larger surface are for evaporation. Other fabrics could be hydrophobic and decrease overall water/air surface area, but the point of a towel is to do the opposite.
Assuming the towel is clean and not causing additional impurities to dissolve into the water, the vapor pressure of the water should still be the same. The towel is then just a mechanical support for lots of capillary channels. I don't think the vapor pressure is lower just because the water is held by capillary action between structures like cloth fibers.
I actually went ahead and spent some hours experimenting. Used two 500ml aluminum beer cans filled with water at room temperature, 21.4°C. One can wrapped in a paper towel soaked with an additional 20ml of water, one left bare as control. Shoved both in my small, non-ventilated house freezer at -14°C and measured temperature and weight every twenty minutes until water in both cans started forming ice. These are the results.
Allowing for some error from my cheap digital food thermometer, the towel-wrapped can cooled quite a bit faster than the control one. In fact, it reached the 4°C serving temperature in about 50 mins, more than an hour earlier than the control can. Notably, by that time it had already lost some 6ml of water, I suppose through evaporation/minor dripping, and ended up losing a total of 10ml by the end of the experiment (the control only lost 2ml).
So yes, the wet paper towel trick does seem to work quite nicely. I'd expect it to work even better if one were to use a ventilated freezer (faster heat exchange) and smaller containers (greater surface/volume ratio).
I also had a properly sealed beer can in there with a wet towel, same starting temperature, and it cooled to serving temp at about the same rate as the other can. A little quicker, because it wasn't taken out of the freezer for measurements until the 1-hour mark, when it was ready to be in my tummy.
Not very academic maybe, but I hope this provides some useful info!
Best Answer
The speaker actually claims it works by "interstitial suspension". I think he is referring to capillary action, which you are aware is involved here. He does not claim this method is faster, only that it is more environment-friendly. Shaking before wiping is a crucial factor.
Folding has 2 advantages : it reduces excess towel at the edges which is not used, and it makes the towel thicker so that the paper between the hands does not become saturated, when it will have no further drying capacity.
Yes, folding twice might be even more effective, depending on the size, thickness and quality of the towel. However, excessive folding leaves paper in the folds usused (ie dry) while the outer surfaces are overused and become saturated. So a single fold might be optimal.
Why not do some personal research and report your findings? (Be quick, or Floris will beat you to it!)