evaporationeveryday-lifetemperaturethermodynamicswater
There are claims like this one that you can improve the cooling speed of beverages when you put them wrapped in a wet paper towel inside the refrigerator/freezer.
I've just tried it by myself and it seems it does not work as expected, although I might have confused freezer with refrigerator.
My question:
Is this kind of an urban legend or does it actually help?
It's not poop. It's fly barf.
A fly spends about 25% of its time re-digesting and it only can eat liquids. It mixes the eaten food with the appropriate enzyme for digestion. The fly does this by retrieving the eaten food from its digestive system (a vomit of sorts), and drop by drop it is placed on the surface on which the fly is sitting. Only then is it sucked back up after they are mixed. The small black dots, that are left in various places, such as the ceiling, are not fly droppings, but actually the remains which are not sucked back up.
There are a variety of fluids that are mixed along with oils from the food. This creates a surface film much like when you write with your finger on a mirror. The growth of the spot is just the fluids slowly flattening out and spreading on the surface. The film prevents condensing water from beading to form a lens shape which defuses the light into the cloudy haze you see elsewhere. This is like an anti-fogging agent which works by minimizing surface tension, resulting in a non-scattering film of water instead of single droplets, an effect called wetting. Anti-fog treatments usually work either by application of a surfactant film, or by creating a hydrophilic surface.
The wetting may appear to spread near the edges of the film during evaporation because the droplets are on the cusp of being completely wetted and have more surface area to evaporate faster which gives the illusion of the spot spreading during evaporation.
As for the poop? Well, the droppings fall to the ground and go undetected.
Capillarity, or capillary action, occurs when intermolecular bonding between water molecules and molecules of another material is greater than the intermolecular bonding among water molecules themselves. This is called adhesion, and when it happens water will tend to spread over the surfaces of the other material, even climbing against gravity to do so. Cohesion among the water molecules will drag along those water molecules not in direct contact with the material, and surface tension will hold the water together as it adheres to the surface. Adhesion may be strong enough to raise the water against gravity. If a material is porous enough, the water can continue to climb for quite a distance. But on a smooth non-porous surface, the water will stop climbing when the weight of the cohered water is greater than the adhesion can support.
Best Answer
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!