Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

I was reading about this water bottle by Botl that behaves like a canvas water bags to keep water cool. I found out that this idea is an old idea and cars would drive with water bags in front as shown below.

enter image description here

Here is what I found out about them: it seeps water through the bag and evaporates, causing the water to cool inside. Here is where I need some help:

  1. Why does the water seep through the bag? Is it because the water molecules have a Maxwell speed distribution and only the fastest molecules seep out?

  2. What mechanism is occurring for heat being removed from the water to evaporate the water on the bag’s surface? Doesn't radiation also contribute here?

  3. Why does moving increases the cooling effect faster?
share|improve this question
Can the person who down voted this question explain why they did so. This way I will know how to better phase my questions in the future. Thanks. –  Jesus Feb 6 at 18:23
add comment

2 Answers

up vote 4 down vote accepted

As far as I understand the idea is very simple:

as the bottle is covered in a film of water (due to a leakage of the container), this can evaporate. Due to the Boltzman distribution this will also happen well below $100°C$. The process of evaporating though takes energy from the system (called latent heat). This cools down the bottle.

If the surrounding air is "filled with water" it is harder for the liquid to evaporate and thus slowing the process, so always bringing fresh air to the system helps.

This is basically the concept of sweating as well, where you can immediately feel the effect of wind blowing on your moist skin.

Hope this helps and I didn't miss something.

share|improve this answer
how is this latent heat taken from the water to vaporize the film of water? –  Jesus Feb 5 at 22:14
I suppose it is fine to imagine that just the fast particles fly away, although this process is most likely more complicated. –  Hagadol Feb 5 at 22:16
So if I understand correctly, the idea is the same as my first point in my question. As the evaporative cooling process continues, the faster molecules will continually seep out and evaporate the new film of water, further cooling the water. –  Jesus Feb 5 at 22:26
I do not think that the fast particles are significantly more likely to seep out but just that they get carried away more easily. –  Hagadol Feb 5 at 22:28
I don't understand your last statement "get carried away more easily." –  Jesus Feb 5 at 22:31
show 3 more comments

This seems to be a design idea, not a real bottle that someone has made, so all we can go on is the information provided by the designer.

I don't see anything about 'seeping'. Instead designer Benjamin Helle says:

If you’ve ever had a cold drink outside in the sun, you’ve see beads of water drip down the side

This refers to water from the atmosphere condensing on the cold glass, not seepage. Since condensation is evaporation in reverse, there would be no overall cooling effect from letting water condense and then evaporate again.

It won't work as described.

share|improve this answer
add comment

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.