This simple IR experiment concerns with putting a piece of paper above a cup of room temperature (nearly) water (Figure 1). I hear you saying, what is the big deal of it? You have probably done that several times in your life, for whatever reasons.
If you happen to have an IR camera and you look at this process through it, you may be surprised. Many of you know that water in an open cup is slightly cooler ( 1-2°C lower) than room temperature because of evaporative cooling: constant evaporation of water molecules from liquid water brings away thermal energy from the cup and causes it to remain a bit cooler than the room environment (which is why you feel cold when you step out of a swimming pool). You may think that the paper would also cool down because at room temperature paper is a bit warmer than the water in the cup and, based on what your science teacher has told you, heat would flow from the warmer paper to the cooler water, causing the paper to cool down.
|Figure 2 (Watch it in YouTube)|
|Figure 3 (Watch it in YouTube)|
|Figure 4: Sensor results.|
But wait, this is not the end of the story!
If you keep observing the paper, you will see that this condensation heating effect will diminish in a few minutes (Figure 3). This trend is more clearly shown in Figure 4 in which the temperature of the paper was recorded for ten minutes using a fast-response surface temperature sensor. What happens?
|Figure 5 (Watch it in YouTube)|
|Figure 6 (Watch it in YouTube)|
|Figure 7 (Watch it in YouTube)|
When will the paper cool down?
Returning to the original purpose of my experiment (looking for cooling due to heat transfer), can we find a situation in which we will indeed see cooling instead of heating? Yes, if the water is cold enough (Figure 7). When the water is cold, the evaporation rate drops. There will be less water molecules hitting the surface of the paper. The energy gain from weaker condensation heating cannot compensate the energy loss due to the heat transfer between the paper and the cold water. (By the way, I think the heat transfer in this case is mostly radiative, because air doesn't conduct heat well and natural convection acts against heat transfer in this situation.)
What if the paper has been atop the water for a long time?
|Figure 8 (Watch it in YouTube)|
If you leave the paper atop the cup of water for a few hours and you come back to examine it, you would probably be surprised again: The paper is now cooler than room temperature (Figure 8). I wouldn't be surprised if you are totally confused now: This heating and cooling business is indeed quite elusive -- even though everything we have done so far has been limited to manipulating paper and water. To keep the story short, I will tell you that this is because water molecules have traveled through the porous layer of the paper through capillary action and shown up on the other side of the paper (this molecular movement is often known as percolation in physics). Their evaporation from the upper side of the paper cools down the paper. The building science guys among us can use this experiment to teach moisture transport through materials. Can the temperature of the upper side be somehow used to gauge the moisture vapor transmission rate (MVTR) of a porous material? If so, this may provide a way to automatically measure MVTR of different materials. The American Society for Testing and Materials already has established a standard based on IR sensors. Perhaps this experiment can be related to that.
Different materials have different dew points?
|Figure 9 (Watch it in YouTube)|
|Figure 10 (Watch it in YouTube)|
What will happen if we add some salt (or baking soda or sugar) to the water? Figure 10 shows that the condensation heating effect becomes weaker. For our chemist friends, this is known as vapor pressure depression. The salt ions do not evaporate themselves, but their presence in a solution slows down the evaporation of water molecules.
A vapor column?
|Figure 11 (Watch it in YouTube)|
What about alcohol?
So far we have used only water. What about other liquids? Alcohol is pretty volatile. So I tried some isopropyl alcohol (91%). Once again, I was baffled. Our experience with applying rubbing alcohol to our skin says that alcohol cools faster than water. So I expected that when the isopropanol molecules condense, they would release more heat. But this is not what Figure 12 suggests! Given the fact that the enthalpies of vaporization of alcohol and water are 44 and 41 kJ/mol, respectively, the only sensible explanation may be that the heating effect is not only due to the condensation of the vapor molecules, but also the interaction between the vapor molecules and the surface molecules of the paper. If the interaction between an alcohol molecule and a paper molecule is weaker, then the adsorption of the alcohol molecule onto the paper surface will produce less heat. I don't know how to prove this now, but this could be a good topic to research.
|Figure 12 (Watch it in YouTube)|
Even if this is a lengthy blog post (and thanks for making it to the end), I am pretty sure that the scientific exploration does not stop here. There are other questions that you can ask yourself. For me, I have been intrigued by the fascinating thermodynamic cycle and have been wondering if that could be used to engineer something that can harvest that latent heat. In other words, could we turn a cup of water into a tiny power plant to charge my cell phone? The evaporation of water molecules from an open cup is a free gift of entropy from Nature. Perhaps something could be done about it.