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Light can make water evaporate without heat

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Re: Light can make water evaporate without heat

#41

Post by ok_computer »

xwkd wrote: post #37

Please excuse my understanding as a layman, but could this be related to the electromagnetic absorption spectrum of water? https://upload.wikimedia.org/wikipedia/commons/1/18/Absorpti... It seems that the "green" wavelength that the article cites is exactly where the lowest point of absorption is. Could this suggest that heat is created as a result of electromagnetic resistance? (Like water molecules vibrating as a r…

That’s what I’m trying to understand too.

Analogously, chemical sunscreens turn UV to heat by absorbing wavelengths with their different bonds and vibrating.

Re: Light can make water evaporate without heat

#42

Post by pdonis »

frud wrote: post #17

I can't wrap my head around this story. What does it mean in thermodynamic terms? Isn't there a fixed amount of energy per mass that it takes to convert liquid water into vapor? Why does it matter that the energy comes from light?

> Isn't there a fixed amount of energy per mass that it takes to convert liquid water into vapor?

Yes.

> Why does it matter that the energy comes from light?

The paper is drawing a distinction between light and "heat", which in the context of these experiments basically involves how you deliver the energy: do you do it by heating up the whole mass of water, or do you do it by shining light at it and having the light interact with individual water molecules?

In a practical sense, this would be expected to potentially increase the efficiency of evaporation, since bulk heating of water involves significant losses--much of the energy you expend doesn't go into the water. If you can find particular wavelengths of light that interact strongly with the water and cause evaporation, you can greatly decrease the amount of input energy that gets lost in the process.

Re: Light can make water evaporate without heat

#43

Post by pdonis »

ethanbond wrote: post #26
frud wrote: post #17

I can't wrap my head around this story. What does it mean in thermodynamic terms? Isn't there a fixed amount of energy per mass that it takes to convert liquid water into vapor? Why does it matter that the energy comes from light?

> In recent years, some researchers have been puzzled upon finding that water in their experiments, which was held in a sponge-like material known as a hydrogel, was evaporating at a higher rate than could be explained by the amount of heat, or thermal energy, that the water was receiving. And the excess has been significant — a doubling, or even a tripling or more, of the theoretical maximum rate. Apparently it evap…

> it evaporates much, much more quickly than you'd expect from purely energy per mass

From purely "thermal" energy per unit mass. But the light is delivering energy too; the total energy per unit mass being delivered is still the same, it's just being put in in a different form. Nothing about this changes the bonding energy between water molecules that has to be overcome for evaporation to occur. It's just a different method of delivering that energy.

Re: Light can make water evaporate without heat

#44

Post by elil17 »

frud wrote: post #17

I can't wrap my head around this story. What does it mean in thermodynamic terms? Isn't there a fixed amount of energy per mass that it takes to convert liquid water into vapor? Why does it matter that the energy comes from light?

Thermodynamics and evaporation are my day job and I think most other explanations here are missing the point. Evaporation normally occurs when individual water molecules have enough thermal energy to break their intermolecular bonds, leaving the bulk liquid and entering the air.

In this case, they found strong evidence that water molecules were being removed in groups of several water molecules. Because intermolecular bonds aren't being broken in these groups, the amount of thermal energy needed to cause them to enter the air is less than if they had evaporated as individual molecules. These groups later break apart in the air, absorbing thermal energy from the air and leading the air temperature to decrease slightly a few millimeters away from the sample surface.

Evaporation happening as clusters of molecules is weird - it's very different from how evaporation usually works. I'm not really sure whether to even call it evaporation since I don't think the clusters would fully qualify as vapor until they are broken apart into individual molecules.

Re: Light can make water evaporate without heat

#45

Post by ankitml »

frud wrote: post #17

I can't wrap my head around this story. What does it mean in thermodynamic terms? Isn't there a fixed amount of energy per mass that it takes to convert liquid water into vapor? Why does it matter that the energy comes from light?

Quantum effects can change thermodynamic parameters. If something seems bizzare in thermodynamic models, next step is to understand quantum physics. This includes modification of energy needed for reaction to phase change energy needs.

Re: Light can make water evaporate without heat

#46

Post by pdonis »

its-summertime wrote: post #24
frud wrote: post #17

I can't wrap my head around this story. What does it mean in thermodynamic terms? Isn't there a fixed amount of energy per mass that it takes to convert liquid water into vapor? Why does it matter that the energy comes from light?

if you leave a container of liquid in a cold dry room, it will eventually become empty and the room will become more humid Heat speeds up this process via excitation, photon bombardment speeds up this process also. I'm guessing its more a matter of, if you heat up stuff, you need to heat up stuff and everything around it. Light can be a lot more controllable and directed. In addition, internal reflection can happen w…

> I'm guessing its more a matter of, if you heat up stuff, you need to heat up stuff and everything around it. Light can be a lot more controllable and directed

Exactly.

Re: Light can make water evaporate without heat

#47

Post by frud »

fspeech wrote: post #25
frud wrote: post #17

I can't wrap my head around this story. What does it mean in thermodynamic terms? Isn't there a fixed amount of energy per mass that it takes to convert liquid water into vapor? Why does it matter that the energy comes from light?

Entropy favors the vapor form. It doesn't necessarily take energy to evaporate.

It takes [40.66 kJ/mol](https://en.wikipedia.org/wiki/Enthalpy_of_vaporization) to vaporize water. There are no shortcuts.

Re: Light can make water evaporate without heat

#49

Post by pdonis »

fspeech wrote: post #25
frud wrote: post #17

I can't wrap my head around this story. What does it mean in thermodynamic terms? Isn't there a fixed amount of energy per mass that it takes to convert liquid water into vapor? Why does it matter that the energy comes from light?

Entropy favors the vapor form. It doesn't necessarily take energy to evaporate.

> Entropy favors the vapor form.

Under the conditions of these experiments (and under most ordinary conditions on Earth), yes. However:

> It doesn't necessarily take energy to evaporate.

Yes, it does. The water molecules in liquid water are bound to each other; that binding energy has to be supplied to enable evaporation. It just doesn't have to be "thermal" energy.

It would be correct to say that it doesn't necessarily take externally applied energy for water to evaporate. Water can evaporate using just its own internal thermal energy. In this case the evaporation process will cause the water to cool.

Re: Light can make water evaporate without heat

#50

Post by frud »

pard68 wrote: post #27
frud wrote: post #17

I can't wrap my head around this story. What does it mean in thermodynamic terms? Isn't there a fixed amount of energy per mass that it takes to convert liquid water into vapor? Why does it matter that the energy comes from light?

My 8th grade understanding is evaporation doesn't always mean steam.

What exactly is the distinction between water vapor and steam?
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