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Light can make water evaporate without heat

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Re: Light can make water evaporate without heat

#51

Post by ndonnellan »

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?

I think the key paragraph is buried:

"Though water itself does not absorb much light, and neither does the hydrogel material itself, when the two combine they become strong absorbers, Chen says. That allows the material to harness the energy of the solar photons efficiently and exceed the thermal limit, without the need for any dark dyes for absorption."

So when water is combined with hydrogel, they absorb more light -> more light = more energy -> more energy = more evaporation.

Re: Light can make water evaporate without heat

#52

Post by sdfghswe »

plugin-baby wrote: post #15
sdfghswe wrote: post #4

Why is it surprising? Light can transfer momentum into the water, which is was matters for the process of evaporation...?

> Light can transfer momentum Momentum is mass x velocity; what’s the mass of a photon?

> > Light can transfer momentum

> Momentum is mass x velocity; what’s the mass of a photon?

Photons have zero mass. What's your point?

Re: Light can make water evaporate without heat

#53

Post by frud »

ankitml wrote: post #45
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.

Quantum or no, there is no shortcut around the 40.66 kJ/mol it takes to evaporate water.

Re: Light can make water evaporate without heat

#55

Post by labcomputer »

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?

> 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?

Yes. The rest of the energy comes from the bulk water/hydrogel in other words, the bulk water is cooled by this process.

What’s happening is that energy is sloshing around between various degrees of freedom of the system (the temperature of the system is not zero). When it sloshes is such a way that a water molecule near the surface has more kinetic energy than the bond strength between it and the bulk, that molecule evaporates. Since the “sloshed” molecule has greater-than-average energy just before evaporation, the average energy of the remaining bulk water is reduced (the bulk cools).

But the interesting thing here is that it seems that they have found a resonance where the photon will not just cause the water molecule to evaporate “early” and also carry with it more excess energy than the phone came in with (hence having an evaporation rate 2x expected).

I wonder if this has something to do with the hydrogel causing the water to behave more like a solid, and enabling some kind of phonon-photon coupling process that isn’t supported in pure bulk water

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

Practically, because they want to make a solar desalination system (though this just raises the question of how do you get monochromatic green light from the solar spectrum).

Scientifically, because it is interesting that the photon will trigger a water molecule to take off with more energy than the photon. Also, it feels entropically weird.

Re: Light can make water evaporate without heat

#57

Post by elil17 »

NikkiA wrote: post #54

But heat IS light, and vice versa.

Light isn't heat (heat is the kinetic energy of the disordered movement of particles). Evaporation normally occurs because molecules have enough of this disordered kinetic energy to break from their intermolecular bonds and enter the air.

In this case, light is physically knocking small clusters of several molecules into the air together in an ordered way.

Re: Light can make water evaporate without heat

#58

Post by floxy »

FourHand451 wrote: post #22
plugin-baby wrote: post #15

Earlier quoted context omitted.

> Light can transfer momentum Momentum is mass x velocity; what’s the mass of a photon?

You can't use classical physics to calculate the momentum of a photon. https://en.wikipedia.org/wiki/Photon#Relativistic_energy_and...

Classical electromagnetic waves from Maxwell's equations (i.e. non-quantized) also carry momentum, right?

https://www.youtube.com/watch?v=bvzr2HbbPC8

(Maxwell's equations are consistent with relativity)

...or another way of looking at it (that I presume Boltzmann would agree with). If your had a single black body mass at some temperature greater than absolute zero in an otherwise empty universe, it would radiate away heat and thus cool off. The cooler body means the individual atoms in the mass have less energy and less momentum. If momentum is conserved, then that momentum must have been carried away from the mass in the mass-less radiation. Another neat thing is that light can also have angular momentum.

Re: Light can make water evaporate without heat

#59

Post by elil17 »

pdonis wrote: post #43
ethanbond wrote: post #26

Earlier quoted context omitted.

> 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 differen…

The paper is accounting for the energy that the light is delivering. The very neat thing about this paper is that it does change the bonding energy between water molecules that has to be overcome for evaporation to occur.

They observed evaporation of clusters of molecules, not individual molecules. Since whole groups of molecules are flung into the air, not all of the intermolecular bonds need to be broken for them to evaporate. Heat from the air is later used to break those clusters apart into individual molecules.

Re: Light can make water evaporate without heat

#60

Post by syntaxing »

elil17 wrote: post #44
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 intermolecula…

But in a closed system, the energy to boil or evaporate the same amount of water is the same right? As in, you still have to pay the energy price but evaporating all the water is probably easier engineering wise?
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