Hacker News Reader

Comment by frud | Hacker News Reader

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?

View on HN

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.

> I'm not really sure whether to even call it evaporation

I can't help but think of ultrasonic humidifiers/misters, which use vibration to do evaporation-adjacent kinds of things.

I also wonder if specific wavelengths of light are involved (sort of how 2.4ghz microwaves work on water)

Water particles produced by an ultrasonic humidifier are larger than those of real steam. I know it because I have one and if I run it for long enough, everything gets covered in a nasty white residue, probably salts from the water. Real evaporation doesn't do that.

I think the difference is that evaporation creates water vapor, whereas an ultrasonic dehumidifier is creating water droplets, some of which are very small, but are still droplets that can carry minerals from the water

Most datacentres ban the use of ultrasonic humidifiers for exactly this reason.

isn't it generally recommended to use distilled water in these humidifiers for exactly this reason?

Yep, both the minerals, and the microorganisms breeding in the water get thrown into the air.

For constant use, I personally recommend "evaporation humidifiers" that use a wick and fan to induce evaporation. The wick will need to be replaced every several weeks.

Cue “burning” salt water video: https://youtu.be/e8utkoK2DhA?si=t2cvuu4V-hFKFjM8

I’m still blown away that radio frequencies can dissociate hydrogen without an electrode. I haven’t read a good explanation of the phenomenon.

Ever heard of a Plasma gun? Or pressurized water cutting? Heck Lasers do the same thing.

In every case, it’s really a collision between different particles at various energy levels, leading to different results.

Take an extremely fast neutron and collide it with an atom, and you could alter the chemical composition of the atom/end up with a different element (basic principle of breeder reactors). Fascinating stuff.

I wonder how strong the radiation was. If strong enough it dissociates and then by burning it gives back the energy... Looks like a fake perpetuum mobile...

Yeah, obviously strong enough to make the energy balance. But still awesome.

Yes it's awesomeness reminds me of the Tcherenkoff light I once saw with my own eyes. It was a deep pool of a test reactor and I even asked the guide if I am allowed to put my index finger into the water. It was warm. I looked long and deep at the fascinating white-blue light in the water.

It is so awesome that no public relations work is needed. The thing speaks for itself and is so convincing.

I suspect you're correct, at least partially.

My personal suspicion is that the light waves are hitting the extremely diffuse binder material and then ejecting the water light a billiard ball or a solar sail effect. It may have a resonance (vibration) with the binder structure. See below image for example of binder material.

https://upload.wikimedia.org/wikipedia/commons/d/d3/Short-pe...

This was also my first thought! And they seem to cool the air nearby. The above explanation makes sense (where bonds are broken in the air).

So a watched kettle might boil faster?

Did you get a green light to make that joke here?

well played

You emmit green light from your eyes?

If you have green eyes, yes.

Being green in the face should be sufficient.

One more thought for how to explain this, for those who don't want to worry about intermolecular bonds and what energy goes where:

Essentially they found that light makes a "splash" when it hits water (at least in certain circumstances).

Worth noting that I’m going off the preprint since I don’t have PNAS access.

https://arxiv.org/pdf/2201.10385.pdf

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?

Yes, that’s exactly right! Although I would caveat that and say that we don’t know whether it is actually useful engineering-wise.

'clumped vapor' might be way more effective for cloudseeding

Yes, although the molecule clusters evaporate rapidly after they leave the surface

Even if they're in a high humidity environment where you could plausibly seed a cloud in the first place?

Thinking about this again, and I'm not sure. On one hand, yes, you could theoretically see a situation where the clusters want to grow because there are so many water molecules around them. But normally we seed clouds with much, much larger aerosols. Larger diameter = different (more favorable, I believe) surface energy.

Speaking as an person ignorant of this entire field, it seems to me that if it's the case that groups of molecules are breaking off rather than individual ones, the total energy required would be less.

But it's comparing apples to oranges, because the "end product" is different. In one, you have a cloud of individual molecules. In the other, you have a cloud of molecule "clumps". If you take it further and break those clumps down to individual molecules as well, I expect the total energy input would match that of evaporating water in the normal way.

To the experts reading this, am I close?

You’re thinking about it right if you’re zoomed into the surface of the water plus a few millimeters above it. But the molecule clusters themselves evaporate after that, which pulls heat from the air.

Gotcha. That heat from the air is also energy input that has to be counted, so it still would equal out in the end, right?

Exactly

So a good analogy might be that it's like a tiny version of what happens in an atomizer. It takes more energy to evaporate water than to turn the same amount of water into a very fine mist. The droplets will then evaporate on their own, using an amount of energy equal to the difference between evaporation and misting.

Thank you, I've got a little clearer view of my world.

Sounds good to me. Energy is a function of state, so if you start and end with the same state, it'll require the same amount of energy. If it takes less light to knock loose bigger clumps, it'll take additional energy from somewhere else to break them up. The remainder will probably come from thermal energy from the air and water, but you could also use something like a laser or chemical reaction.

(armchair science) it seems like if a bigger bunch breaks off, you get better heat transfer from the increased surface area and it would evaporate much faster. Probably the same energy price but much more rapidly applied

Do photons hitting the surface of water transfer any kinetic energy?

Does the evaporation occur as soon as the light is switched on, or does it take a while to get started?

Thank you. This is the first thing I've read about this story that is at all coherent.

What happens if the air is saturated? Does the molecule clump just settle back down where it came from, or would it stay suspended somehow in the air (mist?)?

Alternatively, I wonder if this could be used as a super swamp cooler, I'm picture water dripping or flowing from a tube, a laser causing it to "burst apart", and then the droplets formed rapidly cooling the surroundings due to their surface area.

It seems possible we could improve swamp cooling with this, as with desalination, though in general you want to avoid putting energy into the system, I think. Powering a laser might overcome the benefits... unless you shot lasers in from outside? Even so, it might be best to use [filtered?] natural light for such a system.

Once airborne, the little droplet would be below the critical radius of water and have to absorb heat from air and fully evaporate.

>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.

Sounds like a chance to coin a suitably obtuse and prim science name, like Prosocial Evaporation, as opposed to Solitary Evaporation.

Gregarious and perhaps Convivial Evaporation are also good candidates.

I had this understanding too from university physics but now that I think about it why do we assume each molecule has to break away independently. Why can't lumps of molecules break away as long as the group has enough energy to sever bonds with the rest of the bulk?

Well it’s not that it never happens, it’s just that it’s not particularly likely. Heat is disordered kinetic energy, so most often molecules won’t be traveling in the same direction.

from my understanding, water is typically colloidal, so it would make sense that there's no symmetrical bonding to adjacent molecules and that could easily lead to groups being evaporated.

in many cases, layman's science is oversimplified for the benefit of college science. this might be the case

> 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.

It's simple. You have a small puddle of water on the ground that slowly rains up into the air. ;D

Light Induced Nano Aerosolization

Everything needs an acronym.

ENAA

Is there anyway in which this effect could be used to create a new type of heat pump?

Interesting, reminds me of Microwave Emitter from Batman :D

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

> 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.

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.

> 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.

This sounds more like the light is making thin fog, not water vapor.

Well, sort of. I’m conceptualizing it as an intermediate state between a vapor and an aerosol. The aerosol would have way more intermolecular bonds per molecule on average since most of its molecules are in the insides of droplets. But these molecule clusters have all or almost all their molecules exposed on the clusters surface, so they have many fewer intermolecular bonds.

Evaporation should be a function of surface area. In a large pool of water the evaporation should occur primarily in the upwards direction. Any molecular activity that is sideways or downwards will not lead to the molecule escape.

Once a water particle escapes, depending on the clump size it should have more degrees of freedom in evaporation and I would think surface tension would be reduced.

As I understand it, when you heat water, you give energy to all of the water molecules that start moving faster in their random direction. So molecules that were directed towards outside the water mass are “directly” extracted, and the rest will bounce around in the increasing pressure until they are “indirectly” extracted (I used “directly” and “indirectly” non-scientifically here, just to make a distinction.) It takes quite a lot of energy which leads to evaporation over time.

I think what they found is a set of circumstances where the energy of the light “chips off pieces” of water, so the energy needed is much less. A small broken-off “piece” (or cluster of water molecules) has a very large ratio of surface area over volume, so the rest of the evaporation is taken care of by the surrounding environment as-is.

Yeah, I have the same question. The blurb is too unclear but suggests it's almost more of a mechanical thing: the photon bumps into some molecules that are almost already on the gas side of things and that's enough to turn them into fog. So it's not like the water is heating up and then jumping out, it's more like it's getting knocked to the gas side, like in a humidifier.

> 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.

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.

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

The shortcut is apparently breaking the bonds of groups of molecules, rather than supplying enough energy to break all the bonds of each individual molecule. But still technically correct, with the airborne groups breaking apart into individual modules cooling the air. But you don't have to supply all that energy and get to break some theoretical limits.

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 within water against air, meaning a free second (third, fourth, fifth, etc) attempt at depositing energy somewhere.

> 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.

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.

yes, but that's the exact opposite of everything else that's being said which is that there is no absorption taking place. It's not a good article in terms of explanatory power

> 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.

You don't need monochromatic green light - that was just the test condition to find the best wavelength. Broad spectrum sunlight should do the trick.

Thermodynamics (well, equilibrium thermodynamics) doesn't say too much, because evaporation is a non-equilibrium effect. They're not saying more water vapor is produced for free given the energy, rather that the _rate_ of evaporation is increased — and that is up for grabs.

Because visible light doesn't interact with water very much. It usually just passes through. For many processes the frequency of the light makes a big difference. Longer wavelength microwaves however do interact with water molecules, and that is how microwave ovens work.

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

What exactly is the distinction between water vapor and steam?

Where I'm sitting now, the humidity in the air is about 40%. Would you consider that to be "steam"? I feel like steam generally needs to be hot, or at least somewhat warm. Cold steam exists, but is specifically called out as an exception to the rule. Maybe steam needs to be somewhat translucent?

Humidity is a percentage of the amount of water that could be in the air, which is when the vapor pressure of water is equal to the partial pressure.[1]

So if you're in a room 20 °C, the partial pressure of water is 0.0231 Atmospheres * 40% --> 0.00924 water (0.924%)

Steam is 100% water, and generally can only happen at 100 °C or higher.

The highest humidity/temperature I've experienced is about 100% at 99F, which works out to around 7% water in the air. It was a miserable day, and I was a young/healthy kid at the time. [1] https://en.wikipedia.org/wiki/Vapour_pressure_of_water

Isn’t steam just water vapour off boiling water? I.e., water vapour which, on condensing, yields heat to the surface on which it condenses. Steam is hot water vapour, IOW.

steam is specifically from heating water and often enough is a gaseous water. Water vapor (or "wet steam") is an aerosol of liquid water. But both are means of evaporation.

Temperature

Huh what? I thought steam === water vapor

No steam is specifically from heating water and often enough is a gaseous water. Water vapor (or "wet steam") is an aerosol of liquid water.

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.

Presumably the surrounding air is below 100% humidity. So the light isn’t heating up and vaporizing the water, but rather helping the air pick it up faster.

Reply on news.ycombinator.com