Light can make water evaporate without heat | Hacker News Reader

Light can make water evaporate without heat

news.mit.edu

473 pointsTomte188 comments

> The researchers found that the effect varied with color and peaked at a particular wavelength of green light.

I wonder if this affects photosynthesis in any way? IIRC plants are green because chlorophyll does not absorb that color. Maybe that’s partially because green light induces too much evaporation?

I’ve always wondered why inefficient green is the way plants evolved. Interesting question.

[green] plants are damaged by green light, is the way i've heard it explained. So they "block it".

Maybe, but evaporation is critical for plants. They don't have a heart to pump blood and supply nutrients, they have to rely on evaporation to move nutrients from the roots up

I imagine the leaves being green would encourage this evaporation. The upper layers of cells are probably effectively translucent and might be awash in green light, perhaps increasing the rate of evaporation and improving overall transpiration.

That’s my guess as well. The green chlorophyll is inside of the cell. It absorbs sunlight for food but reflects the green light back through the water-air boundary. Perhaps the effect on evaporation or water movement provides a better benefit for the plant than absorbing the energy of that spectra. For instance, by increasing the efficiency of respiration.

At its simplest, reflecting green light would send the light through the water twice instead of once. Neat trick, plants.

Well, the chloroplast was evolved when all life forms were submerged in the sea

The sea is on the other side of a very carefully policed barrier.

Still, it's less of a challenge to pump water from your roots if you're in the sea. Did the earliest green plants even have roots?

> Did the earliest green plants even have roots?

No. Not even all land plants have roots.

No doubt. I’m just toying with the idea of how this might impact our understanding of how plants work.

That’s a good point actually. I never thought about the fact that a tree is evaporating 100L a day without any heat. They must have figured this out.

It still uses heat. Evaporation actually helps plants keep cool by using up the heat from sun that would otherwise heat the plant too much.

Plants rely on on capillary action to bring water up: https://www.usgs.gov/special-topics/water-science-school/sci....

There's probably some aspect of evaporation, but the density of water is much, much higher when it's kept in liquid form.

That's part of the process, but not all of it. The overall process of water movement through vascular tissue is called transpiration [0]. Yes, capillary action plays a part, but the underlying process is due to differences in water potential. To avoid saturation (which would halt the flow of nutrients), water is released through pores in the leaves called stomata [1], where it evaporates, creating a differential that results in additional water being pulled in from the soil.

[0] transpiration: https://en.wikipedia.org/wiki/Transpiration

[1] stoma: https://en.wikipedia.org/wiki/Stoma

Disclaimer: I'm not a biologist. I'm a third-generation home gardener; my grandfather was the youngest of his family and the to leave the family farm in Kansas to go to college. He always maintained a large garden and loved sharing his knowledge along with his vegetables.

Edit: added links and a disclaimer

Watch Veritasium’s awesome video on this

https://www.youtube.com/watch?v=BickMFHAZR0

It's an old video, more than ten years ago, but it aged well IMHO. I re-watched it and re-learned things I have forgotten.

or the opposite, it likes the green light because reflecting it keeps the plant cool? I also recall a study where trees emitted a sort of mist into the air, perhaps the greenery helps with that process too

No. Plants reflect green to balance luminosity extremes.

Green is by far the most luminous color in the visible spectrum with dark purple being the least luminous. Plants that reflect purple absorb the most visible light and can produce the most sugars. Most plants must undergo a chemical transition to shift from production to dark mode and by choosing to reflect green maximum energy production is sacrificed for internal chemical stability.

Whats the mechanism here?

I found this which seemed informative: http://www.esalq.usp.br/lepse/imgs/conteudo_thumb/Why-did-ch...

C02 levels. We use C02 to turn our weed buds purple.

> most luminous color

What does luminous mean exactly in this context?

I know that our eyes are most sensitive to green. For example a green LED needs less power to seem as bright as a red LED.

But our eyes don't matter in this subject. So I am scratching my head about what you mean that green is the most luminous color?

Here is the math used by W3C for computing acceptable contrast: https://www.w3.org/TR/WCAG20/#relativeluminancedef

I suspect luminosity to mean percentage of brightness for a given frequency from a total light source assuming white light. For a more precise definition I would look to physics.

That’s for sRGB, which as for most colour spaces is defined relative to CIE XYZ, which is itself defined in terms of the perception of a “standard human observer”. In XYZ, the Y coordinate is the luminance (brightness) of the colour and that first equation is computing Y from the RGB value

I think this is the color (wavelength) that corresponds to the sun's emissivity peak (irradiance in W/m²).

The fact that our eyes are most sensitive to green is probably not an accident, but is it due to the color of leaves, or is it due to sunlight?

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 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.

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.

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.

> 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.

> 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.

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

What exactly is the distinction between water vapor and steam?

Temperature

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.

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.

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.

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.

> 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.

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.

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?

(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

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?

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.

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.

Light Induced Nano Aerosolization

Everything needs an acronym.

ENAA

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.

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

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

> 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.

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

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

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

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.

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.

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.

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).

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...

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.

> 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

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

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).

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

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

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?

Interesting, reminds me of Microwave Emitter from Batman :D

>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.

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

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.

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.

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.

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.

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 result of microwave radiation?)

> Could this suggest that heat is created as a result of electromagnetic resistance?

Lightbulbs getting hot would suggest that is correct, but maybe I'm missing what you're saying

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.

"Plausible photomolecular effect leading to water evaporation exceeding the thermal limit" (2023) https://www.pnas.org/doi/abs/10.1073/pnas.2312751120 :

> Abstract: We report in this work several unexpected experimental observations on evaporation from hydrogels under visible light illumination. 1) Partially wetted hydrogels become absorbing in the visible spectral range, where the absorption by both the water and the hydrogel materials is negligible. 2) Illumination of hydrogel under solar or visible-spectrum light-emitting diode leads to evaporation rates exceeding the thermal evaporation limit, even in hydrogels without additional absorbers. 3) The evaporation rates are wavelength dependent, peaking at 520 nm. 4) Temperature of the vapor phase becomes cooler under light illumination and shows a flat region due to breaking-up of the clusters that saturates air. And 5) vapor phase transmission spectra under light show new features and peak shifts. We interpret these observations by introducing the hypothesis that photons in the visible spectrum can cleave water clusters off surfaces due to large electrical field gradients and quadrupole force on molecular clusters. We call the light-induced evaporation process the photomolecular effect. The photomolecular evaporation might be happening widely in nature, potentially impacting climate and plants’ growth, and can be exploited for clean water and energy technologies.

Can low-cost integrated photonics help with e.g. water desalination and sterilization? #Goal6 #CleanWater

> Under certain conditions, at the interface where water meets air, light can directly bring about evaporation without the need for heat, and it actually does so even more efficiently than heat. In these experiments, the water was held in a hydrogel material, but the researchers suggest that the phenomenon may occur under other conditions as well.

Various methods of integrated photonics with various production costs: https://news.ycombinator.com/context?id=38056088

This is really cool, but I'm confused how we haven't stumbled into this before? Wouldn't this be quite obvious under some napkin math or does it have more to do with the specific hydrogels they were using?

This makes me think about a story that Richard Feynman told about experiments and how often people miss the most important part about cargo cult science. Here was him talking about how we got better and better resolution around the charge of an electron:

> Why didn’t they discover that the new number was higher right away? It’s a thing that scientists are ashamed of—this history—because it’s apparent that people did things like this: When they got a number that was too high above Millikan’s, they thought something must be wrong—and they would look for and find a reason why something might be wrong. When they got a number closer to Millikan’s value they didn’t look so hard. And so they eliminated the numbers that were too far off, and did other things like that. We’ve learned those tricks nowadays, and now we don’t have that kind of a disease.

https://calteches.library.caltech.edu/51/2/CargoCult.htm

It seems to be an extreme small effect, which only becomes apparent to someone looking for it under specifically controlled experimental conditions.

This sort of thing happens all the time in chemistry, material science and condensed matter physics. There are infinite possibilities of putting things together, and only some of them are interesting. It takes a lot of manual work to isolate some phenomena like this.

Two things to wonder about, that we didn't see mentioned in article, which I'm sure were considered somewhere. What about the differences in bonding strength between water molecules surrounded by water, as opposed to water in the hydro gel. The other thing, which is sort of related, is partial pressure differences between a pure water surface exposed to air, and a water hydrogel surface exposed to air. Looks like very interesting further studies required.

This is being misinterpreted here. They have found an unknown mechanism where water is absorbing light it would not absorb otherwise while in the boundary layer of a hydrogel and thus evaporating faster than it would if simply heated.

If not a mistake or some unsustainable side reaction, this could mean cheaper things that require evaporation like desalination.

A lot of people here though are saying it means things which would violate conservation of energy.

I wonder how long it’ll be until someone figures out how to utilize this effect to create a heat pump! I would guess that this “photomolecular heat pump” might be easier to miniaturize since it has no need for a compressor. I wonder if other solvents or refrigerant might also be more efficient working fluids than water. Very exciting!!

You'd definitely still need a compressor - otherwise there'd be no change in pressure driving a change in temperature. I guess you're suggesting that the photomolecular effect could be used to boil the refrigerant at the heat pump's evaporator. Boiling is a bulk process caused when the temperature of the fluid rises so much that all it's intermolecular bonds start to get broken. Evaporation is a surface process where a few molecules randomly break free of their intermolecular bonds and mix with surrounding air. Boiling, not evaporation, occurs in a heat pump's evaporator (despite the name). The photomolecular effect doesn't cause boiling, only evaporation. You wouldn't really want evaporation to happen in an evaporator because you'd need to have air around and then you'd be compressing a bunch of air along with your refrigerant, which would waste a lot of energy.

My first thought was some sort of cooling tower application. Cooling towers use evaporating water to cool various process fluids. But, when building a cooling tower, you want to pull down the temperature of the water by having evaporation absorb the heat in the water. This evaporation process actually reduces the amount of heat that gets absorbed from the water because it uses energy from light and heat from the air instead.

If this has engineering applications, it will likely be in places where the end goal is the evaporation of the water itself, such as a drying process or passive desalination.

Of note, Gang Chen was recently slandered and unjustly prosecuted by the US gov for china research collaborations. Good to see he is getting back on his feet.

Related ongoing thread:

Photomolecular effect leading to water evaporation exceeding the thermal limit - https://news.ycombinator.com/item?id=38112574

They give current solar desalination efficiency as 1.5 kilos of water per square meter. Shouldn't there be a time component?

Wouldn't that be rate rather than efficiency?

So maybe it would be better stated as 1.5 kilos per x joules of sunlight (or per area per day average? At the equator?) but as stated I can't make sense of it.

The number is more about the potential productivity of the system across a time period (a full day, I believe), rather than the speed at which it produces purified water at any given moment.

Here is an article from 2020 from MIT using the same units and wording: https://news.mit.edu/2020/passive-solar-powered-water-desali...

I do agree its confusing.

edit: Later in the MIT article they state their device (which is also a 5.8L/m2 system) - "[...] roughly 1-square-meter solar collecting area could meet the daily drinking water needs of one person.

So I will assume the numbers are per day.

edit2: On second thought, the units aren't that confusing. We already use "BTU", for example, to measure air conditioner performance. We just know that means "per hour". Then just the additional complexity of it being based off surface area. "We bought a 6L/m2 solar desalination plant" sounds the same as "We bought a 15,000 BTU A/C unit". Consumer marketing would drop the sizing - "We just bought a 10L desalination plant" and the thing is as big as it needs to be.

Also, do they mean "peak" or "mean"...

They're using Journalist Power Units. You're lucky it's not in houses per year.

How many Libraries of Congress full of water is this?

I love seeing insane units like this in the press. Dishwashers, giraffes, bowling balls, etc. Americans will measure in anything but the metric system.

One such measure stands out to me - they described the accuracy of some computer hardware as “missing one blade of grass while mowing a lawn the size of a football field flying from a helicopter over mile up in the air” (paraphrasing and using quotes to demarcate the interesting bit; not a literal quote).

I guess it’s effective and memorable because even months later, I remember the gist of the accuracy claim (though not the hardware item name)

I am not American, but I love illustrations. A number + an amount of giraffes that illustrate it are better than just the number.

My favourite is an old one from the NZ Herald: "6.85kg - the equivalent of nearly seven 1kg blocks of cheese."

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

Isn't 'transfer momentum' just a round about way of saying "heat up."

I think the interesting aspect here is that the evaporation is greater than what can be explained by heat alone.

Yeh I read it as though it’s a form of catalyst. Ultimately the light is changing the threshold energy at which water can evaporate.

Similar to the photoelectric effect. Similar say to an enzyme.

All these environmental changes to the reaction lower the ‘action’ energy making the reaction vastly more efficient or possible in an environment that it wasn’t possible in previously.

Enzymes catalyze the breaking of molecular bonds.

The embodied energy of a phase transition does not pay attention to what path you took to cross it. There’s no “threshold” between phases of water.

For H20 to move from liquid water to vapor, energy must be added. There’s no catalyst.

So either we’ve discovered some new physics since I last studied thermodynamics, or this isn’t an accurate analogy.

It could be that the energy required to break the hydrogen bond from random collisions is higher than the theoretical minimum because the angle of a typical collision "wastes" energy in imparting vibration or rotation of the water molecule rather than just imparting "escape velocity".

Just a wild guess though. Haven't yet read the article.

> For H20 to move from liquid water to vapor, energy must be added.

I think that is a wrong assumption. Liquids will naturally evaporate even with 0 external energy, assuming there is not too much pressure in the surrounding atmosphere.

The equilibrium partial pressure of water vapor with a source of water in another phase is not zero for most temperature and pressure regimes on earth, that is true.

But thermodynamics still hold; the water vapor is still in a more energetic state than liquid water or ice.

Energy levels are one thing, but liquids are just a fundamentally unstable state of matter. Absent external pressures, they will either evaporate or freeze depending on temperature. Most likely, with very low atmospheric pressures such as in the vacuum of space, the outer parts will quickly vaporize, consuming energy from the inner parts which will then freeze.

Basically, some amount of a liquid will move to the higher energy state, and other parts will move to a lower energy state. The energy to vaporize some of the liquid doesn't need to come from something external to the liquid.

> Light can transfer momentum

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

E = ħf = mc^2.

For photon,

p = hλ

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.

> > Light can transfer momentum

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

Photons have zero mass. What's your point?

Things I am wondering:

1. Is the magnitude of the effect dependent on the angle of incidence?

2. Can the effect be increased by increasing the water surface, e.g. by stirring?

3. totally layman speaking here: is the peak at green somehow explainable by the refractive index at that wavelength? The explanation can't be trivial, otherwise you'd expect the peak to be at either end of the spectrum, not at a specific wavelength, because the refractive index seems to be roughly linear with the wavelength within the visible spectrum, otherwise rainbows wouldn't work that way.

What happens to the light? Wouldn’t it have to lose energy and reduce the frequency (ie change color to the red end of the spectrum)?

Or be absorbed and not transmitted.

I dont see the breakthrough. All light causes evaporation, even at near-zero heat. Every light ray has the possibility of knocking off a molecule of water, transfering enough energy to turn it into gas. If the angles are just right, the water molecule vibrating just the right way, it gets lauched off the surface to become gas. Whether it stays gas depends on other things, but i dont see how observing the process is anything new; Better defining the process but not worthy of the article title.

'All light causes evaporation' is not really true. IR heating works because the wavelength resonates with the vibration modes of the intramolecular bonds. Water has low absorption of visible wavelengths, so you would not expect light absorption in the visible spectrum to provide enough energy to knock individual water molecules free. Water is an interesting substance and a lot of the properties of water come from intermolecular hydrogen bonding and polarity. Even though it's a liquid, there are transient molecular structures that spontaneously occur in bulk due to hydrogen bonding between water molecules. The polarity of water causes interesting effect at interfaces (surface tension, electrical double layer, etc). It's possible that the water in the hydrogel is forming some hydrogen bonded structure that interacts with green light. Where the individual water molecules won't strongly interact, the larger structure does. That could lead to the ejection of 'packets' of water molecules as discussed in the paper. Why and what these multi-molecular structures are? No idea. But this is a very interesting effect.

Doesn’t the light itself transfer heat? Like with a magnifying glass and an ant, it amplifies and possibly focuses the light to burn the ant. Or even more simply, sunburns.

From the paper:

For the wavelength-dependent evaporation measurement, we used LED with different wavelengths. LED lamps were purchased from Chanzon with rated power of 100 W and different wavelengths: purple 390nm, blue 440nm, green 520nm, yellow 590nm, red 650nm, and IR 850nm.

Note that rated power is not the same as energy. E=hf. Since green had the most impact on evaporation, this phenomenon is not similar to the photoelectric effect.

> evaporation rates are wavelength dependent, peaking at 520 nm

Basically green is apparently a great wavelength to evaporate water at. Interesting

I'm skeptical. There is a "latent heat of evaporation" that has to be put in; some of it is coming from the light.

So can sound, almost as if anything with energy can move or excite particles.

In other news scientist discovers water is wet.

Sound would literally mean waves of kinetic energy hitting both the surface of the water and traveling inside it. Does that really count as "without heat"?

And what type of energy is light?

One which can get absorbed by a single molecule individually as opposed to sound which has to affect large groups of them, increasing movement / heat.

Sound is movement, it’s not a particle. It’s your way of sensing stuff smashing together by feeling the ripple that comes off it

Both kinetic energy.

Imagine a material, or a passive device, that can evaporate a glass of water fast, using only the heat of the environment, as the energy source. That is possible, and would be huge.

It would not be. If you do not replace the "heat" absorbed by the evaporating water, the room cools down. The kinetic energy needed to maintain water molecules as a gas instead of condensing comes from somewhere.

This is a very complicated swamp cooler.

Any one else familiar with “solar vapor drive” in the building science community (vapor being driven into buildings from the outside simply due to solar exposure)? My understanding is that it is expected even in the absence of warmth.

Is this the same effect?

> … even in the absence of warmth.

Tangent (adjacent?): my ice “cubes” evaporate. (They’re hollow cylinders) If I put into my freezer more ice than I can use in a week, about half of the top layer of blocks has lost quite a bit of their substance.

Your ice is sublimating. Cold dry air absorbs vapour directly from the solid water. The more surface area is exposed to air (hollow cylinders) the more it'll sublimate.

I must be missing something. My ice cubes in my freezer evaporate with no light and no heat. My understanding of the situation was the surrounding air was pulling off moisture because of some sort of osmosis effect.

So why is this news?

There is heat in your freezer, unless you cool it to 0 Kelvin.

This is news because we thought that light by itself doesn't actually affect water evaporation. We found a new mechanism for evaporation, that's exciting, scientists love when they find something that allows them to find out even more about how things work.

This technology leads directly to cloud lasers. Yesss

Cloud what?

lasers that make clouds, I assume - cloud lasers

Would this be useful for desalination or laundry?

This is a massive discovery!

It's hard to even imagine the multitude of applications of this.

Funny enough, the foggy season has the least heat.. and the least light

Now I know the reason for my clothes drying when hanging them outside.

is this the reason plants are mostly green? :?

Why stop at water

Yea, I wonder if it could work on AI based Blockchain.

We can use NFTs to pay for it in the Metaverse.

true. skin evaporates quicker in the hot desert sun.

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.

test

And heat can make water evaporate without light! Wow!

I can make water evaporate too (by drinking it and making it disappear;)

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