[HN Gopher] In a surprising finding, light can make water evapor...
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In a surprising finding, light can make water evaporate without
heat
Author : Tomte
Score : 138 points
Date : 2023-11-02 17:59 UTC (5 hours ago)
(HTM) web link (news.mit.edu)
(TXT) w3m dump (news.mit.edu)
| smolder wrote:
| They give current solar desalination efficiency as 1.5 kilos of
| water per square meter. Shouldn't there be a time component?
| lcnPylGDnU4H9OF wrote:
| Wouldn't that be rate rather than efficiency?
| smolder wrote:
| 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.
| pests wrote:
| 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.
| klyrs wrote:
| Also, do they mean "peak" or "mean"...
| IshKebab wrote:
| They're using Journalist Power Units. You're lucky it's not in
| houses per year.
| hinkley wrote:
| How many Libraries of Congress full of water is this?
| suprjami wrote:
| I love seeing insane units like this in the press.
| Dishwashers, giraffes, bowling balls, etc. Americans will
| measure in anything but the metric system.
| Sai_ wrote:
| 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)
| dr_dshiv wrote:
| This technology leads directly to cloud lasers. Yesss
| _boffin_ wrote:
| Cloud what?
| sdfghswe wrote:
| Why is it surprising? Light can transfer momentum into the water,
| which is was matters for the process of evaporation...?
| vivekd wrote:
| 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.
| ace2358 wrote:
| 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.
| mercutio2 wrote:
| 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.
| catskul2 wrote:
| 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.
| tsimionescu wrote:
| > 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.
| plugin-baby wrote:
| > Light can transfer momentum
|
| Momentum is mass x velocity; what's the mass of a photon?
| ace2358 wrote:
| E = hf = mc^2.
|
| For photon,
|
| p = hl
| FourHand451 wrote:
| You can't use classical physics to calculate the momentum of
| a photon.
|
| https://en.wikipedia.org/wiki/Photon#Relativistic_energy_and.
| ..
| floxy wrote:
| 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.
| sdfghswe wrote:
| > > Light can transfer momentum
|
| > Momentum is mass x velocity; what's the mass of a photon?
|
| Photons have zero mass. What's your point?
| bo-tao wrote:
| Why stop at water
| geodel wrote:
| Yea, I wonder if it could work on AI based Blockchain.
| glitchc wrote:
| We can use NFTs to pay for it in the Metaverse.
| Eduard wrote:
| true. skin evaporates quicker in the hot desert sun.
| westurner wrote:
| "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
| croes wrote:
| Related https://news.ycombinator.com/item?id=38112574
| yeknoda wrote:
| 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.
| frud wrote:
| 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?
| its-summertime wrote:
| 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.
| pdonis wrote:
| _> 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.
| fspeech wrote:
| Entropy favors the vapor form. It doesn't necessarily take
| energy to evaporate.
| frud wrote:
| It takes [40.66 kJ/mol](https://en.wikipedia.org/wiki/Enthalp
| y_of_vaporization) to vaporize water. There are no shortcuts.
| pdonis wrote:
| _> 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.
| ethanbond wrote:
| > 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.
| pdonis wrote:
| _> 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.
| elil17 wrote:
| 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.
| tzot wrote:
| 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.
| pard68 wrote:
| My 8th grade understanding is evaporation doesn't always mean
| steam.
| frud wrote:
| What exactly is the distinction between water vapor and
| steam?
| wnoise wrote:
| Temperature
| px43 wrote:
| 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?
| Sai_ wrote:
| 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.
| dheera wrote:
| Huh what? I thought steam === water vapor
| zwieback wrote:
| 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.
| ummonk wrote:
| 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.
| pdonis wrote:
| _> 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.
| elil17 wrote:
| 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.
| syntaxing wrote:
| 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?
| klysm wrote:
| (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
| elil17 wrote:
| Yes, that's exactly right! Although I would caveat that and
| say that we don't know whether it is actually useful
| engineering-wise.
| JohnFen wrote:
| 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?
| galangalalgol wrote:
| Light Induced Nano Aerosolization
|
| Everything needs an acronym.
| godshatter wrote:
| ENAA
| hawk_ wrote:
| So a watched kettle might boil faster?
| tzot wrote:
| Did you get a green light to make that joke here?
| elil17 wrote:
| Worth noting that I'm going off the preprint since I don't
| have PNAS access.
|
| https://arxiv.org/pdf/2201.10385.pdf
| m463 wrote:
| > 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)
| ankitml wrote:
| 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.
| frud wrote:
| Quantum or no, there is no shortcut around the 40.66 kJ/mol
| it takes to evaporate water.
| ndonnellan wrote:
| 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.
| labcomputer wrote:
| > 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.
| kortex wrote:
| 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.
| bilsbie wrote:
| Would this be useful for desalination or laundry?
| Whooping7116 wrote:
| I can make water evaporate too (by drinking it and making it
| disappear;)
| xwkd wrote:
| 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?)
| throwawaaarrgh wrote:
| > 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
| ok_computer wrote:
| 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.
| NikkiA wrote:
| But heat _IS_ light, and vice versa.
| elil17 wrote:
| 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.
| kazinator wrote:
| I'm skeptical. There is a "latent heat of evaporation" that has
| to be put in; some of it is coming from the light.
| klysm wrote:
| 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?
| 101011 wrote:
| 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
| montjoy wrote:
| > 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?
| dang wrote:
| Related ongoing thread:
|
| _Photomolecular effect leading to water evaporation exceeding
| the thermal limit_ -
| https://news.ycombinator.com/item?id=38112574
| colechristensen wrote:
| 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.
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