[HN Gopher] Light can make water evaporate without heat
___________________________________________________________________
Light can make water evaporate without heat
Author : Tomte
Score : 432 points
Date : 2023-11-02 17:59 UTC (1 days 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)
| bmacho wrote:
| I am not American, but I love illustrations. A number +
| an amount of giraffes that illustrate it are better than
| just the number.
| Nition wrote:
| My favourite is an old one from the NZ Herald: "6.85kg - the
| equivalent of nearly seven 1kg blocks of cheese."
| dr_dshiv wrote:
| This technology leads directly to cloud lasers. Yesss
| _boffin_ wrote:
| Cloud what?
| stevemk14ebr wrote:
| lasers that make clouds, I assume - cloud lasers
| 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.
| mercutio2 wrote:
| 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.
| tsimionescu wrote:
| 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.
| 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.
| stubish wrote:
| 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.
| 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.
| pdonis wrote:
| _> 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.
| elil17 wrote:
| 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.
| pixl97 wrote:
| 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.
| 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?
| mikewarot wrote:
| 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 degC, 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
| degC 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
| 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.
| pard68 wrote:
| 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.
| dheera wrote:
| Huh what? I thought steam === water vapor
| pard68 wrote:
| 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.
| 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.
| depereo wrote:
| 'clumped vapor' might be way more effective for
| cloudseeding
| elil17 wrote:
| Yes, although the molecule clusters evaporate rapidly
| after they leave the surface
| gpm wrote:
| Even if they're in a high humidity environment where you
| could plausibly seed a cloud in the first place?
| elil17 wrote:
| 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.
| 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?
| itishappy wrote:
| 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.
| elil17 wrote:
| 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.
| JohnFen wrote:
| 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?
| elil17 wrote:
| Exactly
| JohnFen wrote:
| 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.
| 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?
| dfex wrote:
| well played
| aatd86 wrote:
| You emmit green light from your eyes?
| LoganDark wrote:
| If you have green eyes, yes.
| layer8 wrote:
| Being green in the face should be sufficient.
| 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)
| grishka wrote:
| 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.
| jordan_curve wrote:
| isn't it generally recommended to use distilled water in
| these humidifiers for exactly this reason?
| tjmc wrote:
| Most datacentres ban the use of ultrasonic humidifiers
| for exactly this reason.
| ponyboy123 wrote:
| 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
| Modified3019 wrote:
| 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.
| dr_dshiv wrote:
| 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.
| _nalply wrote:
| 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...
| dr_dshiv wrote:
| Yeah, obviously strong enough to make the energy balance.
| But still awesome.
| foota wrote:
| 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.
| thaumasiotes wrote:
| > 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
| geoffcampbell64 wrote:
| Is there anyway in which this effect could be used to create
| a new type of heat pump?
| elil17 wrote:
| 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).
| farhanhubble wrote:
| 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?
| elil17 wrote:
| 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.
| cyanydeez wrote:
| 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
| thsksbd wrote:
| Once airborne, the little droplet would be below the critical
| radius of water and have to absorb heat from air and fully
| evaporate.
| bloopernova wrote:
| 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?
| pomtato wrote:
| Interesting, reminds me of Microwave Emitter from Batman :D
| Modified3019 wrote:
| >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.
| frud wrote:
| Thank you. This is the first thing I've read about this story
| that is at all coherent.
| 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.
| fsckboy wrote:
| 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
| 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.
| thayne wrote:
| 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.
| movpasd wrote:
| 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.
| 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
| abdullahkhalids wrote:
| 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.
| 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?
| canadianfella wrote:
| I've always wondered why inefficient green is the way plants
| evolved. Interesting question.
| genewitch wrote:
| [green] plants are damaged by green light, is the way i've
| heard it explained. So they "block it".
| malfist wrote:
| 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
| chimpanzee wrote:
| 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.
| xigency wrote:
| 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.
| deanputney wrote:
| At its simplest, reflecting green light would send the
| light through the water twice instead of once. Neat
| trick, plants.
| bdd8f1df777b wrote:
| Well, the chloroplast was evolved when all life forms were
| submerged in the sea
| jacquesm wrote:
| The sea is on the other side of a very carefully policed
| barrier.
| geysersam wrote:
| 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?
| eru wrote:
| > Did the earliest green plants even have roots?
|
| No. Not even all land plants have roots.
| montjoy wrote:
| No doubt. I'm just toying with the idea of how this might
| impact our understanding of how plants work.
| MagicMoonlight wrote:
| 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.
| yetihehe wrote:
| 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.
| SkyPuncher wrote:
| 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.
| mechanicalpulse wrote:
| 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
| iamgopal wrote:
| Watch Veritasium's awesome video on this
| _nalply wrote:
| 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.
| luxuryballs wrote:
| 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
| austin-cheney wrote:
| 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.
| kalium-xyz wrote:
| Whats the mechanism here?
| austin-cheney wrote:
| I found this which seemed informative:
| http://www.esalq.usp.br/lepse/imgs/conteudo_thumb/Why-did-
| ch...
| bhewes wrote:
| C02 levels. We use C02 to turn our weed buds purple.
| _nalply wrote:
| > 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?
| austin-cheney wrote:
| 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.
| 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.
| gtbcb wrote:
| 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)?
| jagged-chisel wrote:
| Or be absorbed and not transmitted.
| pomian wrote:
| 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.
| geoffcampbell64 wrote:
| 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!!
| elil17 wrote:
| 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.
| ezconnect wrote:
| Now I know the reason for my clothes drying when hanging them
| outside.
| spandextwins wrote:
| And heat can make water evaporate without light! Wow!
| waldothedog wrote:
| 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?
| jagged-chisel wrote:
| > ... 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.
| lovemenot wrote:
| 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.
| souplantation wrote:
| test
| Sparkyte wrote:
| So can sound, almost as if anything with energy can move or
| excite particles.
|
| In other news scientist discovers water is wet.
| viraptor wrote:
| 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"?
| Sparkyte wrote:
| And what type of energy is light?
| viraptor wrote:
| 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.
| MagicMoonlight wrote:
| 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
| bmacho wrote:
| 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.
| mrguyorama wrote:
| 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.
| bethekind wrote:
| > evaporation rates are wavelength dependent, peaking at 520 nm
|
| Basically green is apparently a great wavelength to evaporate
| water at. Interesting
| dclowd9901 wrote:
| 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?
| quickthrower2 wrote:
| There is heat in your freezer, unless you cool it to 0 Kelvin.
| yetihehe wrote:
| 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.
| mfeldheim wrote:
| Funny enough, the foggy season has the least heat.. and the least
| light
| nikster wrote:
| This is a massive discovery!
|
| It's hard to even imagine the multitude of applications of this.
| moring wrote:
| 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.
| sandworm101 wrote:
| 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.
| waiquoo wrote:
| '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.
| hk1337 wrote:
| 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.
| 1970-01-01 wrote:
| 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.
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