In a Surprising Finding, Light Can Make Water Evaporate Without Heat 85
David L. Chandler reports via MIT News: 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. After carrying out a series of new experiments and simulations, and reexamining some of the results from various groups that claimed to have exceeded the thermal limit, a team of researchers at MIT has reached a startling conclusion: 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.
The phenomenon might play a role in the formation and evolution of fog and clouds, and thus would be important to incorporate into climate models to improve their accuracy, the researchers say. And it might play an important part in many industrial processes such as solar-powered desalination of water, perhaps enabling alternatives to the step of converting sunlight to heat first. The new findings come as a surprise because water itself does not absorb light to any significant degree. That's why you can see clearly through many feet of clean water to the surface below. The findings have been published in the journal PNAS.
The phenomenon might play a role in the formation and evolution of fog and clouds, and thus would be important to incorporate into climate models to improve their accuracy, the researchers say. And it might play an important part in many industrial processes such as solar-powered desalination of water, perhaps enabling alternatives to the step of converting sunlight to heat first. The new findings come as a surprise because water itself does not absorb light to any significant degree. That's why you can see clearly through many feet of clean water to the surface below. The findings have been published in the journal PNAS.
Re:non story (Score:5, Informative)
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If it is a particular wavelength, assuming we can reproduce said wavelength, I would think we would still use some process to convert to said wavelength. Currently that would entail converting to electricity and then back to light. A really big breakthrough would be a process in which we can efficiently ramp up or down the wavelength in light to produce the correct wavelength.
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I didn't read the paper at the time of posting, so I find it better to make no assumptions.
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OK, but how efficiently can we make it? Do we make it directly from light of other wavelengths, or do we need to first convert the incoming light to electricity?
That the evaporation would happen I find unsurprising. Ditto that we can reproduce 520nm light. But I don't off hand know of any way to convert light from one frequency to another other than Doppler or gravitational wave shift. (Most ways I can think of are just filters, which isn't useful here.)
Green is one of the more efficient lasers (Score:2)
Diode lasers, typically infrared are efficient and doubling their wavelength is also easy and efficient. Will it be efficient enough? I can't say. But it's a very sweet spot for light generation at single frequencies
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Diode lasers are electrically powered, even if they are pumped by light. Which I don't think they typically are. (Dye lasers ARE pumped by light, but also require electric power.)
I think the process will not be efficient. However, if you're going to model interactions that occur anyway, like sunlight on the ocean surface, then you need to include this effect. (Well, actually they haven't shown that it occurs in that context, but I'd be surprised if it didn't.)
I think that this is basically a process tha
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I mean, creating a system that includes or maximizes this effect would be a step up in efficiency, however.
Re: non story (Score:2)
See: dye laser
https://en.m.wikipedia.org/wik... [wikipedia.org]
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That strikes me as a rather expensive AND inefficient process. Perhaps things have changed, since it's been decades since I looked into that at all, but I have my doubts. I'd guess that it would be cheaper and more efficient to use solar cell-> electricity -> semiconductor laser -> light. (And many places you could get the electricity without the solar cell.) OTOH, even using a semiconductor laser for the light is probably rather inefficient.
I notice that the summary was mainly speculating abou
Re: non story (Score:2)
I wasn't aware that we were going for efficiency, my comment was just in response to "if we can make light at that wavelength". We can make light at any wavelength in the visible, near UV, and throughout the IR regions with lasers. Dye lasers are a standard way to make a *lot* of light of a particular wavelength because dyes have broad gain curves. It's possible that you could get a semiconductor laser to emit at 520 nm. But yes lasers in general aren't very efficient.
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Re:non story (Score:5, Informative)
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Even at the same temperature, laundry dries faster in sunlight than on a cloudy day.
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On a day of the same temperature, clothes will be hotter in the sun.
Re:non story (Score:5, Informative)
What? No.
The "temperature" of which you speak is the ambient air temperature. The sunlight -- diffuse on a cloudy day, intense on a sunny day -- applies direct thermal energy in addition to the ambient air.
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Snow slowly sublimates even when it's below freezing.
No news there.
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Snow slowly sublimates even when it's below freezing.
No news there.
Snow slowly sublimates in the dark even when below freezing.
This is different.
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Everyone who has ever dried laundry on a clothesline knew about it.
Even at the same temperature, laundry dries faster in sunlight than on a cloudy day.
Tell us you don't understand what is being talked about without saying you don't understand what is being talked about.
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Plants should be looked at more closely (Score:5, Informative)
The surprising surprise behind that is: if maximum evaporation happens at 520nm, a very lush green, does it have by any chance any relation to plants, who use chlorophyll for their photosynthesis and thus appear mostly green?
Look at the absorption spectrum of the different chlorophyll variants and you will notice that all those different variants leave out the same wavelength region. You probably guess which one: all chlorophyll variants exclude the 520nm region, some even drop off very sharply before that point.
https://en.wikipedia.org/wiki/... [wikipedia.org]
https://www.atophort.com/files... [atophort.com]
As if every species that evolved some version of cholorophyll for their environment specifically evolved around that. For some reason, it seems as if it was better for all photosynthesizing species to NOT absorb 520nm light but to even REFLECT as much of it as possible from specifically this wavelength. As this is wasting potential solar energy income for that cell, they probably have a good reason to do it that far outweighs the wasted potential. If conserving water and minizing unwanted evaportation was the reason, it could make some sense for evolution to come to this point.
The study didn't concern itself with plants at all, but several biologists will probably home in on it next Monday, I guess.
Re:Plants should be looked at more closely (Score:4, Interesting)
If conserving water and minizing unwanted evaportation was the reason, it could make some sense for evolution to come to this point.
I think you may have that backwards. Reflecting green light doubles the chances of green photons evaporating water from the water-air boundary - once on the way in, and again on the way out. It is thought that wet leaves reduce photosynthesis as it blocks leaf respiration.
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Mod parent 'interesting' :-)
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> by directly spalling off lumps of water molecules at the water-air interface.
I think my knock-off microwave uses this same technology. It only heats the very outside of food while the inside remains ice cold.
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The outside is melting from your kitchen's warmth.
So, the outside has liquid water, molecules that can be rapidly spun up (excited) by the microwave's particular wave frequency. Spinners then have collisions that pass around energy.
The inside has frozen ice. The molecules have a rigid crystal structure and don't really spin up.
And then there's probably some noise about "hot spots" and wavelength regions that benefit from the rotating plate.
Turn down the power level to 2/10 or 3/10 and you can "safely" run f
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The headline is a bit dumb.
"Without heat" is eye-rolling.
Evaporation of water is an increase in its kinetic energy. An exchange of any kind from the photon's energy to the kinetic energy of the water molecule is literally heat.
They should have said, "in a not-directly thermodynamic manner".,br>
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So are radio waves; and neutrino radiation. Doesn't mean you can evaporate water with them.
Energy is the *capacity* to do work. That doesn't make all forms of energy interchangeable at performing specific *tasks*.
Re:Not surprising if you have an education. (Score:4, Interesting)
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Well if it's so not surprising, why haven't you or anyone else mentioned it before?
If it is so surprising, why did I guess it would be about wavelength before reading that it is indeed about wavelength. I am assuming the OP had the same idea. Odd that.
Relax bro. You are not in charge of keeping the Universe free from mistaken ideas.
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> If you have water at 0C or 32F is it solid or liquid
Yes. It is also gas as that's the triple point (adjusted to 0.01C recently)
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Not surprising at all. When they say "without heat" they aren 't properly speaking of temperature, but of temperature changes.
Water will always evaporate without additional heat being applied. Think about snow in the winter that is sheltered from the sun. The temperatures may be 20-30 degrees F. yet the snow melst away. Or better yet, think of how freezer burn occurs. No additional "Heat" is being applied.
But with light, it IS energy even if not measured as thermal energy. If you have water at 0C or 32F is it solid or liquid. If energy is subtracted from the atoms the water will become a solid. If energy is added it will be a liquid.
Many things evaporate by themselves without anything needed, even black holes apparently. The ice in my freezer evaporates without any light nor heat. Ice can evaporate without having to turn to water first. They must have experimented with light taking into account the thermal energy light gives vs no light. Or maybe, they miscalculated the thermal energy light gives, I think it's a possibility as well. That said, I wonder how fast a rock evaporate but I wouldn't be surprised if rock evaporation was non ze
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The ice in my freezer evaporates without any light nor heat. Ice can evaporate without having to turn to water first.
Your refrigerator probably is an anti-frost version, which permits the cabinet and its contents to "warm" and then the dry air collects the water only to be drained outside the fridge. To suggest this means it's done without heat is... disingenuous.
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To suggest this means it's done without heat is... disingenuous.
I think you are disingenuous, the same will happen on a dry and very cold day if you put a block of ice outside shielded from sunlight and wind. It's simply related to the room air has for water, less relative humidity more room for water, more room for water ice evaporates faster without even having to turn into water first. No need for "warm" air at all as you said in a disingenuous way.
Anti-frost freezer just act as dehumidifiers taking the humidity out, nothing disingenuous there. The same happens in na
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To suggest this means it's done without heat is... disingenuous.
I think you are disingenuous
Touche
the same will happen on a dry and very cold day if you put a block of ice outside shielded from sunlight and wind. It's simply related to the room air has for water, less relative humidity more room for water, more room for water ice evaporates faster without even having to turn into water first.
Which is all related to... drumroll... heat, and energy. What... You think it takes zero energy to generate the environment?
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Which is all related to... drumroll... heat, and energy. What... You think it takes zero energy to generate the environment?
drumroll: irrelevant since I never said otherwise, you mentioned warm air and it was very disingenuous, the only component a frost-free freezer needs and an AC doesn't have is a filament to mildly warm the cooling coils enough once in a while just enough so frozen water stuck to the coils melts and drops *outside* of the freezer where it evaporates with the heat the compressor makes! Apart from that, it's the very same thing as an air conditioner.
Obviously, air isn't circulating at all during that process s
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Which is all related to... drumroll... heat, and energy. What... You think it takes zero energy to generate the environment?
drumroll: irrelevant since I never said otherwise, you mentioned warm air and it was very disingenuous, the only component a frost-free freezer needs and an AC doesn't have is a filament to mildly warm the cooling coils enough once in a while just enough so frozen water stuck to the coils melts and drops *outside* of the freezer where it evaporates with the heat the compressor makes! Apart from that, it's the very same thing as an air conditioner.
Obviously, air isn't circulating at all during that process so, again, no warm air involved at all anywhere in the process!
Dude... you wrote:
"The ice in my freezer evaporates without any light nor heat."
There is _heat_ in the refrigerator, whether or not the refrigerator is running.
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Dude... you wrote:
"The ice in my freezer evaporates without any light nor heat."
There is _heat_ in the refrigerator, whether or not the refrigerator is running.
Yes, exactly what I said and I see you are too disingenuous to get it still!
Heat is irrelevant to the ice evaporating in this scenario! Ice will evaporate naturally without any heat if the air around it has a very low relative humidity level. No heat needed at all! It's another type of evaporation unrelated to heat. It's the same thing as a towel absorbing water, no heat needed!
Do you get it now?
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Here, next time, do your homework before calling somebody disingenuous and looking disingenuous yourself. Water and even ice can evaporate directly without any heat!
https://en.wikipedia.org/wiki/... [wikipedia.org]
https://www.scienceabc.com/nat... [scienceabc.com]
https://www.quora.com/How-can-... [quora.com]
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LOL! OK, I'll give you that: sublimation is endothermic! But absolutely no "warm air" circulating in your frost-free freezer as OP suggested (just in case it wasn't you). It's only the low relative humidity level in your freezer that cause sublimation of the ice! No external heat source required. So the low relative humidity level kept in your freezer because it extract water out and thus dehumidify the air in your freezer cause sublimation which in turn helps to keep your food at the right temperature whil
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Furthermore, if you have air with 100% relative humidity around the ice, zero sublimation will occur, don't matter how much what you call "heat" is in the air. LOL again...
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You could also insulate the cooling coils from the freezer and close a valve to the freezer and open another valve to let warm air in to melt the ice into water. Then, no warming filament and obviously, again, no warm air in the freezer! That'd be pretty bad for food and kind of disingenuous.
https://duckduckgo.com/?q=anti... [duckduckgo.com]
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I think that requires UV light, though. Visible light isn't energetic enough. (OTOH, that's just a potential energy barrier, so it might happen *really* slowly...like diode tunneling.)
Well, sublimation requires a solid (Score:2)
Any EM wave that can add energy... (Score:2)
... to atoms or molecules will effectively add heat regardless of whether the EM is in the infrared band. If this wasn't the case microwave ovens wouldn't work.
Re:Any EM wave that can add energy... (Score:5, Informative)
The point was that light need not be converted to heat to cause evaporation. From the article:
"Though water itself does not absorb much light, and neither does the hydrogel material itself, when the two combine they become strong absorber"
There you have it, but with no heat.
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Causing evaporation is adding heat. Heat is the kinetic energy of the molecules so when you smack one with a photon hard enough to blow it off into the air, you're adding heat. The article doesn't say "without adding heat" anywhere, it talks about many different kinds of heating. Specifically, this effect doesn't involve the bulk heating of the material, which explains an observation that certain hydrogels seem to exhibit more evaporation than would be expected from their bulk temperature.
Re:Any EM wave that can add energy... (Score:5, Insightful)
Especially the molecules that exist on the edge of a phase boundary. They are subject to a type of vacuum-like tension that increases their willingness to pop off into another phase. That why we have vapor pressures and such. So any energetic nudge will statistically increase that willingness.
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Amazing! (Score:5, Funny)
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"Obvious Science" is a huge industry.
There was a major paper this week "proving" that people are happier when they're not in solitary confinement.
The people with no wisdom would have you believe that we could not have known that before advanced statistical techniquies were available.
They consider themselves "credentialed experts".
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"Obvious Science" is a huge industry.
There was a major paper this week "proving" that people are happier when they're not in solitary confinement.
The people with no wisdom would have you believe that we could not have known that before advanced statistical techniquies were available.
There is significant value in scientifically testing the obvious, because it occasionally happens that careful scrutiny shows that "obvious", "common sense" truths are in fact false.
Also, I'm skeptical of your claim about the solitary confinement study. I think it's likely that you're mischaracterizing its goals and results, because the psychological and physical effects of solitary confinement have been studied for decades and there's no way that any research group today would bother testing something so
The Stargate Effect (Score:3)
There's a Dr. McKay in every universe!
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Evaporation requires _energy_ but it does not necessarily require _heat_ as such, and as far as I am aware no reasonably informed person ever thought that it did, certainly not within my lifetime. Heat is energy, sure, but light is also energy. We *knew* this, in principle, decades ago. Maybe we didn't realize the extent to which it would be applicable in a particular specific experimental scenario involving aerogel or whatever, but we did know _in ge
Small wavelength ultrasonic humidifier (Score:3)
It's an interesting finding.
The article talks about the photoelectric effect being an analogy, though it reminds me more of how ultrasonic humidifiers work.
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<morbo>Ultrasonic humidifiers don't work that way!</morbo> They break water into tiny droplets, which then evaporate naturally more easily than bulk water (see Köhler theory / Kelvin effect). The effect of the article is about evaporation itself.
Ultrasonic humidifiers use less energy than traditional ones that rely on direct evaporation, but the droplets can be a health hazard. If the water has any pathogens (and it likely does), they may be carried about along the droplets.
Discovered in the 1990's or before (Score:2)
Scientists discovered this basic effect when they were investigating the effects of global cooling from soot pollution. They did not see the high rates they're getting now with the hydrogels, but it was discovered as a process in the 80's or 90's. Maybe scientists should search previous discoveries before posting new discoveries. Of course, then their would not be as many new discoveries.
baffling (Score:2)
I'm a little puzzled by the language, the complexity of the setup, and the conclusions drawn.
First, as far as I understand light doesn't carry HEAT, it carries ENERGY.
HEAT is the term we use for energy transferred from one thing to another, specifically the measure of the exciting of a thing as a result of that energy transfer.
Heat cannot transfer from one system to another (by itself) through a vacuum, as the lack of contact prevents the transmission of energy-as-heat.
However, we have a pretty obvious cons
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So why does sunlight feel hot on Earth?
How does the heat from the sun get here across millions of miles of space?
Isn't it just transfer of energy by radiation?
Re:baffling (Score:4, Informative)
Heat is the kinetic energy of the individual molecules in a bulk substance. What they hypothesize (they haven't really shown it conclusively) is that in certain situations light can knock water molecules off a liquid surface directly, rather than being absorbed into the bulk material and heating it up.
The "without heating" is an over simplification by the article.
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Thanks, I appreciate the reply.
And that was kind of my point; I didn't get from the article anyway, that anything in their setup could distinguish the difference between:
1) photons knocking water molecules from the surface directly -- their hypothesis
and
2) photons heating water molecules directly enough to energize them enough to depart the liquid state. (Or, photons heating air heating water, but that's even less likely).
Feels like #1 is the much-more-extraordinary outcome, requiring inordinate proof. Oc
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Your #1 and #2 are the same thing.
"Heating" is the process of a particle smacking into another and increasing its kinetic energy. "Temperature" is a measure of the average kinetic energy of the particles that make up a bulk substance. The article is describing a situation where, under certain circumstances, there's more evaporation than you would expect from the temperature, and their hypothesis is that light can be selectively absorbed locally by water molecules rather than more or less being absorbed even
Desalination is the exciting part. (Score:2)
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scurry (Score:2)
wha? (Score:1)
Evaporation is a thermal phenomenon. I.e., molecules achieve escape velocity from the liquid. This means by definition that those molecules were heated. Maybe what's new is that the distribution of thermal energy doesn't follow the expected boltzmann or whoever distribution?
why not (Score:2)
ArXiv link (Score:2)