From the MASSACHUSETTS INSTITUTE OF TECHNOLOGY
Evaporation is happening all around us all the time, from the sweat cooling our bodies to the dew burning off in the morning sun. But science’s understanding of this ubiquitous process may have been missing a piece all this time.
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 findings are published this week in a paper in PNAS, by MIT postdoc Yaodong Tu, professor of mechanical engineering Gang Chen, and four others.
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. So, when the team initially began exploring the process of solar evaporation for desalination, they first put particles of a black, light-absorbing material in a container of water to help convert the sunlight to heat.
Then, the team came across the work of another group that had achieved an evaporation rate double the thermal limit — which is the highest possible amount of evaporation that can take place for a given input of heat, based on basic physical principles such as the conservation of energy. It was in these experiments that the water was bound up in a hydrogel. Although they were initially skeptical, Chen and Tu starting their own experiments with hydrogels, including a piece of the material from the other group. “We tested it under our solar simulator, and it worked,” confirming the unusually high evaporation rate, Chen says. “So, we believed them now.” Chen and Tu then began making and testing their own hydrogels.
They began to suspect that the excess evaporation was being caused by the light itself —that photons of light were actually knocking bundles of water molecules loose from the water’s surface. This effect would only take place right at the boundary layer between water and air, at the surface of the hydrogel material — and perhaps also on the sea surface or the surfaces of droplets in clouds or fog.
In the lab, they monitored the surface of a hydrogel, a JELL-O-like matrix consisting mostly of water bound by a sponge-like lattice of thin membranes. They measured its responses to simulated sunlight with precisely controlled wavelengths.
The researchers subjected the water surface to different colors of light in sequence and measured the evaporation rate. They did this by placing a container of water-laden hydrogel on a scale and directly measuring the amount of mass lost to evaporation, as well as monitoring the temperature above the hydrogel surface. The lights were shielded to prevent them from introducing extra heat. The researchers found that the effect varied with color and peaked at a particular wavelength of green light. Such a color dependence has no relation to heat, and so supports the idea that it is the light itself that is causing at least some of the evaporation.
The puffs of white condensation on glass is water being evaporated from a hydrogel using green light, without heat. Image: Courtesy of the researchers
The researchers tried to duplicate the observed evaporation rate with the same setup but using electricity to heat the material, and no light. Even though the thermal input was the same as in the other test, the amount of water that evaporated never exceeded the thermal limit. However, it did so when the simulated sunlight was on, confirming that light was the cause of the extra evaporation.
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.
Having discovered this effect, which they have dubbed the photomolecular effect, the researchers are now working on how to apply it to real-world needs. They have a grant from the Abdul Latif Jameel Water and Food Systems Lab to study the use of this phenomenon to improve the efficiency of solar-powered desalination systems, and a Bose Grant to explore the phenomenon’s effects on climate change modeling.
Tu explains that in standard desalination processes, “it normally has two steps: First we evaporate the water into vapor, and then we need to condense the vapor to liquify it into fresh water.” With this discovery, he says, potentially “we can achieve high efficiency on the evaporation side.” The process also could turn out to have applications in processes that require drying a material.
Chen says that in principle, he thinks it may be possible to increase the limit of water produced by solar desalination, which is currently 1.5 kilograms per square meter, by as much as three- or fourfold using this light-based approach. “This could potentially really lead to cheap desalination,” he says.
Tu adds that this phenomenon could potentially also be leveraged in evaporative cooling processes, using the phase change to provide a highly efficient solar cooling system.
Meanwhile, the researchers are also working closely with other groups who are attempting to replicate the findings, hoping to overcome skepticism that has faced the unexpected findings and the hypothesis being advanced to explain them.
The research team also included Jiawei Zhou, Shaoting Lin, Mohammed Alshrah, and Xuanhe Zhao, all in MIT’s Department of Mechanical Engineering.
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The paper: https://www.pnas.org/doi/10.1073/pnas.2312751120
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Temperature being a necessity to evaporate water doesn’t seem to me to compute.
I dry laundry outside on a cloths line. It does dry faster in summer when there is more sun and the temperature is higher, but an adequate breeze seems more important. Certainly, neither the air nor the sheets and towels, nor the water in them, ever reaches anywhere near 100C.
70%, roughly, of the globe is ocean. Evaporation occurs constantly over much of it but neither air temperature or water surface temperature ever come near the boiling point of water. Wind creates spray. Water droplets in the air get further dispersed through turbulence. Small enough quantities, with a limit of single molecules, can stay mixed with the air for an extended period of time. Dust particles, much larger than molecular size, are readily raised to considerable height by air movement, so small water droplets could certainly also be.
Does 20C water in my laundry or at the ocean surface actually somehow heat to 100C, then gain another 540 calories per gram, to become vapor? Isn’t this the same puzzle as colder air heating a warmer surface to even a higher temperature through IR emission at air temperature?
“Does 20C water in my laundry or at the ocean surface actually somehow heat to 100C, then gain another 540 calories per gram, to become vapor?”
Don’t confuse evaporation with boiling point. Even snow evaporates. Swamp coolers use evaporation to cool.
That’s my point. It seems to me that in various situations liquid water can change to the vapor state on much less energy gain than the 540 calories per gram heat of vaporization of water, which starts at 100̊C. This would also mean that the calculations of water condensing to liquid at high altitude might release much less energy than what seems to be generally believed.
The latent heat of vaporization of 540 cal/gm as well as the phase change (i.e., “boiling”) temperature of 100 °C for water are only applicable under ambient pressure of 14.7 psia. And yes, such values are less for “high altitudes” due to reduced ambient pressure.
Take liquid water at any given temperature in a sealed container and start evacuating the overhead gas in that container . . . as the pressure is reduced the water will start to “boil”, even at bulk water temperatures as low as 1 °C.
To find the latent heat of vaporization or corresponding heat released from condensation for specific temperature and pressure boundary conditions, one needs to consult P-h or T-s phase diagrams for water, or the “on-line calculator” equivalents of such.
(ref: https://en.wikipedia.org/wiki/Phase_diagram# )
You have to have some source of energy to change state of water. In the case of your laundry this source is the internal energy of the air passing through and by your clothing — the air emerges cooler from the evaporation.
Yes, some energy, but maybe not as much energy as the heat of vaporization of water is claimed to require.
I’m surprised that chemists haven’t weighted in on much of this nonsense. I can place a glass of water on my kitchen counter, and in days or weeks it will all evaporate. There’s no need to boil it, and everything remains at room temperature.
Well, not quite: there will be a small be detectable decrease in the temperature of the liquid water that remains in the glass. That is because the process of water evaporation from the liquid’s surface extracts energy from the liquid. That temperature drop will reach a steady state “equilibrium” value since it also establishes a temperature gradient between the water and the ambient air (across the glass wall) that will transfer heat into the water that offsets the heat being lost due to evaporation.
The water continues to evaporate from the glass over “days or weeks” because it never fully saturates the air in the hypothetical kitchen.
However, if you were to place a glass of water, say, containing 300 cc of liquid water, into a hermetically-sealed box of, say, 1000 cc total internal volume, you would find that the water will never completely evaporate from the glass, independent of the ambient temperature being anywhere in the range 1 to 99 °C, because such a volume ratio does allow the evolved water vapor to reach 100% relative humidity level of the gas inside the box. At that point, the temperature gradients will disappear.
Heh! It’s another nit-picker!
I would like to try that box experiment, but I don’t have anything that will measure cubic centimeters. Is it alright that I use milliliters? When the metric system was first set up, milliliters and cubic centimeters were supposed to refer to the same volume, but they aren’t exactly the same now. My measuring device is only plus or minus 5%. Is 300 milliliters plus or minus 15 milliliters okay?
I guess I could assemble a box that’s 10 centimeters by 10 centimeters by 10 centimeters. My carpentry skills aren’t really that great. I believe that would be approximately 1000 cc. I found a glass that would fit into that box, but when I poured 300 ml (plus or minus 15 ml) into it, it overflowed. Apparently, the glass can only hold 280 ml (plus or minus 14 ml). Probably 250 ml is more likely (plus or minus 12 to 13 ml). Is that okay?
Another problem is that the box is probably not hermetically sealed. Can the box be partially hermetically sealed or is that like being partially pregnant?
When I place the glass of water on my kitchen counter, how long do I need to wait before it equalizes with room temperature? Charging a capacitor through a resistor takes an infinite amount of time. The capacitor is never fully charged. However, after five time constants, the capacitor IS considered fully charged. What is the time constant of equalizing the temperature of the room temperature with the glass of water?
“. . . because it never fully saturates the air in the hypothetical kitchen.”
I didn’t think my kitchen was hypothetical. Thanks for letting me know. Is my kitchen counter hypothetical too?
One of the problems I have is trying to figure out the physics around my glass of water. There’s a water vapor presence over the water in the glass. If saturation is achieved, then when a water molecule leaves the liquid, a water molecule in the overlying vapor will return to the the liquid. That would be called equilibrium. However, as the vapor diffuses into the room air, equilibrium is never achieved.
“. . . there will be a small be detectable decrease in the temperature of the liquid water that remains in the glass.”
I love this silly statement. You previously stated that more than four significant figures is laughable. I would think it would take more than four significant figures to measure that “so-called” detectable decrease in temperature. And when a water molecule left the water in the glass, it would immediately un-equalize the energy in the glass, and by definition, it would no longer have a thermodynamic temperature.
I have a thermostat that sets the temperature back at night. And my dogs love to have the patio doors open. My kitchen counter is 191 inches from the patio doors—that’s about 485 centimeters.
So when does that glass of water ever reach equilibrium?
So many wasted words . . . so little rational thought . . . so many absurd, tangential statements.
But thank you providing such humor for me and others!
“So many wasted words . . . so little rational thought . . . so many absurd, tangential statements.”
I see you’re projecting again. This easily describes most of your comments. I’m still trying to get around the fact that I have a hypothetical kitchen.
Here, I’ll help you out:
First you post “I can place a glass of water on my kitchen counter, and in days or weeks it will all evaporate. There’s no need to boil it, and everything remains at room temperature.”
—> That’s one amazing kitchen you have there that over the period of “days or weeks” “everything remains at room temperature” (quoting your words, and noting specifically the use of the word “temperature”, not “temperatures” that would indicate a range of variability).
Then you post “I have a thermostat that sets the temperature back at night. And my dogs love to have the patio doors open.”
—> Again, what an amazing real (not hypothetical) kitchen you have that remains at room temperature despite a change in thermostat setting (assuming of course that you have working HVAC) and where inside/outside convection from open patio doors have no effect on your real kitchen temperature that remains unchanged.
You should patent the design of your “real” kitchen . . . or maybe not :-))
Thus spoke whats-his-name!
I’m sorry, bad pronoun. I should have said, “Thus spoke whats-its-name!”
Bad grammar. Should be “Thus spoke what’s-its-name.”
LOL!
Mr. Layman here.
I remember back in one my first classes on science (grade school? HS?) being surprised to learn that a pot of boiling water will still pick up some H2O molecules from the air. And also that a chunk of frozen ice will still release some H2O molecules into the air. Liquid H2O does the same, releasing and picking up at about equal rates.
If I remember correctly, it had to with molecules always moving, sometimes from one state to another.
Some form of energy causes that motion. Light is energy. Heat is energy. All light doesn’t produce heat.
Why couldn’t it increase the motion of the H2O molecules (with more molecules changing state) without measurably producing heat?
Again, “Mr. Layman here”.
All solids and liquids have a property known as their “vapor pressure” at a given temperature. “Vapor pressure” encompasses the atomic/molecular scale thermodynamics (maybe even quantum mechanics) that drives matter to move from a volume of very high density (a solid or liquid state) toward a volume of very low density (a vacuum or low pressure ambient gas) . . . commonly known as a “concentration gradient”. The gas phase, by nature, requires that the associated atoms or molecules have greater kinetic energy (contained in all available degrees of freedom) than they have in solid or liquid form.
There is no getting around the fact that such delta-energy is either taken from the bulk material that is not evaporated or is provided by external input of energy.
“Heat” in a substance (solid, liquid or gas) is just a convenient term to encompass the total kinetic energy that is tied up in the rotational, bending and stretching vibrational modes of atom-atom bonds of poly-atomic molecules as well as the translational (linear velocity) modes of both atoms (e.g., He) and poly-atomic molecules, collectively referred to as mechanical degrees-of-freedom.
“Heat” does necessarily reflect the total energy carried by an atom or molecule, as energy can also be “tied up” in their electron shells, such as being elevated to an “excited state” (as in a laser) or to different levels of ionization (as in a fluorescent light).
4 related Preprints https://scholar.google.com/scholar?oi=bibs&hl=en&cites=821270015002238579&as_sdt=5 e.g.
Photomolecular Effect Leading to Water Evaporation Exceeding Thermal Limit
https://arxiv.org/ftp/arxiv/papers/2201/2201.10385.pdf
On the Molecular Picture and Interfacial Temperature Discontinuity During Evaporation and Condensation
https://arxiv.org/ftp/arxiv/papers/2201/2201.07318.pdf
Maybe it is a discovery, but why is the hydro gel important? Water can unzip glass, so it would not be surprising if it has some undiscovered secrets.
Using light instead of heat to evaporate water? Aren’t we really talking about energy, which at the lowest level is photons absorbed or released, even in solids, because you cannot have fractional photons regardless of how the energy is transmitted/conducted.
As such, making a distinction between light and heat with regard to evaporation seems a false distinction.
An interesting problem perhaps unresolved. What happens to the momentum of light at the air-water interface. Could this explain the observations?
Researchers find that light energy alone can cause water to change state, blimey, they will be discovering sublimation next!
Sublimation can occur in the complete absence of incident light. All that is required, thermodynamically, is that the ambient pressure be lower than the vapor pressure-at-the-given-temperature of the substance being considered.
Simple home experiment: wrap a piece of “dry ice” (solid CO2) in totally opaque paper . . . you will find that is still sublimates quite rapidly at typical room temperature and room pressure conditions.
QED.
You can’t exceed the thermal limit, i.e. heat of vaporization of water. If the researchers do a careful energy balance, the amount of water evaporated will equal the amount of heat and light energy added to the system. There may be an effect where water is volatilized as a very small droplet, so not a true vapor, and this could require less energy. As with other things that sound too good to be true, they are.
I would recommend watching Dr. Gerald Pollack’s video – https://risingtidefoundation.net/2023/10/15/the-fourth-phase-of-water-beyond-the-three-you-already-know-rtf-lecture-with-dr-gerald-pollack/
Anthony,
I know better than to dispute Willis’s very shaky grasp of Heat Transfer. Heat is energy. The way to measure Heat is Temperature, which is the Average Kinetic Energy of the molecules of an object.
Light is Radiation. There are three ways to transfer energy between objects: conduction, convection, and radiation. If there is one thing that has been studied extensively, it is how to make water evaporate/boil.
i think that “evaporation rate” actually refers to Pan Evaporation, dependent on many variables. A veteran engineer, this is the first time I have ever seen the phrase “evaporation rate,” meaningless without specifying many’ many other variables.
A real clunker here.
MR Moon
“The way to measure Heat is Temperature, which is the Average Kinetic Energy of the molecules of an object.”
This is not what the kinetic theory of gases says. Heat is not temperature. Using SI units, Kelvin does not equal joule. And temperature does not equal average kinetic energy. Heat is energy and can be measured by BTUs, calories, joules, foot-pounds, watt-hours, newton-meters, ergs, dyne-centimeters and so on. But temperature is not energy. The MORE correct statement is that temperature is PROPORTIONAL to the average kinetic energy. Even that statement isn’t exactly correct.
I didn’t say Temperature is Enegy. Work on your reading skills
I’m sorry. I guess I have extremely poor reading skills. So when you say “heat is temperature,” you didn’t say that heat (which is energy) is temperature? And the clause “which is the Average Kinetic Energy . . .” doesn’t refer to the preceding word which is “temperature?” I should really work on my poor reading skills.