MIT Uncovers Photomolecular Effect: Light Can Vaporize Water Without Heat

Abstract Light Water Evaporation Art

Researchers at MIT have discovered that evaporation can occur through exposure to light, not just heat. This process, observed on various water surfaces, has profound implications for climate modeling and innovative technologies like solar-driven water purification. (Artist’s concept.) Credit:

MIT researchers have uncovered that light can induce evaporation, not just heat, demonstrating this across various natural and synthetic water surfaces. This discovery could impact climate modeling and lead to innovations in solar energy and water purification technologies.

It’s the most fundamental of processes — the evaporation of water from the surfaces of oceans and lakes, the burning off of fog in the morning sun, and the drying of briny ponds that leaves solid salt behind. Evaporation is all around us, and humans have been observing it and making use of it for as long as we have existed.

And yet, it turns out, we’ve been missing a major part of the picture all along.

Light-Induced Evaporation Discovery

In a series of painstakingly precise experiments, a team of researchers at MIT has demonstrated that heat isn’t alone in causing water to evaporate. Light, striking the water’s surface where air and water meet, can break water molecules away and float them into the air, causing evaporation in the absence of any source of heat.

The astonishing new discovery could have a wide range of significant implications. It could help explain mysterious measurements over the years of how sunlight affects clouds, and therefore affect calculations of the effects of climate change on cloud cover and precipitation. It could also lead to new ways of designing industrial processes such as solar-powered desalination or drying of materials.

Light Can Vaporize Water Without Heat

Researchers at MIT have discovered a new phenomenon: that light can cause evaporation of water from its surface without the need for heat. Pictured is a lab device designed to measure the “photomolecular effect,” using laser beams. Credit: Bryce Vickmark

The findings, and the many different lines of evidence that demonstrate the reality of the phenomenon and the details of how it works, are described today in the journal PNAS, in a paper by Carl Richard Soderberg Professor of Power Engineering Gang Chen, postdocs Guangxin Lv and Yaodong Tu, and graduate student James Zhang.

The authors say their study suggests that the effect should happen widely in nature— everywhere from clouds to fogs to the surfaces of oceans, soils, and plants — and that it could also lead to new practical applications, including in energy and clean water production. “I think this has a lot of applications,” Chen says. “We’re exploring all these different directions. And of course, it also affects the basic science, like the effects of clouds on climate, because clouds are the most uncertain aspect of climate models.”

A Newfound Phenomenon

The new work builds on research reported last year, which described this new “photomolecular effect” but only under very specialized conditions: on the surface of specially prepared hydrogels soaked with water. In the new study, the researchers demonstrate that the hydrogel is not necessary for the process; it occurs at any water surface exposed to light, whether it’s a flat surface like a body of water or a curved surface like a droplet of cloud vapor.

Because the effect was so unexpected, the team worked to prove its existence with as many different lines of evidence as possible. In this study, they report 14 different kinds of tests and measurements they carried out to establish that water was indeed evaporating — that is, molecules of water were being knocked loose from the water’s surface and wafted into the air — due to the light alone, not by heat, which was long assumed to be the only mechanism involved.

Light Can Vaporize Water Without Heat Photomolecular Effect

The authors say their study suggests that the photomolecular effect should happen widely in nature, from clouds to fogs, ocean to soil surfaces, and plant transpiration. “I think this has a lot of applications,” Gang Chen, pictured in center, says. Chen stands with authors Guangxin Lv, on left, and James Zhang. Author Yaodong Tu is not pictured. Credit: Bryce Vickmark

One key indicator, which showed up consistently in four different kinds of experiments under different conditions, was that as the water began to evaporate from a test container under visible light, the air temperature measured above the water’s surface cooled down and then leveled off, showing that thermal energy was not the driving force behind the effect.

Other key indicators that showed up included the way the evaporation effect varied depending on the angle of the light, the exact color of the light, and its polarization. None of these varying characteristics should happen because at these wavelengths, water hardly absorbs light at all — and yet the researchers observed them.

The effect is strongest when light hits the water surface at an angle of 45 degrees. It is also strongest with a certain type of polarization, called transverse magnetic polarization. And it peaks in green light — which, oddly, is the color for which water is most transparent and thus interacts the least.

Chen and his co-researchers have proposed a physical mechanism that can explain the angle and polarization dependence of the effect, showing that the photons of light can impart a net force on water molecules at the water surface that is sufficient to knock them loose from the body of water. But they cannot yet account for the color dependence, which they say will require further study.

Photomolecular Effect Light Can Vaporize Water Without Heat

“We’re exploring all these different directions,” Chen says. “And of course it also affects the basic science, like the effects of clouds on climate, because clouds are the most uncertain aspect of climate models.” Credit: Bryce Vickmark

They have named this the photomolecular effect, by analogy with the photoelectric effect that was discovered by Heinrich Hertz in 1887 and finally explained by Albert Einstein in 1905. That effect was one of the first demonstrations that light also has particle characteristics, which had major implications in physics and led to a wide variety of applications, including LEDs. Just as the photoelectric effect liberates electrons from atoms in a material in response to being hit by a photon of light, the photomolecular effect shows that photons can liberate entire molecules from a liquid surface, the researchers say.

“The finding of evaporation caused by light instead of heat provides new disruptive knowledge of light-water interaction,” says Xiulin Ruan, professor of mechanical engineering at Purdue University, who was not involved in the study. “It could help us gain new understanding of how sunlight interacts with cloud, fog, oceans, and other natural water bodies to affect weather and climate. It has significant potential practical applications such as high-performance water desalination driven by solar energy. This research is among the rare group of truly revolutionary discoveries which are not widely accepted by the community right away but take time, sometimes a long time, to be confirmed.”

Gang Chen

Because the effect is so new and unexpected, Chen says, “this phenomenon should be very general, and our experiment is really just the beginning.” Credit: Bryce Vickmark

Solving a Cloud Conundrum

The finding may solve an 80-year-old mystery in climate science. Measurements of how clouds absorb sunlight have often shown that they are absorbing more sunlight than conventional physics dictates possible. The additional evaporation caused by this effect could account for the longstanding discrepancy, which has been a subject of dispute since such measurements are difficult to make.

“Those experiments are based on satellite data and flight data,“ Chen explains. “They fly an airplane on top of and below the clouds, and there are also data based on the ocean temperature and radiation balance. And they all conclude that there is more absorption by clouds than theory could calculate. However, due to the complexity of clouds and the difficulties of making such measurements, researchers have been debating whether such discrepancies are real or not. And what we discovered suggests that hey, there’s another mechanism for cloud absorption, which was not accounted for, and this mechanism might explain the discrepancies.”

Chen says he recently spoke about the phenomenon at an American Physical Society conference, and one physicist there who studies clouds and climate said they had never thought about this possibility, which could affect calculations of the complex effects of clouds on climate. The team conducted experiments using LEDs shining on an artificial cloud chamber, and they observed heating of the fog, which was not supposed to happen since water does not absorb in the visible spectrum. “Such heating can be explained based on the photomolecular effect more easily,” he says.

Lv says that of the many lines of evidence, “the flat region in the air-side temperature distribution above hot water will be the easiest for people to reproduce.” That temperature profile “is a signature” that demonstrates the effect clearly, he says.

Zhang adds: “It is quite hard to explain how this kind of flat temperature profile comes about without invoking some other mechanism” beyond the accepted theories of thermal evaporation. “It ties together what a whole lot of people are reporting in their solar desalination devices,” which again show evaporation rates that cannot be explained by the thermal input.

The effect can be substantial. Under the optimum conditions of color, angle, and polarization, Lv says, “the evaporation rate is four times the thermal limit.”

Practical Applications and Future Research

Already, since the publication of the first paper, the team has been approached by companies that hope to harness the effect, Chen says, including for evaporating syrup and drying paper in a paper mill. The likeliest first applications will come in the areas of solar desalinization systems or other industrial drying processes, he says. “Drying consumes 20 percent of all industrial energy usage,” he points out.

Because the effect is so new and unexpected, Chen says, “This phenomenon should be very general, and our experiment is really just the beginning.” The experiments needed to demonstrate and quantify the effect are very time-consuming. “There are many variables, from understanding water itself, to extending to other materials, other liquids and even solids,” he says.

“The observations in the manuscript points to a new physical mechanism that foundationally alters our thinking on the kinetics of evaporation,” says Shannon Yee, an associate professor of mechanical engineering at Georgia Tech, who was not associated with this work. He adds, “Who would have thought that we are still learning about something as quotidian as water evaporating?”

“I think this work is very significant scientifically because it presents a new mechanism,” says University of Alberta Distinguished Professor Janet A.W. Elliott, who also was not associated with this work. “It may also turn out to be practically important for technology and our understanding of nature, because evaporation of water is ubiquitous and the effect appears to deliver significantly higher evaporation rates than the known thermal mechanism. … My overall impression is this work is outstanding. It appears to be carefully done with many precise experiments lending support for one another.”

Reference: “Photomolecular effect: Visible light interaction with air–water interface” by Guangxin Lv, Yaodong Tu, James H. Zhang and Gang Chen, 23 April 2024, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2320844121

The work was partly supported by an MIT Bose Award. The authors are currently working on ways to make use of this effect for water desalination, in a project funded by the Abdul Latif Jameel Water and Food Systems Lab and the MIT-UMRP program.

6 Comments on "MIT Uncovers Photomolecular Effect: Light Can Vaporize Water Without Heat"

  1. Eric M. Jones | May 5, 2024 at 8:12 am | Reply

    My physics professor says…Show me the ENERGY. So count me as a skeptic.

  2. So. Just have to point out. Heat is from um.. oh yeah thermal radiation that comes from light…. We knew this already…. It’s called sunlight causes evaporation…. Doesn’t even have to be hot out.

  3. Jericho Simon | May 5, 2024 at 3:14 pm | Reply

    I suspect that pressure alters the rate of evaporation by light with water. The greater the pressure the higher the rate of light vaporation occurs. I would be interested to know if pressure was taken into consideration for this experiment.

    Congratulations and well done.

    Thank you.

  4. Daniel Price | May 5, 2024 at 6:30 pm | Reply

    Hmmm, subatomic ballistic object traveling at the speed of light hitting another object in this case water…….. that’s basic ballistic with a fancy name. It’s the same thing as throwing a rock at a 45° angle into a pond. It displaced the water making a splash and some of that water converted into its gaseous form due to the pressure of the impact. It’s just at a subatomic level. It’s common sense, it’s fundamental cause and effect. it’s the law of conservation, energy/matter cannot be created or destroyed. A photon is energy, it has mass though is nearly incalculable it has mass. It is going to PHYSICALLY interact with something it hits. Just because no one took the time to name a common sense effect doesn’t make it a new discovery.

  5. Light has momentum, can be shown by two equations both known in the 1920s. So it is actually unsurprising the the photomolecular effect is happening.

  6. Yagub hasanpur | May 8, 2024 at 5:17 pm | Reply

    The temperature of the thin layer between water and air decreases, so we conclude that first the water molecules are separated from the water surface due to vibration, then they quickly absorb the temperature of the air and enter the gas phase. This phenomenon is similar to the Raman effect, in which liquid molecules are made to vibrate with a 532 nm laser in order to find out the type of molecules from the vibration frequency spectrum of the liquid. It is possible that at a certain frequency of light, which is called green light in the mentioned article, water molecules reach resonance, although the angle of 45 degrees and the polarization of light can all be proof of this claim.
    I think that by changing the pressure, the optimal frequency of evaporation may also change, in which case this can be another evidence to prove my point.

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