How light evaporates water without the need for heat

It’s the most basic process – water evaporates from the surfaces of oceans and lakes, fog burns off in the morning sun, and saltwater ponds dry up leaving behind solid salt. Evaporation is all around us, and humans have been observing and exploiting it for as long as we have existed.

However, it turns out that we have been missing a major part of the picture.

In a series of painstakingly precise experiments, a team of MIT researchers demonstrated that heat isn’t the only factor that causes water to evaporate. Light shining on the surface of the water where air and water meet can break up the water molecules and float them into the air, causing evaporation without any source of heat.

This startling new discovery could have widespread and significant implications. It could help explain mysterious measurements of how sunlight affects clouds over the years, affecting 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 desalination or materials drying.

These findings, along with many different pieces of evidence proving the reality of the phenomenon and the details of its operation, are published today in Proceedings of the National Academy of Sciencesin a paper by Carl Richard Soderberg Professor of Power Engineering Gang Chen, postdocs Guangxin Lu and Yaodong Tu, and graduate student James Zhang.

The authors say their study shows that this effect should occur widely in nature – from clouds to fog to ocean surfaces, soil and plants – and that it could lead to new practical applications, including energy and clean water Production.

“I think this has a lot of applications,” Chen said. “We’re exploring all these different directions. Of course, it also impacts basic science, such as the impact of clouds on climate, because clouds are the most uncertain aspect of climate models.”

a newly discovered phenomenon

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

Because this effect was unexpected, the team worked hard to prove its existence with as much different evidence as possible. In the study, they report on 14 different tests and measurements they performed to determine that water is indeed evaporating—that is, water molecules break off the surface and float into the air—solely due to the action of light , not through heat, which was long thought to be the only mechanism involved.

One key metric that emerged consistently across four different experiments under different conditions was that as the water began to evaporate from the test container under visible light, the air temperature measured above the water cooled down and then leveled off, showing thermal energy and is not the driving force behind the effect.

Other key indicators that emerge include the way the evaporation effect changes with the angle of the light, the exact color of the light and its polarization. None of these different features should occur because water absorbs almost no light at these wavelengths, but the researchers observed them.

The effect is strongest when the light hits the water at a 45-degree angle. It is also strongest with a certain type of polarization called transverse magnetic polarization. It peaks under green light – which, oddly enough, is where water is most transparent and therefore interacts the least.

Chen and his colleagues proposed a physical mechanism that could explain the angle and polarization dependence of the effect, showing that photons of light can exert a net force on water molecules at the surface, enough to knock them off the surface. But they can’t yet explain the color dependence, which they say requires further study.

They named the phenomenon the photomolecular effect, similar to the photoelectric effect discovered by Heinrich Hertz in 1887 and eventually explained by Albert Einstein in 1905. The effect was one of the earliest demonstrations that light also had particle properties, and was of great significance to the field of physics and led to a wide range of applications including LEDs. Just as the photoelectric effect releases electrons from atoms in a material when struck by a photon, the photomolecular effect shows that photons can release entire molecules from the surface of a liquid, the researchers said.

“The discovery that evaporation is caused by light, not heat, provides new and disruptive knowledge about the interaction of light and water,” said Xiulin Nguyen, a professor of mechanical engineering at Purdue University who was not involved in the study.

“It can help us gain new insights into how sunlight interacts with clouds, fog, oceans and other natural bodies of water to influence weather and climate. It has important potential practical applications, such as solar-powered high-performance desalination. This project Research This is one of those rare truly revolutionary discoveries that are not immediately widely accepted by society but take time, sometimes a long time, to be confirmed.

Solve cloud challenges

The discovery could solve an 80-year-old mystery in climate science. Measurements of how clouds absorb sunlight often show that they absorb more sunlight than conventional physics dictates is possible. Additional evaporation caused by this effect may be responsible for the long-term differences, but this has been a controversial topic because such measurements are difficult to make.

“These experiments are based on satellite data and flight data,” Chen explained. “They flew aircraft above and below clouds, and also based on data on ocean temperature and radiation balance. They all concluded that clouds absorb more than theoretically calculated. However, due to the complexity of clouds and conducting this Researchers have debated whether this difference is real due to the difficulty of class measurements, and our findings suggest that there is another, as yet unexplained, cloud absorption mechanism that may explain these differences.

Chen said he recently spoke about the phenomenon at a meeting of the American Physical Society, where a physicist who studies clouds and climate said they had never considered the possibility, which could influence the complex effects of clouds on climate. calculation. The team conducted experiments using LED illumination of an artificial cloud chamber, and they observed heating of the fog, which should not occur because water does not absorb the visible spectrum.

“This heating can be more easily explained in terms of photomolecular effects,” he said.

Among the many pieces of evidence, Lu said, “the flat area of ​​the temperature distribution on the air side above the hot water will be the easiest for people to reproduce.” That temperature curve “is a landmark” that can clearly demonstrate the effect, he said.

Zhang added, “Besides the accepted theory of thermal evaporation, it is difficult to explain how this flat temperature distribution arises without invoking other mechanisms.” He continued, “It will make many people think that in solar desalination devices reported,” which again shows an evaporation rate that cannot be explained by heat input.

The effects can be huge. Under optimal conditions of color, angle and polarization, “the evaporation rate is four times the thermal limit,” Lv said.

Chen said that since the first paper was published, the team has been approached by companies looking to exploit the effect, including evaporating syrup and drying paper at paper mills. Most likely, he said, the first applications will be in solar desalination systems or other industrial drying processes.

“Drying consumes 20 percent of all industrial energy consumption,” he points out.

Because the effect is so novel and unexpected, Chen said, “This phenomenon should be very common, and our experiment is really just the beginning.” The experiments required to prove and quantify the effect are very time-consuming. “There are a lot of variables, from understanding water itself to extending it to other materials, other liquids and even solids,” he added.

“The observations in the manuscript point to a new physical mechanism that fundamentally changes our view of evaporation dynamics,” said Shannon Yee, an associate professor of mechanical engineering at Georgia Tech who was not associated with the work. “Who would have thought that we would still be learning about something as commonplace as the evaporation of water?”

“I think this work is scientifically very important because it proposes a new mechanism,” said Janet AW Elliott, a distinguished professor at the University of Alberta who was not involved in the work. “It could also be very important for technology and our understanding of nature, because water evaporates everywhere and at rates that appear to be much higher than known thermal mechanisms. … My overall impression is that this work is very outstanding.

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