Revolutionary discovery: They show that light can evaporate water without the need for heat in the process

They discover that not only heat can evaporate water. The photomolecular effect is the process by which light also evaporates water without the presence of heat. This finding is fundamental for meteorology and other branches of applied sciences.

Laser, light.
MIT researchers have discovered a new phenomenon: that light can cause water to evaporate from its surface without the need for heat. In the image, a laboratory device designed to measure the photomolecular effect using laser rays. Image: MIT

When we think about the process of water evaporation, we associate this with the presence of heat. By increasing the temperature, water can go from the liquid state to the gaseous state, that is, it can become water vapour. This process is one of the most common in the atmosphere in which we live below freezing levels. Now, scientists at the Massachusetts Institute of Technology (MIT) have discovered that light can cause water to evaporate from a surface without the need for heat.

The discovery could solve an 80-year-old mystery in climatology. Measurements of how clouds absorb sunlight have often shown that they absorb more sunlight than conventional physics dictates is possible. The photomolecular effect would explain this mystery.

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reports that in recent years, some researchers have been surprised to see that the water in their experiments, which was kept in a sponge-like material known as a hydrogel, evaporated at a faster rate than could be explained by the amount of heat, or thermal energy, that the water received. And the excess has been significant: double, triple or even more, the theoretical maximum rate.

So the question that arose was more than logical. What other factor was influencing evaporation beyond what heat could produce? The results of the research that led to answering this question were published on 23 April in an article published by MIT News. It is indicated that the surprising “photomolecular effect” discovered by researchers could affect climate change calculations and could lead to improvements in desalination and drying processes.

It would help to better understand the physics of clouds

The technical details of the work led by Yaodong Tu were published in the journal PNAS. What was demonstrated is that under certain conditions, at the interface between water and air, light can directly cause evaporation without the need for heat, and in fact it does so even more effectively than heat. In these experiments, water was held in a hydrogel-based material, but the researchers believe that the phenomenon can also occur under other conditions.

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Specifically, light, when hitting the surface of the water where air and water meet, can break up water molecules and make them float in the air, causing evaporation in the absence of any heat source. This discovery could help explain mysterious measurements made 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 desalination or solar-powered material drying.

The scientists explain that they used 14 different experiments to demonstrate the existence of the photomolecular effect. They used the laser to study individual air-water interfaces and showed responses dependent on polarisation, angle of incidence and wavelength, peaking in the green, where bulk water does not absorb.

In the summary of the work they indicate that “we suggest that the photomolecular effect provides a mechanism to solve the long-standing enigma that the measured solar absorbance of clouds is greater than theoretical predictions based on the optical constants of bulk water, and we demonstrate that the Visible light can warm clouds."

Photomolecular evaporation is common in nature

Among the research conclusions, it is indicated that the work suggests that photomolecular evaporation is frequent in nature. That this process occurs everywhere, from clouds to fog, passing through the surfaces of the oceans, soils and plants. And it could also lead to new practical applications, such as in the production of energy and clean water.

Green light.
The researchers used green light to achieve water evaporation from a hydrogel. Image from the MIT research team

The new work is based on research published last year, which described this new “photomolecular effect” but only under very specialised conditions: on the surface of specially prepared hydrogels soaked in water. In the new study, the researchers show that the hydrogel is not necessary for the process; It occurs on any water surface exposed to light, whether a flat surface like a body of water or a curved surface like a cloud vapour droplet.

A key indicator, which was consistently observed in four different types of experiments under different conditions, was that when water began to evaporate from a test container under visible light, the air temperature measured above the water surface cooled and it then stabilised, showing that thermal energy was not the driving force of the effect.

Other key indicators that emerged were how the evaporation effect varied depending on the angle of the light, the exact colour of the light, and its polarisation. None of these variations should occur because, at those wavelengths, water barely absorbs light, and yet the researchers observed them.

Reference of the news:

Guangxin Lv,, Photomolecular effect: Visible light interaction with air–water interface, PNAS, 2024.