MIT researchers unveil how light alone can cause water to evaporate

New research conducted by scientists from the Massachusetts Institute of Technology suggests that light can evaporate water without the help of heat. Researchers indicate that this effect is common and could lead to new, practical applications.

For millennia, people have observed the process of water evaporation, during which water transforms from a liquid into steam. It is one of the most basic processes. Rivers, lakes, and oceans evaporate – this process occurs everywhere. It might seem that evaporation, which we encounter daily, is well understood physically. However, a recent discovery by researchers from the Massachusetts Institute of Technology (MIT) challenges our traditional understanding of this process. It turns out that heat is not necessary for the evaporation of water; light alone is sufficient.

The work's description and results appeared in the pages of the journal Proceedings of the National Academy of Sciences.

The team of MIT scientists, led by Professor Gang Chen, demonstrated that heat is not the only cause of water evaporation. Light hitting the surface of the water can directly release water molecules, causing them to evaporate into the air. The researchers named this effect the photomolecular effect, and it occurs independently of heat, which challenges our beliefs.

Given the unexpected nature of this effect, the scientists conducted 14 different experiments and measurements, collecting evidence of light-induced evaporation under various conditions to confirm their findings. A key indicator obtained in four different experiments was measuring air temperature over the water during evaporation in visible light. Rather than increasing, the temperature stabilized, indicating that heat energy was not the cause. Other vital indicators included the change in the evaporation effect depending on the angle of light incidence, the exact colour of light, and its polarization. For instance, the effect was strongest when green light, which minimally absorbs in the wavelength range of 495 to 566 nanometers, hit the water at a 45-degree angle.

The authors believe this effect should occur commonly in nature – from clouds to fogs, ocean surfaces, or soils- and may also lead to new practical applications, including energy production and clean water supply.

"I think this has a lot of applications," says Chen. "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," he adds.

This fascinating discovery could have a wide range of implications. It might help explain strange measurements made over the years, suggesting that clouds absorb more light than conventional models predict, thus affecting climate change calculations. Chen suggests that this newly discovered mechanism could account for this excessive absorption, potentially improving climate calculations. It might also lead to new ways of designing industrial processes, such as desalination or drying materials powered by solar energy.

The research builds on findings from a year ago. The scientists noticed and described the photomolecular effect, but only under very specialized conditions: on the surface of specially prepared hydrogels saturated with water. In their new work, they demonstrated that the hydrogel is not necessary for this process, and the effect occurs on any water surface exposed to light, regardless of whether it is a flat surface, like a water body, or a curved surface, like a cloud droplet.

The scientists determined that the photomolecular effect is most potent when light hits the water surface at a 45-degree angle. It is also strongest with a specific type of polarization, known as transverse magnetic (TM) polarization, and peaks in green light.

Chen and his colleagues proposed a physical mechanism that may explain the effect's dependence on angle and polarization. They suggest that light photons provide sufficient force to water molecules on the surface, displacing them from the liquid body. However, the dependence on the colour of light remains a mystery, which the scientists believe will require further research.

"The finding of evaporation caused by light instead of heat provides new disruptive knowledge of light-water interaction" says Xiulin Ruan from Purdue University, who was not involved in the research. "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," he adds.

"The observations in the manuscript points to a new physical mechanism that foundationally alters our thinking on the kinetics of evaporation," says Shannon Yee from Georgia Tech, who also was not involved in the research. "Who would have thought that we are still learning about something as quotidian as water evaporating?" he adds.