Light alone can provoke water evaporation, MIT study finds

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

For millennia, the process of water evaporation, during which water transforms from a liquid to steam, has been observed. This basic process occurs everywhere - in rivers, lakes, and oceans. It might appear that evaporation, a daily phenomenon, is well-understood physically. However, a recent discovery by researchers from the Massachusetts Institute of Technology (MIT) challenges the conventional understanding of this process. They found that heat is not necessary for the evaporation of water; light alone can do the job.

The details and results of the work have been published in the journal "Proceedings of the National Academy of Sciences" (DOI: 10.1073/pnas.2320844121).

The MIT team, led by Professor Gang Chen, showed that light striking the water's surface could directly release water molecules, causing them to evaporate into the air. This novel effect, the photomolecular effect, as they named it, happens independently of heat, upending conventional wisdom.

In light of this unexpected discovery, the scientists carried out up to 14 different experiments and measurements to verify their findings under various conditions. A key piece of evidence found in four separate experiments was the air temperature measurement over the water during evaporation in visible light. Instead of increasing, the temperature stabilized, indicating that heat energy was not responsible. Another significant finding was that the evaporation effect varied according to the angle at which light hit the water, the specific colour of light, and its polarization. Notably, the effect was strongest when green light, about 495 to 566 nanometres, struck the water at a 45-degree angle despite the water's minimal absorption in this wavelength range.

The authors suggest that this effect should commonly occur in nature—from clouds and fogs to ocean surfaces and soils—and that it could also lead to new practical applications, including energy production and clean water.

"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 remarkable discovery could have wide-ranging implications. It may help explain odd readings taken over years, suggesting clouds absorb more light than conventional models predict, influencing climate change calculations. Chen believes this newly identified mechanism could explain this excessive absorption, possibly refining climate models. It might also pave the way for new industrial processes, such as desalination or drying materials, powered by solar energy.

The research builds on findings from a year ago when the scientists observed and described the photomolecular effect, but only under highly specialized conditions: on the surface of specially prepared hydrogels saturated with water. In their recent work, they demonstrated that the hydrogel isn't necessary for this process to occur and that the effect happens on any water surface exposed to light, regardless of whether it's a flat surface, like a lake, 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 and when there’s a specific type of polarization known as transverse magnetic (TM) polarization, peaking in green light.

Chen and his team have proposed a physical mechanism that might explain the effect's dependence on angle and polarization. They argue that photons of light provide enough force to displace water molecules from the surface, although the influence of light's colour remains a mystery, requiring further investigation.

"The finding of evaporation caused by light instead of heat provides new disruptive knowledge of light-water interaction," states 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," mentions Shannon Yee from Georgia Tech, who also was not part of the study. "Who would have thought that we are still learning about something as quotidian as water evaporating?" he comments.