For generations now, people have been put in the belief that evaporation of water is due to heat. Now, a group of researchers from MIT discovered a phenomenon they call the “photomolecular effect,” where light alone initiates the water evaporation process without using thermal energy. This opens up great possibilities for applications from climate science to industries.
This fundamentally changes our understanding of how light and water might interact, in the sense that light, with no heat, could manage to evaporate water. A new effect has been studied recently by a team led by the Carl Richard Soderberg Professor of Power Engineering at MIT, Gang Chen, along with postdoctoral researchers Guangxin Lv and Yaodong Tu, as well as graduate student James Zhang. Their work, published in the Proceedings of the National Academy of Sciences, holds important implications for weather forecasting, climate change models, and new technologies for energy and water production.
The photomolecular effect explains a mysterious phenomenon that puzzled scientists in climatology for quite some time: why clouds on Earth absorb more sunlight than conventional physics predicts. Xiulin Ruan, a professor at Purdue University in mechanical engineering and not associated with the work, told Physics World, that to make things clear, they carried out 14 types of experiments to verify this hypothesis. The key indicator, however, was the cooling and subsequent stabilization of air temperature above visible light at the water surface, which was evidence that ruled out the involvement of thermal energy. Lv highlighted, “The flat region in the air-side temperature distribution above hot water will be the easiest for people to reproduce.” “The finding of evaporation caused by light instead of heat provides new disruptive knowledge of light-water interaction. It could help us gain new understanding of how sunlight interacts with clouds, fog, oceans, and other natural water bodies to affect weather and climate.”
One prediction of the photomolecular effect is that photons of light are able to free full water molecules from the surface of a liquid in much the same way that the photoelectric effect liberates electrons from atoms. It was shown by the experiments that the light enables the evaporation of water four times more efficiently than the thermal limit. “It kind of ties together what a whole lot of people are reporting in their solar desalination devices,” said Zhang. According to him, it showed an evaporation rate that was way higher than what the heat alone could explain.
This discovery may also provide the answers to puzzling cloud absorption observations. Measurements for many years have shown that clouds absorb more sunlight than what theoretical models predict. “Those experiments are based on satellite data and flight data. 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” Gang Chen explained. The complexity of clouds and the problems of making such measurements have led researchers to argue about whether such discrepancies are real or not. “And what we found out suggests, ‘Hey, there’s this other mechanism for cloud absorption,’ which wasn’t accounted for, and this mechanism might just explain the discrepancies.”
It doesn’t stop here the implication of this discovery goes much beyond the theoretical. Developments related to the photomolecular effect could improve solar desalination and other industrial drying processes. Drying, which uses a large fraction of industrial energy, could become far more efficient. Gang Chen pointed out that “drying consumes 20 percent of all industrial energy usage,” so the potential economic and environmental benefits are huge.
This photomolecular effect could also completely twist conventional wisdom regarding natural evaporation processes. Shannon Yee, associate professor of mechanical engineering at Georgia Tech, said, “The observations in the manuscript point toward some new physical mechanism that fundamentally changes our thinking about the kinetics of evaporation. Who would have thought we still had so much to learn about something as prosaic as evaporating water?”
Though revolutionary, this discovery is just the tip of the iceberg. As Chen close-endedly mentioned, “This phenomenon should be very general, and our experiment is really just the beginning.” Future research will delve into the myriad variables, from different materials to other liquids and even solids. The team is already working with companies to help apply it in practice, especially in solar desalination systems.
In other words, the photomolecular effect is something of a paradigm shift in what we call the evaporating process of water. This was quite a breakthrough, breaking major theories and providing new hopes for the future of scientific investigations and technological changes. Since research is continuing, more efficient industrial procedures can easily be discovered, even better climate models for a deeper understanding of the natural world.