Paradigm shift: Light causes water evaporation without the need for heat

Water evaporation is a fundamental process that occurs all around us, from the surfaces of oceans and lakes to the burning off of fog in the morning light from the sun.

For centuries, humans have observed and utilized this phenomenon, assuming that heat was the sole cause of evaporation. However, a truly groundbreaking discovery by a team of scientists has revealed that there is more to evaporation than meets the eye.

Photomolecular effect: How light breaks water molecules

In a series of meticulous experiments, a team led by Gang Chen, the Carl Richard Soderberg Professor of Power Engineering at MIT, along with postdocs Guangxin Lv and Yaodong Tu, and graduate student James Zhang, has demonstrated that light can also cause water to evaporate, even in the absence of heat.

Their findings, published in the journal PNAS, suggest that this effect, which they have named the photomolecular effect, could have far-reaching implications in various fields, from climate science to industrial applications.

The researchers have named this phenomenon the photomolecular effect, drawing an analogy with the photoelectric effect discovered by Heinrich Hertz in 1887 and later explained by Albert Einstein in 1905.

Just as the photoelectric effect demonstrates that photons of light can liberate electrons from atoms in a material, the photomolecular effect shows that photons can also liberate entire molecules from a liquid surface.

Solving an 80-year-old mystery in climate science

This discovery may hold the key to solving an 80-year-old mystery in climate science, which has puzzled scientists for decades. Measurements of cloud absorption of sunlight have often shown discrepancies, with clouds absorbing more sunlight than conventional physics dictates possible.

“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,” Gang Chen explains.

“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,” Chen continued.

Rigorous testing and evidence

Due to the unexpected nature of the effect, the research team conducted 14 different kinds of tests and measurements to establish that water was indeed evaporating due to light alone, not heat.

One key indicator, consistently observed in four different experiments under varying conditions, was the cooling and leveling off of air temperature above the water’s surface as evaporation began under visible light. This finding suggests that thermal energy was not the driving force behind the effect.

Lv, one of the postdocs involved in the study, highlights that among the many lines of evidence, “the flat region in the air-side temperature distribution above hot water will be the easiest for people to reproduce.” This temperature profile “is a signature” that demonstrates the effect clearly, he says.

Zhang, the graduate student on the team, adds: “It is quite hard to explain how this kind of flat temperature profile comes about without invoking some other mechanism” beyond the accepted theories of thermal evaporation.

According to Zhang, “It ties together what a whole lot of people are reporting in their solar desalination devices,” which again show evaporation rates that cannot be explained by the thermal input.

Substantial impact: Four times the thermal limit

The photomolecular effect can have a substantial impact on evaporation rates. Under the optimum conditions of color, angle, and polarization, Lv says, “the evaporation rate is four times the thermal limit.”

“I think this has a lot of applications,” Chen says. “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.”

Since the publication of the first paper on this topic, the team has been approached by companies hoping to harness the effect for various applications, including evaporating syrup and drying paper in a paper mill.

Chen believes that the likeliest first applications will come in the areas of solar desalinization systems or other industrial drying processes. “Drying consumes 20 percent of all industrial energy usage,” he points out.

New frontier in light-water interactions and evaporation

As the photomolecular effect is a new and unexpected phenomenon, Chen emphasizes that “this phenomenon should be very general, and our experiment is really just the beginning.”

The experiments needed to demonstrate and quantify the effect are very time-consuming, and there are many variables to explore, from understanding water itself to extending the research to other materials, liquids, and even solids.

“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 cloud, fog, oceans, and other natural water bodies to affect weather and climate,” enthused Xiulin Ruan, professor of mechanical engineering at Purdue University, who was not involved in the study.

“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,” Ruan concluded.

How the photomolecular effect could transform our world

In summary, the photomolecular effect represents a paradigm shift in our understanding of water evaporation and light-water interaction.

This truly groundbreaking discovery by Gang Chen and his team at MIT challenges long-held assumptions and opens up a wealth of possibilities for future research and applications.

As scientists continue to explore the variables and implications of this phenomenon, we can anticipate a new era of innovation in fields ranging from climate science to industrial processes.

The photomolecular effect reminds us that even the most fundamental and seemingly well-understood processes in nature can still hold surprises, and that the pursuit of scientific knowledge is an ongoing journey of discovery and wonder.

The full study was published in the journal Proceedings of the National Academy of Sciences.

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