TechMIT researchers prove light alone can evaporate water, hinting at new technologies

MIT researchers prove light alone can evaporate water, hinting at new technologies

To evaporate water, you don't need heat.
To evaporate water, you don't need heat.
Images source: © Bryce Vickmark
5:34 PM EDT, May 9, 2024
New research from the Massachusetts Institute of Technology (MIT) suggests that light can evaporate water without the help of heat. This discovery, prevalent in nature, opens up potential for novel applications.

Water evaporation has been observed for thousands of years, where water turns from liquid to steam. This process takes place across rivers, lakes, and oceans. While seemingly well-understood, MIT's recent findings challenge the notion that heat is necessary for evaporation; light alone can do the job.

The study's findings were published in the "Proceedings of the National Academy of Sciences" (DOI: 10.1073/pnas.2320844121).

Photomolecular Effect

An MIT team led by Professor Gang Chen has shown that light, not just heat, can cause water to evaporate. When light hits water, it can directly cause water molecules to transition into the air, an occurrence the researchers call the photomolecular effect. This surprising revelation indicates that heat is not the only trigger for evaporation.

To confirm their unexpected findings, the team conducted 14 different experiments under various conditions, proving that light can cause evaporation. One key discovery was that air temperature above the water didn't increase during evaporation in visible light; it stabilized, demonstrating that heat wasn't responsible. Other factors, such as the light's angle, color, and polarization, also affected the evaporation. Notably, a 45-degree angle with green light showed the strongest effect, even though water absorbs this wavelength minimally.

The authors believe this widespread effect could lead to new practical uses, including energy production and water purification.

Better Climate Models

Chen sees extensive potential for this discovery. "I think this has a lot of applications. 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 says.

This breakthrough could resolve longstanding anomalies in cloud behavior, aiding climate change estimations. It also opens up possibilities for new industry processes, such as solar-powered desalination or drying.
A year back, the photomolecular effect was observed on specially prepared hydrogels. The latest research demonstrates that this effect isn't limited to hydrogels; it occurs on any water surface exposed to light.

Interactions of Light with Water

The effect is strongest under specific conditions: a 45-degree angle, transverse magnetic (TM) polarization, and green light.
The team proposes that photons from light exert enough force to displace water molecules, although the effect's dependency on light color remains a mystery, suggesting further research is needed.

"The finding of evaporation caused by light instead of heat provides new disruptive knowledge of light-water interaction," says Xiulin Ruan from Purdue University, not part of the study. It hints at innovative approaches to water desalination powered by solar energy, representing a pioneering leap in scientific knowledge.

Shannon Yee from Georgia Tech, also not involved in the study, highlights this research as a paradigm shift in understanding water evaporation kinetics, emphasizing the continuous potential for learning in science.
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