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How light can vaporize water without the need for heat

MIT scientists uncover a surprising phenomenon where light directly triggers water evaporation, with potential implications for climate modeling and desalination technology.

How light can vaporize water without the need for heat

For centuries, the process of evaporation – water transforming from liquid to vapor – was understood to be solely driven by heat. However, our understanding of this fundamental phenomenon is being rewritten thanks to groundbreaking research conducted at MIT. A team of scientists, led by Professor Gang Chen, has demonstrated that light itself can directly cause water to evaporate, even in the absence of heat. This astonishing discovery, termed the “photomolecular effect,” has far-reaching implications for various scientific and technological fields.

The research builds upon earlier work by the same team, where they observed this light-driven evaporation under specific conditions using hydrogels. The recent study, however, reveals that the photomolecular effect is not limited to specialized materials but occurs on any water surface exposed to light. Whether it’s a serene lake, a swirling fog bank, or even a tiny droplet suspended in the air, light can directly liberate water molecules, sending them on their airborne journey.

To solidify their findings, the researchers meticulously conducted a series of experiments, employing various techniques to confirm that the observed evaporation was indeed caused by light and not by any unintended heat sources. One key observation was the cooling of the air above the water surface as evaporation occurred under illumination. This temperature drop contradicts the conventional heat-driven evaporation process, where the air would be expected to warm up.

Further investigations revealed intriguing details about the photomolecular effect. The rate of evaporation was found to be dependent on the angle and color of the incident light, as well as its polarization. For instance, the effect was most pronounced when green light struck the water surface at a 45-degree angle with a specific polarization known as transverse magnetic polarization. These observations, particularly the influence of green light (color water readily transmits), present exciting puzzles for scientists to unravel.

The team proposes a theoretical framework to explain the dependence on light angle and polarization. They suggest that photons, the fundamental particles of light, can impart momentum to water molecules at the surface, providing enough force to eject them from the liquid. However, the observed color dependence remains a mystery, requiring further investigation.

Drawing parallels with the revolutionary photoelectric effect, where light liberates electrons from materials, the researchers believe that the photomolecular effect could similarly lead to groundbreaking applications. The potential impact is vast, ranging from advancements in solar desalination and industrial drying processes to a deeper understanding of cloud formation and climate modeling.

The discovery may finally resolve a long-standing discrepancy in climate science. For decades, measurements of sunlight absorption by clouds have consistently exceeded theoretical predictions. The photomolecular effect, with its ability to enhance evaporation, could account for this puzzling observation, potentially leading to more accurate climate models and predictions.

Professor Chen and his team are optimistic about the future applications of their discovery. They envision the development of energy-efficient desalination systems and drying technologies that harness the power of light. The photomolecular effect represents a significant leap in our understanding of light-matter interactions and holds immense promise for addressing pressing challenges in energy, water resources, and climate change.

For a deeper dive into the scientific details, you can access the research paper published in the journal PNAS here.

The original research cover story can be read at MIT Research News.

The image is courtesy of Bryce Vickmark.

Written by


Dr. Ravindra Shinde is the editor-in-chief and the founder of The Science Dev. He is also a research scientist at the University of Twente, the Netherlands. His research interests include computational physics, computational materials, quantum chemistry, and exascale computing. His mission is to disseminate cutting-edge research to the world through succinct and engaging cover stories.

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