Light Can Make Water Evaporate Without Heat

Runaway1956 writes:

https://news.mit.edu/2023/surprising-finding-light-makes-water-evaporate-without-heat-1031
https://www.pnas.org/doi/full/10.1073/pnas.2312751120 Note that the paper is not paywalled.

A newly identified process could explain a variety of natural phenomena and enable new approaches to desalination.

Evaporation is happening all around us all the time, from the sweat cooling our bodies to the dew burning off in the morning sun. But science's understanding of this ubiquitous process may have been missing a piece all this time.

In recent years, some researchers have been puzzled upon finding that water in their experiments, which was held in a sponge-like material known as a hydrogel, was evaporating at a higher rate than could be explained by the amount of heat, or thermal energy, that the water was receiving. And the excess has been significant - a doubling, or even a tripling or more, of the theoretical maximum rate.

After carrying out a series of new experiments and simulations, and reexamining some of the results from various groups that claimed to have exceeded the thermal limit, a team of researchers at MIT has reached a startling conclusion: Under certain conditions, at the interface where water meets air, light can directly bring about evaporation without the need for heat, and it actually does so even more efficiently than heat. In these experiments, the water was held in a hydrogel material, but the researchers suggest that the phenomenon may occur under other conditions as well.

The findings are published this week in a paper in PNAS, by MIT postdoc Yaodong Tu, professor of mechanical engineering Gang Chen, and four others.

The phenomenon might play a role in the formation and evolution of fog and clouds, and thus would be important to incorporate into climate models to improve their accuracy, the researchers say. And it might play an important part in many industrial processes such as solar-powered desalination of water, perhaps enabling alternatives to the step of converting sunlight to heat first.

The new findings come as a surprise because water itself does not absorb light to any significant degree. That's why you can see clearly through many feet of clean water to the surface below. So, when the team initially began exploring the process of solar evaporation for desalination, they first put particles of a black, light-absorbing material in a container of water to help convert the sunlight to heat.

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