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Evaporation without heat

A surprising “photomolecular effect” could affect calculations of climate change and may lead to improved desalination and drying processes.

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This lab device uses laser beams to measure the “photomolecular effect” the MIT team has identified and confirmed, in which light striking a surface where water meets air frees water molecules and lets them float away.BRYCE VICKMARK

In a series of painstakingly precise experiments, a team of researchers at MIT has confirmed an astonishing discovery: light can cause water to evaporate without involvement from any source of heat.

The phenomenon can occur at any surface where air and water meet, whether it’s flat like a pond or curved like a droplet of cloud vapor: light striking the surface breaks water molecules free and lets them float away. The researchers call it the “photomolecular effect,” by analogy to the photoelectric effect discovered by Heinrich Hertz and explained by Albert Einstein. The strength of the effect depends on the angle of the light, the exact color of the light, and its polarization.

The finding could help explain a phenomenon that has mystified climate scientists for more than 80 years, in which clouds are measured to be absorbing more sunlight than conventional physics holds possible: the additional evaporation could account for the discrepancy. This realization could alter calculations of how climate change affects cloud cover and precipitation. The photomolecular effect could also be harnessed for industrial processes such as desalinating water or drying materials.

“I think this has a lot of applications,” says engineering professor Gang Chen, who wrote a paper on the work with postdocs Guangxin Lv and Yaodong Tu and graduate student James Zhang, SM ’21. Noting that drying consumes 20% of all industrial energy usage, he says the team has already been approached by companies looking to use the effect for evaporating syrup and drying paper in a paper mill. 

“We’re exploring all these different directions,” he adds. “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.”

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