Evaporation is a fundamental process; whether it is fog burning off in the morning or water vapor rising from a boiling pot, evaporation is a primary component of the planet's water cycle. And now scientists have discovered something about it that we never knew: water molecules can break away when light strikes the point where the water meets the air, and this process can cause evaporation to occur even in the absence of a heat source. The findings have been reported in the Proceedings of the National Academy of Sciences (PNAS).
The study authors suggested that this effect could be widespread in nature, and could be observed in places such as the ocean's surface, fog, clouds, soil, and plants. It could also have an array of uses, they added, such as in climate change modeling and desalination.
"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,” said Gang Chen, a Professor at the Massachusetts Institute of Technology (MIT).
This work builds on previous research reported by this group in PNAS, in which light was shown to cause water to evaporate from a hydrogel without a heat source. But those were highly specialized laboratory conditions, while this latest work has determined that this is also a natural and common phenomenon.
Because of the surprising conclusion, the team used many lines of evidence to prove their hypothesis.
They also showed that the evaporation effect depends on different properties of the light that hits the water, including the color of the light, its angle, and polarization. The strongest effect happens when the light is striking the surface of water at a 45 degree angle, when the color of the light is green, and in transverse magnetic polarization.
The findings could help explain why measurements of the sunlight absorbed by clouds has been confounding; clouds seem to absorb more sunlight than the laws of physics allow. However, these latest findings on the evaporative effect of light could resolve the problem.
These cloud absorption measurements are made with satellites and flight data, and there has seemed to be too much sunlight absorption by clouds based on current theories, Chen explained. "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.”
The investigators have called this phenomenon the photomolecular effect, which is analogous to the photoelectric effect, discovered in 1887 by Heinrich Hertz. In the photoelectric effect, electrons are released from atoms in a surface that's struck by a photon of light.
Sources: MIT, Proceedings of the National Academy of Sciences (PNAS)
