Contrary to what we learned in school, scientists at MIT found that light, not just heat, can make water evaporate. This discovery is surprising because we usually think heat is needed for evaporation. This finding could help us understand nature better and make desalination systems work better.
Evaporation happens when water molecules at the surface of a liquid get enough energy to turn into a gas, which we call water vapor. Usually, heat is the source of this energy, and sunlight provides heat in the Earth’s water cycle.
However, in recent years, some scientists noticed that water in certain gels was evaporating faster than expected, even when there wasn’t much heat. To figure out why, MIT scientists did experiments. They thought that light might be causing the extra evaporation, even though water doesn’t normally absorb light.
To test their idea, they put a gel in a container and shone different colors of light on it. They made sure no heat was involved. The water still evaporated more than it should have, and it depended on the color of light, with green light being the best.
Then, they did the same experiment in the dark but used electricity to add the same amount of heat as the light experiment. The water evaporated much less.
They called this new discovery the “photomolecular effect.” They think that light might be able to break apart clusters of water molecules at the surface. They believe this could happen in nature, like in clouds or on the sea’s surface, but heat is still the main cause in those situations.
This discovery might help improve desalination and cooling systems. The scientists got a grant to study using this effect in solar-powered desalination, which could make it three or four times more efficient. They also want to see if this effect affects climate models.