Researchers from Massachusetts Institute of Technology (MIT) believe they have made an unusual and surprising discovery: Light appears to be capable of evaporating water without the involvement of heat.

The team were interested in making desalination – removing salts and minerals from water by evaporating it and then cooling down the vapor into liquid once more – more efficient. They were using black, light-absorbing materials placed into water to try and help speed up the heating process. As always [curses Newton], they were held back by a number of physical principles, in this case the thermal limit, which is the amount of evaporation that should occur for a given input of heat, taking into account physical principles such as the law of conservation of energy. 

In recent years, however, other teams had reported evaporation rates that go beyond that limit. In one experiment where the water was contained in hydrogels – three-dimensional networks of water-insoluble polymer chains that can hold large amounts of water – the evaporation rate was twice the thermal limit. This team, though skeptical, decided to experiment for themselves, including testing material provided by the previous researchers.

“We tested it under our solar simulator, and it worked,” professor of mechanical engineering Gang Chen said in a press release. “So, we believed them now.”

From here, they began testing their own hydrogels. Working on the hypothesis that photons of light were knocking out water molecules on the surface of the water, they began exposing these hydrogels to different wavelengths of light, precisely measuring the amount of liquid lost to evaporation. Sure enough, they were again able to exceed the thermal limit, with the most efficient evaporation taking place under a particular wavelength of green light. Under the same experiment without light, where electricity was used to heat the material instead, the evaporation rate did not exceed the thermal limit.

"We interpret these observations by introducing the hypothesis that photons in the visible spectrum can cleave water clusters off surfaces due to large electrical field gradients and quadrupole force on molecular clusters," the team wrote in their paper. "We call the light-induced evaporation process the photomolecular effect."

Water does not absorb much light, and neither do hydrogels, but combined they become good absorbers without the need to add dyes to aid absorption, Chen explained in the press release, aiding the effect. 

It could have useful real-world applications, including improving desalination and making it far cheaper. As well as this, if water evaporates under light it may impact climate models, and improve them by being incorporated in future.

The study is published in Applied Physical Sciences.