Unexpectedly, light has the ability to evaporate water without heat.

Everything around us is undergoing evaporation, from the sweat that cools our bodies to the dew that burns off in the early morning light. It’s possible that science’s comprehension of this commonplace process has been lacking a component for quite some time.

When they discovered that the water in their experiments—which were contained in a hydrogel, a material that resembles a sponge—was evaporating more quickly than could be explained by the heat, or thermal energy, the water was receiving, several researchers became perplexed in recent years. Furthermore, there has been a notable excess—a doubling, tripling, or even triple of the theoretical maximum rate.

A team of researchers at MIT has come to the astonishing conclusion that, under certain circumstances, light can directly cause evaporation at the interface where water meets air without the need for heat, and it can do so even more efficiently than heat. They have done this by conducting a number of new experiments and simulations, as well as by reexamining some of the results from various groups that claimed to have exceeded the thermal limit. Although the water in these studies was contained in a hydrogel substance, the researchers hypothesize that the phenomena might also happen in other situations.

The results are presented this week in a PNAS publication coauthored by Gang Chen, a professor of mechanical engineering at MIT, and four other authors, including postdoc Yaodong Tu.

The researchers claim that since the phenomena may have an impact on the development and growth of fog and clouds, it would be crucial to include it in climate models in order to increase their accuracy. Additionally, it can be crucial to many industrial operations, such the solar-powered desalination of water, opening up possibilities to skip the initial step of turning sunlight into heat.

The latest findings are unexpected because water can not significantly absorb light on its own. This explains why you can see the surface below through several feet of crystal-clear water. Thus, in order to aid in their initial investigation of the sun evaporation method for desalination, the scientists first added particles of a dark, light-absorbing substance to a container of water to help convert the sunlight to heat.

The researchers then discovered the work of another group that had doubled the thermal limit, or the maximum quantity of evaporation that may occur for a given input of heat, according to fundamental physical concepts like energy conservation. In these investigations, water was encapsulated in a hydrogel. Despite their initial skepticism, Chen and Tu began investigating hydrogels on their own, utilizing some of the other group’s material. According to Chen, “we tested it under our solar simulator, and it worked,” proving the remarkably high evaporation rate. “So, we believed them now.” Then Chen and Tu started creating and experimenting with their own hydrogels.

Chen states that he believes this light-based method might theoretically allow for a three- or four-fold increase in the maximum amount of water generated by solar desalination, which is now 1.5 kg/m³. “This could potentially really lead to cheap desalination,” according to him.

According to Tu, this phenomena may also be used to evaporative cooling procedures, where the phase change would enable the creation of an extremely effective solar cooling system.

In the meantime, the researchers are collaborating closely with other teams that are trying to duplicate the results in an effort to dispel doubts about the surprising results and the theory being put up to explain them.