MIT Researchers Make an Unexpected Discovery: Light-Induced Water Evaporation Without Heat

Researchers at the Massachusetts Institute of Technology (MIT) have uncovered a surprising phenomenon: light can trigger the evaporation of water at rates surpassing those achievable by heat alone, a discovery that has particular relevance for water contained within hydrogels. This “photomolecular effect” holds the promise of transforming solar desalination and enhancing the accuracy of climate models, as it could potentially increase water production in desalination processes by threefold and propel advancements in solar cooling technology.

A new process has been pinpointed that may illuminate various natural occurrences and pave the way for novel desalination methods.

We constantly witness evaporation, whether it be through the cooling mechanism of sweat or the morning dew dissipating in the sunlight. However, our comprehension of this omnipresent process may have overlooked an essential aspect until now.

Some scientists have been confounded in recent years by observing that water confined within a hydrogel—a material similar to a sponge—was evaporating at a rate exceeding what could be accounted for by the heat it was exposed to. This excess was not trivial, amounting to a doubling or even tripling of the anticipated maximum rate.

A study from MIT suggests that under specific conditions, light alone can induce evaporation at the junction of water and air, bypassing the need for thermal energy, and it does so more effectively than heat. The researchers found this to occur with hydrogel-bound water and postulate that it might also take place under different circumstances.

The research has been documented in a recent publication in the Proceedings of the National Academy of Sciences (PNAS) by MIT postdoctoral researcher Yaodong Tu, mechanical engineering professor Gang Chen, and their colleagues.

In the laboratory, the team observed the surface of a hydrogel—a gelatin-like structure composed mainly of water within a mesh of fine membranes. They prepared hydrogel samples, documenting both their frozen or dry states and their water-saturated “swollen states.” This work is credited to the MIT research group.

The study suggests that this phenomenon may contribute to the development and transformation of fog and clouds, making it a critical factor to be integrated into climate predictions to enhance their precision. It could also play a significant role in numerous industrial applications, such as solar desalination, offering an alternative to the step of converting sunlight into heat.

Unpacking the Implications of the Discovery

The novelty of these findings lies in the fact that water typically does not significantly absorb light—hence the clarity of clean water over great depths. Initially, when examining solar evaporation for desalination, the team included particles of a dark, light-absorbing substance in water to facilitate the transformation of sunlight into heat.

However, upon encountering research that demonstrated an evaporation rate twice the thermal limit—the maximal evaporation rate dictated by a given amount of heat based on energy conservation laws—the MIT team, though initially skeptical, began their own hydrogel experiments. Confirming the heightened evaporation rate, they concluded that photons of light were likely dislodging water molecule clusters directly from the surface. This phenomenon seemed to occur precisely at the water-air boundary, and potentially also on the surfaces of the ocean, clouds, or fog.

In their lab experiments, they scrutinized the hydrogel’s surface, monitoring its response to simulated sunlight of specific wavelengths. Through different light colors, they gauged the rate of evaporation, employing scales to measure mass loss and temperature checks above the hydrogel surface, ensuring no additional heat from the lighting.

The team observed that the evaporation effect varied with light color and peaked with a certain green wavelength, which further substantiated the theory that light, not heat, was the catalyst for the evaporation.

Despite water and hydrogel’s low light absorption, their combined form showed a high capacity to absorb solar photons, thus exceeding the thermal limit without the need for light-absorbing dyes.

Exploring Potential Applications and Continued Collaborations

With the discovery of the photomolecular effect, the research team is now investigating how to leverage this in practical applications. They have received funding from MIT’s Abdul Latif Jameel Water and Food Systems Lab to enhance solar desalination efficiency using this phenomenon and from a Bose Grant to study its impact on climate modeling.

Tu outlines the traditional two-step desalination process: evaporation followed by vapor condensation into fresh water. With this discovery, he posits a significant efficiency increase on the evaporation front. Chen envisions the potential to multiply the current solar desalination water yield—1.5 kilograms per square meter—by three or four times, potentially resulting in cost-effective desalination methods.

Moreover, this effect could also be employed in evaporative cooling systems, using the phase change to achieve efficient solar cooling.

The research team continues to collaborate with others in an effort to replicate their findings and to address the skepticism faced by such unexpected results and their proposed explanation.

Reference: “Plausible photomolecular effect leading to water evaporation exceeding the thermal limit” by Yaodong Tu, Jiawei Zhou, Shaoting Lin, Mohammed Alshrah, Xuanhe Zhao, and Gang Chen, 30 October 2023, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2312751120

The investigative team also comprises Jiawei Zhou, Shaoting Lin, Mohammed Alshrah, and Xuanhe Zhao, all affiliated with MIT’s Department of Mechanical Engineering.

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