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Light can drive water evaporation without heat

Evaporation is a constant phenomenon in our daily lives, ranging from perspiration on our skin to the early morning dew. However, there might be aspects of this universal process that science has yet to fully comprehend.

Unexpected evaporation rate

In recent experiments, scientists observed that water confined within a hydrogel, a sponge-like structure, was evaporating at rates unexpectedly higher than what the applied thermal energy could account for. This discrepancy wasn’t trivial; the evaporation rate was sometimes two or three times the theoretical maximum.

Surprising conclusion

A team of researchers from the Massachusetts Institute of Technology (MIT) embarked on a journey to unravel this mystery. Through a series of experiments and simulations, and revisiting data from various groups that had previously reported surpassing the thermal limit of evaporation, the scientists arrived at a surprising conclusion. 

They discovered that under specific circumstances, light interacting with the water’s surface could initiate evaporation without necessitating heat, and it does so more effectively than heat does. Although these experiments utilized hydrogel, the team believes this phenomenon could extend to other scenarios as well.

Light evaporation implications

The implications of this discovery are vast. It might influence our understanding of atmospheric phenomena like fog and cloud formation, necessitating revisions in climate models for enhanced accuracy. 

Additionally, it could revolutionize industrial processes, such as solar-powered water desalination, potentially allowing for a direct conversion of sunlight to evaporation, bypassing the need for heat conversion.

Light as a direct catalyst 

The research journey began with solar evaporation experiments for desalination, where the team initially used black, light-absorbing particles to facilitate the conversion of sunlight to heat in water. 

However, upon encountering research showcasing evaporation rates double the thermal limit using hydrogel, the team was initially skeptical. It was only after replicating these experiments and achieving similar results that they began to entertain the possibility of light being a direct catalyst for evaporation.

Photomolecular effect 

Through meticulous lab work, the team monitored the hydrogel’s surface response to simulated sunlight of varying wavelengths, finding a peak in evaporation efficiency at a specific green wavelength. This color-dependent behavior was a strong indicator that light, not heat, was driving the evaporation.

Despite water and hydrogel’s inherent low light absorption properties, their combination surprisingly exhibited strong absorption, enabling the efficient utilization of solar energy to exceed thermal evaporation limits. The experts referred to this phenomenon as the “photomolecular effect.”

Future research on light evaporation

Armed with this newfound knowledge, the researchers are now exploring practical applications, from improving solar desalination efficiency to studying the impact of these findings on climate change models. This light-induced evaporation process could potentially enhance solar desalination production limits, contribute to drying processes, and offer a novel approach to solar cooling.

As the MIT team continues their work, they are also collaborating with other research groups, aiming to validate their findings and establish the photomolecular effect as a cornerstone in the understanding of evaporation.

The study is published in the journal Proceedings of the National Academy of Sciences.

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