Researchers at the Massachusetts Institute of Technology (MIT) have discovered that light can cause water to evaporate without the need for heat. This unexpected phenomenon challenges our understanding of the evaporation process and opens up new possibilities for various applications.
The researchers initially noticed that water held in a hydrogel material was evaporating at a rate that exceeded the amount of heat it was receiving. Further experiments and simulations conducted by the MIT team confirmed that under specific conditions, light can directly trigger evaporation at the interface where water meets air. Astonishingly, this light-induced evaporation was found to be even more efficient than heat-induced evaporation.
The implications of this discovery are far-reaching. The phenomenon could have a significant impact on the formation and evolution of fog and clouds, making it essential to incorporate this finding into climate models for improved accuracy. Moreover, in industrial processes such as solar-powered desalination of water, this discovery could potentially eliminate the need to convert sunlight into heat before initiating evaporation.
One of the most intriguing aspects of this research is that water itself does not absorb light to a significant extent. This explains why we can see clearly through clean water. To investigate solar evaporation for desalination, the researchers initially introduced light-absorbing particles into the water to convert sunlight into heat. However, they later stumbled upon the work of another group that achieved an evaporation rate that exceeded the thermal limit without the use of heat. These experiments involved water bound within a hydrogel, prompting the MIT team to explore this phenomenon further.
The researchers hypothesized that light could be responsible for the excess evaporation, with photons knocking water molecules loose from the surface. They conducted experiments using hydrogels, closely monitoring the surface’s response to simulated sunlight of different colors. The evaporation rate was measured by directly weighing the mass lost to evaporation and monitoring the temperature above the hydrogel surface. The researchers discovered that the effect varied with color, peaking at a particular wavelength of green light. This color dependence strongly indicates that it is the light itself, not heat, causing the additional evaporation.
To confirm their findings, the researchers repeated the experiments using electricity to heat the material, without light. Surprisingly, the evaporation never exceeded the thermal limit, further supporting the notion that light is responsible for the increased evaporation rate.
Although water and the hydrogel material individually do not absorb much light, their combination creates a strong absorber. This allows the material to efficiently harness the energy of solar photons, surpassing the thermal limit without the need for dark dyes for absorption.