Is heat necessary for evaporation?

Everyone knows that when you heat water hard enough, it evaporates into vapour. But is heat the only stimulus that can trigger this process?

Researchers at MIT have recently found that, in the absence of heating, certain wavelengths of light itself can be used to cause the evaporation of water from materials known as hydrogels – porous, sponge-like substances capable of storing large volumes of liquid. Intriguingly, not only has light been shown to be more effective than heat at this task, but the rate of light-induced evaporation was observed to exceed the thermal limit; this is the highest theoretical amount of thermal evaporation possible from a given area, calculated using fundamental principles of physics. 

Although the exact mechanism for this phenomenon is not yet understood, the researchers posit that photons (effectively packets of light) are able to dislodge clusters of water molecules from the surface of these so-called hydrogels, and thus cause the molecules to vaporise under certain conditions – this process has been coined the photomolecular effect. Notably, the rate of evaporation was found to vary upon exposure to different colours of LEDs; a particular shade of green was shown to be the most effective. 

So why is this finding important?

It is possible that this effect occurs constantly throughout nature, from the evaporation of water in oceans, clouds and fogs, through to the soil and surfaces of plants, and could prove to be a fundamental component in our understanding of these processes. 

Further, given the highly efficient nature of this effect when applied to hydrogels, the use of light as a means of evaporation could be beneficial in streamlining industrial processes such as desalination (the removal of salts and minerals from seawater). During thermal desalination, saline water is purified by a procedure of heating, evaporating and condensing; instead of using heat, however, employing light to induce evaporation could provide a greener, cheaper and more efficient alternative to a crucial process. 

Finally, and perhaps most prudently, factoring the role of light into climate modelling could serve to increase the accuracy of our predictions in the ongoing fight against global warming.

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