New discovery: Light can cause water to evaporate without heat

Credit: Bryce Vickmark.

Researchers at MIT have made a groundbreaking discovery that light alone can cause water to evaporate, challenging the long-held belief that heat is necessary for evaporation.

This finding, detailed in a recent study published in the journal PNAS, could significantly impact our understanding of climate science and lead to innovative technologies in solar energy and water desalination.

Traditionally, evaporation has been understood as a process driven by heat — the sun heats the water, and the water turns into vapor.

However, the MIT team’s experiments reveal that under certain conditions, light can directly cause water molecules to break away and evaporate without any increase in temperature.

This phenomenon, named the “photomolecular effect,” shows that photons, the particles of light, can exert a force strong enough to separate water molecules from surfaces.

This research, led by Gang Chen, a professor at MIT, alongside postdocs Guangxin Lv and Yaodong Tu, and graduate student James Zhang, found that this effect could explain previously mysterious aspects of how sunlight interacts with clouds and influences the weather and climate.

The study could revise how scientists calculate the heating effects of clouds on Earth’s climate, which has been a puzzle due to inconsistencies in data collected over the years.

The team conducted 14 different types of experiments to confirm their findings, ensuring a robust and reliable conclusion.

One significant indicator of the photomolecular effect was observed when water under visible light began evaporating while the air temperature above the water cooled down, proving that thermal energy was not driving the evaporation.

The researchers also discovered that the evaporation effect varied depending on the light’s angle, color, and polarization.

Remarkably, the evaporation was most effective when the light was green — a color in which water is usually most transparent, indicating minimal absorption and interaction with light.

The implications of this discovery are vast.

Not only does it add a critical piece to the puzzle of how sunlight affects Earth’s water cycle, but it also opens doors to developing high-performance, solar-powered water desalination technologies and other industrial processes that involve drying.

For example, the research has already attracted interest from industries looking to use this effect for more energy-efficient drying methods in processes like paper production.

Chen emphasizes that this new understanding of light-water interaction is just the beginning.

The photomolecular effect appears to be a general phenomenon that could extend beyond water to other materials. As such, it invites a broader exploration of its implications across various fields and materials.

The discovery challenges existing theories and shows that our understanding of natural processes like evaporation is still evolving. It underscores the importance of scientific inquiry and experimentation in uncovering the full complexities of the natural world.

As researchers continue to explore and quantify this effect, the initial findings herald a significant shift in how we perceive the interaction between light and water.

This could reshape technologies and our understanding of nature, proving that even in the most fundamental processes like evaporation, there is still much to learn.