Visible light does not speed up water evaporation, rigorous experiments suggest
In recent experiments involving hydrogels and water droplets, researchers have proposed that exposure to visible light could make water evaporate faster without heating it. However, direct evidence that light acts on water itself has proven elusive.
A recent study published in the Proceedings of the National Academy of Sciences (PNAS) has challenged the notion that visible light speeds up water evaporation. Researchers at the Max Planck Institute for Polymer Research in Mainz, Germany, conducted a series of rigorous tests to investigate whether light could act directly on water molecules, a phenomenon referred to as the photomolecular effect.
The study found that exposure to visible light, including blue, green, and red lasers, had no measurable impact on the evaporation rate of pure water, even under conditions similar to sunlight. Surface molecular vibrations remained unchanged, and even extremely short laser pulses, which reached peak brightnesses over 10 billion times higher than conventional lasers, did not disturb the bonds between surface molecules.
The researchers suggest that the previously observed enhanced evaporation in hydrogels and water droplets may be due to factors such as light-absorbing materials warming up, heat spreading through them, water vapor gathering near the surface, the gentle push of light, or the shapes of droplets and gel pores. Their findings provide a benchmark for future studies to uncover the true cause of enhanced evaporation rates in water-related technologies.
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