This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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.
Through new research published in PNAS, Mischa Bonn and colleagues at the Max Planck Institute for Polymer Research in Mainz, Germany, have put the idea through a rigorous series of tests. They found that visible light has no measurable effect on the evaporation of pure water—hinting at an alternative influence on the evaporation rate that has yet to be discovered. In previous experiments, researchers investigated the impact of visible light exposure on water-soaked gels called hydrogels, as well as tiny water droplets.
Surprisingly, the water in these samples seemed to evaporate more quickly than could be explained by warming alone. One possible explanation was that light could be directly knocking water molecules free from the surface, a process dubbed the "photomolecular effect." If this were the case, it could change how scientists model water loss from oceans and clouds, and could lead to new solar-powered systems for purifying water. However, important questions remained about whether light was really acting on the water molecules themselves or if the results came from the materials and measuring techniques being used.
To answer these questions, Bonn's team used blue, green and red lasers to illuminate a flat surface of pure water at brightnesses similar to sunlight and over a range of humidities. With a highly precise sensor, they tracked how quickly the water's surface sank as it evaporated, comparing periods with the light on and off. At the same time, they used a technique that detects the vibrations of molecules in just the top two or three layers of water.
This would reveal whether light was loosening the bonds holding these molecules together. Finally, they hit the surface with extremely short laser pulses, each lasting around a trillionth of a second. These pulses reached peak brightnesses more than 10 billion times higher than conventional lasers but were too brief to heat the water.
In each case, evidence for light-driven evaporation came up short. The water's surface sank at exactly the same rate whether the light was on or off, and the vibrations of the surface molecules were unchanged. Even the intense pulses left the bonds between surface molecules undisturbed.
Extract — continue reading at the source.
