Visible Light Does Not Speed Up Water Evaporation, Max Planck Experiments Find
Three experiments, including laser pulses billions of times brighter than conventional lasers, found no sign of the 'photomolecular effect' that earlier work proposed.
A popular idea in surface physics says visible light can make water evaporate faster without heating it, by knocking molecules free from the surface. New experiments at the Max Planck Institute for Polymer Research in Mainz, Germany, found no evidence that this happens.
The team, led by Mischa Bonn, tested the proposed "photomolecular effect" directly and reported the results in the Proceedings of the National Academy of Sciences. The effect was suggested after earlier experiments found that water in hydrogels and in droplets evaporated faster when it was lit. Researchers proposed that light could directly detach water molecules from the surface, a mechanism that would be a way of producing vapor with far less energy than heating. If real, it would offer a way to make vapor with far less energy than heating requires.
Bonn's group did three different tests. In the first, they shone blue, green and red lasers on pure water surfaces under a range of humidity levels and measured the evaporation rate with precision sensors. They found that the surface dropped at exactly the same rate whether the light was on or off.
In the second, they looked at the top two or three layers of water molecules using a technique that detects molecular vibrations, to see whether light changed how those surface molecules behaved. It did not.
The third test aimed to rule out heating. The researchers used laser pulses lasting a trillionth of a second, with brightness more than 10 billion times that of conventional lasers. Such short pulses deliver intense light before heat can spread through the water. The surface vibrations stayed unchanged even under that extreme illumination.
The authors do not say the earlier measurements were wrong. They say those results probably have another cause. Their list of alternatives includes the way gels absorb light, how heat is distributed through the material, the buildup of vapor near the surface, radiation pressure from the light, and the geometry of droplets and pores. Sorting out which of these is responsible requires further work.
The result is a reminder of how easily evaporation experiments can mislead. Light almost always warms something, even slightly, and evaporation is sensitive to small temperature changes, so it is hard to separate a purely optical effect from a thermal one. Pure water at a clean interface is the simplest possible case, which is why Bonn's team started there.
For engineers, the finding cuts off one hoped-for shortcut. Systems that use sunlight to evaporate water, such as solar stills, will still rely on heat, and anyone designing a device around the photomolecular effect should check it against these results first. For physicists, the question of why gels and droplets sometimes dry faster under light is now open again, and it may turn out to have a more ordinary answer than a new light-matter interaction.
