ദൃശ്യപ്രകാശം ജലത്തിന്റെ ബാഷ്പീകരണത്തെ വേഗത്തിലാക്കുന്നുവെന്ന വാദം തെറ്റാണെന്ന് മാക്സ് പ്ലാങ്ക് ഇൻസ്റ്റിറ്റ്യൂട്ടിന്റെ പുതിയ പഠനം വ്യക്തമാക്കുന്നു. ഹൈഡ്രോജെല്ലുകളിലെ ബാഷ്പീകരണം ചൂടുകൊണ്ട് മാത്രമാണെന്നും പ്രകാശം നേരിട്ട് ബാധിക്കുന്നില്ലെന്നും കൃത്യമായ പരീക്ഷണങ്ങളിലൂടെ തെളിഞ്ഞു. ജലബാഷ്പീകരണത്തെക്കുറിച്ചുള്ള മുൻധാരണകളെ തിരുത്തുന്നതാണ് ഈ കണ്ടെത്തൽ.
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.
Unexplained evaporation
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.
Rigorous experiments
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.
Looking for other explanations
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.
The result means that the fast evaporation seen in earlier experiments needs a different explanation. Bonn's team points to several possibilities: gels absorbing light and warming up, heat spreading through these materials, water vapor gathering near the surface, the gentle push of light itself, and the shapes of droplets and gel pores.
For now, their measurements offer a clear benchmark for future studies and could help engineers designing light-powered water technologies better understand where any gains are really coming from.