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A new study is shedding light on why solar radiation is more effective than other forms of energy at causing water to evaporate. The key factor turns out to be the oscillating electric field inherent to sunlight itself:
"It's well established that the sun is exceptionally good at causing water to evaporate – more efficient than heating water on the stove, for instance," says Saqlain Raza, first author of a paper on the work and a Ph.D. student at North Carolina State University. "However, it has not been clear exactly why. Our work highlights the role that electric fields play in this process."
"This is part of a larger effort in the research community to understand this phenomenon, which has applications such as engineering more efficient water-evaporation technologies," says Jun Liu, co-corresponding author of the paper and an associate professor of mechanical and aerospace engineering at NC State.
To explore questions related to sunlight's efficiency at evaporating water, the researchers turned to computational simulations. This allowed them to alter different parameters associated with sunlight to see how those characteristics influence evaporation.
"Light is an electromagnetic wave, which consists – in part – of an oscillating electric field," Liu says. "We found that if we removed the oscillating electric field from the equation, it takes longer for sunlight to evaporate water. But when the field is present, water evaporates very quickly. And the stronger the electric field, the faster the water evaporates. The presence of this electric field is what separates light from heat when it comes to evaporating water."
But what exactly is the oscillating electric field doing?
"During evaporation, one of two things is happening," Raza says. "Evaporation either frees individual water molecules, which drift away from the bulk of liquid water, or it frees water clusters. Water clusters are finite groups of water molecules which are connected to each other but can be broken away from the rest of the liquid water even though they are still interconnected. Usually both of these things happen to varying degrees."
"We found that the oscillating electric field is particularly good at breaking off water clusters," says Liu. "This is more efficient, because it doesn't take more energy to break off a water cluster (with lots of molecules) than it does to break off a single molecule."
[...] "This work substantially advances our understanding of what's taking place in this phenomenon, since we are the first to show the role of the water clusters via computational simulation," says Liu.
Journal Reference:
Saqlain Raza, Cong Yang, Xin Qian, et al. Oscillations in incident electric field enhances interfacial water evaporation [open], Materials Horizons (DOI: 10.1039/D5MH00353A)
(Score: 5, Interesting) by PhilSalkie on Thursday July 03, @06:04PM
The paradoxical thing here is that ultrasonic humidifiers don't evaporate water at all. They just throw tiny droplets of water into the air, so that the heat of the air can evaporate the water more quickly by exposing more water surface area to the air molecules. That's why the air coming out of the devices feels colder than the surrounding air - it _is_ colder, because the heat of evaporation has been absorbed by the water as part of the process of changing from liquid phase to gaseous phase (phase changes are where the big energy gains and losses are - easy to heat water to the boiling point, much harder to actually boil it.)
If you don't add heat to the equation, you just get colder and colder air in the room, eventually you hit the dew point and the "humidifier" just throws droplets of water onto the floor.
Thus, the reason ultrasonic humidifiers are so "efficient" is that they rely on separate external heat sources to actually do the task they're claiming to do - and those aren't accounted for in peoples' minds, therefore they don't realize the extra cost of heating that's required to evaporate the water that their ultrasonic humidifier is flinging into the air.