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."
The researchers demonstrated this by simulating how evaporation works in a model of pure water and how evaporation works in a model where water saturates a hydrogel.
"In pure water, you don't find many water clusters near the surface - where evaporation can take place," says Raza. "But there are lots of water clusters in the second model, because they form where the water comes into contact with the hydrogel. Because there are more water clusters near the surface in the second model, evaporation happens more quickly. Basically, there are more water clusters that the oscillating field can cleave off from the liquid water."
"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.
The paper, "Oscillations in Incident Electric Field Enhances Interfacial Water Evaporation," is published in the journal Materials Horizons. The paper was co-authored by Cong Yang, a Ph.D. graduate from NC State. Co-corresponding author of the paper is Xin Qian of the Huazhong University of Science and Technology.
This work was done with support from the American Chemical Society's Petroleum Research Fund, under award ND5-65609.
