Sunlight’s hidden electric field linked to faster water evaporation

Water has always responded well to sunlight, evaporating faster under the sun than when heated by other energy sources. For years, scientists knew this happened but didn’t fully understand why. Now, a new study combining advanced simulations and molecular physics offers a clear explanation: sunlight’s hidden electric field plays a powerful role.

Researchers from North Carolina State University and Huazhong University of Science and Technology have found that it’s not just the heat in sunlight that drives water to evaporate quickly. It’s also the light’s natural oscillating electric field. Their findings help explain why solar energy is so efficient for evaporation and could improve the design of future water purification and energy systems.

Sunlight isn’t just warm—it’s a form of electromagnetic radiation. That means it comes with an electric field that constantly changes direction. This back-and-forth movement, or oscillation, is what gives sunlight a special edge over other heat sources.

Saqlain Raza, the study’s lead author and a Ph.D. student at NC State, said, “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. However, it has not been clear exactly why. Our work highlights the role that electric fields play in this process.”

To dig deeper, the team used non-equilibrium molecular dynamics simulations. These computer models allowed them to tweak and study the effects of various light-related factors in a virtual environment. By doing this, they could isolate what really speeds up evaporation.

Jun Liu, co-corresponding author and associate professor of mechanical and aerospace engineering at NC State, explained, “Light is an electromagnetic wave, which consists – in part – of an oscillating electric field. 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.”

So, what exactly does this electric field do? It doesn't just heat up the water. It directly interacts with the molecules at the surface of the liquid. Evaporation happens when water molecules break free from the liquid and escape into the air. This can happen one molecule at a time, or in small clusters of molecules stuck together. Both methods occur, but the electric field seems especially good at freeing clusters.

“Evaporation either frees individual water molecules, which drift away from the bulk of liquid water, or it frees water clusters,” Raza said. “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.” Liu added, “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.”

These clusters are easier for the electric field to grab and pull apart. And when there are more of them near the surface, evaporation speeds up. That’s why understanding where and how these clusters form is so important.

To test this idea further, the researchers compared evaporation in two different situations: one with plain water and one with water held inside a hydrogel. Hydrogels are soft, sponge-like materials made from long chains of polymers. In this case, they used polyvinyl alcohol hydrogels.

Water inside the hydrogel behaves differently than in open space. Because of how it interacts with the material, more water clusters tend to gather near the surface. This makes it easier for the electric field to cleave them off.

“In pure water, you don’t find many water clusters near the surface – where evaporation can take place,” Raza said. “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.”

This finding challenges previous ideas that focused only on changes in the water’s structure or energy state within hydrogels. Some earlier studies suggested that water inside hydrogels existed in a special “intermediate state” with a lower latent heat of evaporation. Others believed that light might knock entire water clusters loose through direct contact.

However, the new research shows that evaporation rates stay consistent as long as the same amount of interfacial heat is applied, regardless of the presence of a hydrogel. This means that changes in water state alone do not explain the speed. Instead, the interaction between the electric field and the water clusters is the key factor.

This study could have real-world impacts. Clean water is in high demand around the globe, and solar-powered water purification is a growing field. Knowing that electric fields can help speed up evaporation allows engineers to design better, more energy-efficient systems. Jun Liu highlighted this point: “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.”

By simulating the exact molecular actions involved, the team has helped solve a puzzle that stood for decades. Their paper, “Oscillations in Incident Electric Field Enhances Interfacial Water Evaporation,” was published in Materials Horizons. Co-authors also included Cong Yang, a Ph.D. graduate from NC State, and Xin Qian from Huazhong University of Science and Technology.

“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,” said Liu.

The next step may involve testing these theories in the lab. For now, these virtual results already give scientists a sharper view of how nature turns sunlight into one of the most effective water evaporators we know.

Note: The article above provided above by The Brighter Side of News.

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