Why the sun is so good at evaporating water

The sun evaporates water far more efficiently than a stovetop burner, and scientists at North Carolina State University now understand why.

Their computational simulations reveal that sunlight’s oscillating electric field excels at breaking off clusters of water molecules from liquid surfaces—a process that requires less energy than liberating individual molecules one by one.

The discovery helps explain why solar-powered water purification systems often exceed theoretical limits based purely on thermal energy. It also opens new possibilities for engineering more efficient desalination and water treatment technologies that harness electromagnetic effects beyond simple heating.

Electric Fields vs. Pure Heat

Light consists of oscillating electric and magnetic fields, but researchers found the electric component drives water evaporation enhancement. When they removed this oscillating field from their simulations, sunlight took longer to evaporate water. Stronger electric fields produced faster evaporation rates.

“Light is an electromagnetic wave, which consists – in part – of an oscillating electric field,” explains Jun Liu, co-corresponding author and associate professor of mechanical and aerospace engineering at NC State. “We found that if we removed the oscillating electric field from the equation, it takes longer for sunlight to evaporate water.”

The team used molecular dynamics simulations to isolate different aspects of light-water interactions. This computational approach allowed them to test variables impossible to control in real experiments, such as comparing identical conditions with and without electric field oscillations.

Water Clusters Hold the Key

Water molecules at surfaces don’t always evaporate individually. Sometimes they escape as connected clusters—groups of molecules linked by hydrogen bonds that break away together from the liquid bulk. The research revealed that oscillating electric fields particularly excel at cleaving these clusters.

Key findings about cluster dynamics include:

• Breaking off a water cluster requires similar energy to liberating a single molecule
• Clusters provide multiple molecules per liberation event, improving efficiency
• Large clusters often break into smaller fragments before evaporating completely
• Hydrogels promote cluster formation at water-air interfaces
• Electric field frequency matters—direct current showed no enhancement

“During evaporation, one of two things is happening,” notes Saqlain Raza, first author and Ph.D. student at NC State. “Evaporation either frees individual water molecules, which drift away from the bulk of liquid water, or it frees water clusters.”

The researchers demonstrated this using two models: pure water and water saturated within polyvinyl alcohol hydrogels. Pure water surfaces contained fewer clusters available for liberation, while hydrogel interfaces promoted cluster formation through interactions between water and polymer networks.

Hydrogels Amplify the Effect

Hydrogels—water-absorbing polymer networks used in many solar evaporation systems—proved especially responsive to oscillating electric fields. These materials disrupt normal hydrogen bonding patterns among water molecules, creating more clusters susceptible to electromagnetic liberation.

The simulations tracked individual water molecules and clusters over nanosecond timescales, revealing complex dynamics. Large clusters rarely evaporated directly but instead broke into smaller fragments that could escape. The process resembled molecular-scale demolition, with electric fields providing the energy to fracture cluster bonds.

Importantly, the research challenges previous theories about “intermediate water states” in hydrogels. Some scientists proposed that confined water molecules required less energy to evaporate, but the simulations showed similar interaction energies between different water types.

“We found that the oscillating electric field is particularly good at breaking off water clusters,” Liu emphasizes. “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.”

From Theory to Technology

These insights could guide development of next-generation water purification systems. Rather than relying solely on thermal heating, engineers might design devices that optimize electromagnetic field interactions with water-material interfaces.

The research connects to broader efforts understanding “photomolecular effects”—direct interactions between light and liquid-vapor interfaces that bypass traditional thermal pathways. While water appears transparent to visible light, these interfaces can strongly interact with electromagnetic radiation.

Future applications might include materials engineered to promote beneficial cluster formation, electromagnetic field configurations that enhance liberation efficiency, or hybrid systems combining thermal and electromagnetic approaches for maximum water treatment rates.