Sunlight's hidden power supercharges water evaporation

Sunlight is nature’s most efficient tool for evaporating water. A recent study from North Carolina State University reveals that the key lies not in heat, but in the oscillating electric field that light carries.

This field interacts with water molecules in a unique way, helping them escape into the air faster than when heated by conventional sources.

Study lead author Saqlain Raza, a PhD student at NC State, noted that 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,” said Raza.

The researchers simulated water evaporation using molecular dynamics. These simulations allowed them to pinpoint how sunlight’s electric field speeds up the evaporation process by acting on clusters of water molecules at the surface.

Heat doesn’t boost water evaporation

The team compared evaporation from pure water and water contained in hydrogels under thermal conditions. When both systems received the same heat input, the evaporation rate remained the same.

This showed that the presence of hydrogel alone does not boost evaporation beyond thermal limits.

The researchers also investigated the idea that hydrogels reduce water’s latent heat through the formation of intermediate water states.

However, the interaction energy of intermediate water was only about twelve percent lower than free water. This small difference does not justify the much higher evaporation rates seen in previous experiments.

The simulations confirmed that intermediate water states do not make evaporation significantly easier. Most of the enhanced evaporation seen in experiments must result from another factor.

Sunlight has a secret weapon

Sunlight is not just thermal energy. It is an electromagnetic wave with an electric field that oscillates. This field can interact with polar water molecules.

The researchers mimicked this effect by applying an alternating electric field during their simulations. They found that when this field was active, water evaporated much faster.

“Light is an electromagnetic wave, which consists – in part – of an oscillating electric field,” said study co-author Jun Liu.

“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.”

In the simulation, water in hydrogels exposed to the electric field evaporated 2.3 times faster than with heat alone. Pure water evaporated 1.44 times faster under the same conditions. These results point to the electric field as the primary driver of enhanced evaporation.

How sunlight boosts water evaporation

Water at the surface does not always evaporate as single molecules. It can also leave the surface as clusters of molecules connected by hydrogen bonds. The study found that electric fields are especially effective at breaking these clusters off.

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

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

Hydrogels create conditions where more clusters form near the surface. Their polymer networks disturb the normal hydrogen bonding of water, making clusters more common and easier to cleave under the electric field.

Hydrogels help but only with light

The simulations showed that the presence of a hydrogel alone did not increase evaporation under heat. However, when an electric field was added, the rate increased significantly. Hydrogels seem to help by promoting the formation of surface clusters that the field can act upon.

In one analysis, the researchers found that nearly half of the clusters formed in hydrogel systems condensed back into the water without evaporating. But with the electric field turned on, many of these clusters broke into smaller groups or single molecules that could escape more easily.

The electric field not only helped free clusters but also increased the number of clusters at the surface. This likely happens because the field breaks large clusters into smaller ones that are more prone to evaporation.

Photomolecular effect in action

This work supports the photomolecular effect, a theory that explains how light can eject water molecules and clusters from the surface through evaporation.

The study showed that the electric field enables water evaporation by exciting water at the interface, causing it to separate and escape.

Simulations also showed that under real sunlight frequencies, evaporation increased, but only at very high electric field strengths. This limitation stems from the computational model, which could not capture the full spectrum of light’s interaction with water.

Still, the pattern is clear. Light acts not only through heat, but through field-driven interaction with water molecules.

Looking toward future applications

This study opens the door for new technologies in solar desalination and clean water production. Understanding the role of the electric field means engineers can design materials that enhance this effect.

The researchers suggest testing evaporation across different frequencies and materials, and using tools like infrared and Raman spectroscopy to track water clusters.

Experiments with antenna-driven electric fields could also help measure how water responds to oscillating energy at different scales. These insights will be essential for building efficient and sustainable water systems powered by sunlight.

“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.

By focusing on how sunlight affects water molecules at the interface, this study lays the foundation for more effective water evaporation methods and clean water solutions.

The study is published in the journal Materials Horizons.

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