Why the Sun is good at evaporating water

The sun evaporates water more efficiently than stove burners, and scientists at North Carolina State University now understand why.

Their computational simulations show that the oscillating electric field of sunlight is effective when it breaks away from the water molecular cluster from the liquid surface, and this process is less energy than the release of individual molecules one by one.

This finding helps explain why purely thermal-based solar water purification systems often exceed theoretical limits. It also opens up new possibilities for engineering more efficient desalination and water treatment technologies that utilize electromagnetic effects rather than simple heating.

Electric field and pure heat

Light consists of an oscillating electric and magnetic field, but the researchers found that the electric component drives water evaporation enhancement. When they took this oscillation field from the simulation, it took longer for the sun to evaporate the water. Stronger electric fields produce faster evaporation rates.

“Light is electromagnetic waves, partly an oscillating electric field,” explained Jun Liu, co-author and associate professor of mechanical and aeronautics engineering at North Carolina State University. “We found that if we take the oscillating electric field out of the equation, it takes longer for the sun to evaporate.”

The team used molecular dynamics simulations to isolate different aspects of light water interactions. This calculation method allows them to test variables that are uncontrollable in real experiments, such as comparing the same conditions with or without electric field oscillation.

Water clump holds the key

Water molecules on the surface do not always evaporate alone. Sometimes they escape with the linked clusters, a group of molecules connected by hydrogen bonds that break down together from large blocks of liquid. The study shows that the oscillating electric field is particularly outstanding in cutting these clusters.

Key findings about cluster dynamics include:

• Decomposition of water clusters requires similar energy to liberate individual molecules
• Clusters provide multiple molecules for each liberation event, improving efficiency
• Large clusters usually break down into smaller pieces and then evaporate completely
• Hydrogel promotes cluster formation at the water interface
• Electric field frequency is important – DC current is not enhanced

“During the evaporation, one of two things is happening,” said first author and PhD student in North Carolina. “Evaporation releases individual water molecules, which flow from most of the liquid water, and can also release clusters of water.”

The researchers demonstrated this using two models: pure water and water are saturated in polyvinyl alcohol hydrogels. Pure water surfaces contain fewer clusters that can be liberated, while the hydrogel interface promotes cluster formation through interactions between water and polymer networks.

Hydrogel enlargement effect

Hydrogels – A network of water-absorbing polymers used in many solar evaporation systems – are particularly responsive to the reaction of oscillating electric fields. These materials disrupt the normal hydrogen bonding pattern between water molecules, resulting in more clusters that are susceptible to electromagnetic release.

These simulations track individual water molecules and clusters on the nanosecond time scale, revealing complex dynamics. Large clusters rarely evaporate directly, but break into smaller pieces that may escape. This process is similar to molecular scale removal, where the electric field provides energy for the broken cluster bonds.

Importantly, the study challenges previous theories about the “intermediate water state” in hydrogels. Some scientists have suggested that limiting water molecules requires less energy to evaporate, but simulations show similar interaction energies between different water types.

“We found that the oscillating electric field was especially good at breaking water clusters,” Liu stressed. “This is more efficient because it does not require more energy to break down water clusters (with many molecules) than breaking up a single molecule.”

From theory to technology

These insights can guide the development of next-generation water purification systems. Engineers can not only rely on heat heating, but also design equipment that optimizes the interaction between the electromagnetic field and the water substance interface.

The study is linked to a broader effort to understand the “photomolecular effects” – the contact interaction between light and liquid vapor interfaces bypassing traditional thermal pathways. Although water appears to be transparent to visible light, these interfaces can strongly interact with electromagnetic radiation.

Future applications may include materials designed to promote beneficial cluster formation, electromagnetic field configurations that can improve emancipation efficiency, or hybrid systems combining thermal and electromagnetic methods to achieve maximum water treatment rates.

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