Every 7 years or so, all 330 m of the Eiffel Tower must be repainted, by hand, to protect the iron symbol of French ingenuity from corrosion—a grueling process that can take over a year.
Why this coating needs to be replaced so often has long been a mystery. The obvious answer, of course, is degradation caused by rain. But how exactly raindrops keep breaking down the tower’s protective paint is less clear. Some scientists have theorized that rain is somehow physically abrasive or acidic because of air pollutants.
A new study says there’s something else driving raindrops’ caustic power: electricity.
The work finds that electrically charged raindrops can deteriorate surfaces treated with Teflon (Nature 2026, DOI: 10.1038/s41586-026-10941-6).
For years, researchers at the Max Planck Institute for Polymer Research have studied how water droplets from rain, dew, ocean waves, and melting snow become electrically charged. They say the process is like tribocharging, which causes static when we rub certain objects together, such as a balloon on our hair or a sweater on our skin.
“Water drops spontaneously become electrically charged when moving on different surfaces, such as plant leaves, insect wings, building walls, window glass, and plastic. This process, known as contact or sliding electrification, is analogous to tribocharging between solids,” says study coauthor Zhongyuan Ni, a PhD candidate at Max Planck.
“Water drops getting charged is not a new phenomenon, but for the past 200 years, people have not paid attention to this,” Ni says.
Rain check: Studying electricity in droplets
In 2023, Ni and colleagues set out to determine whether electrically charged water droplets affect the surfaces they contact. To find out, the team poured water over four common surfaces—a plant leaf; polyvinyl chloride foam board; polystyrene glass; and perfluoro octadecyltrichlorosilane (PFOTS), a commonly used water-repelling coating—and let it drip onto sheets of Teflon-coated copper tilted at 50 degrees.
When the experiments began, the team would allow only a few dozen drops to fall onto the copper before using electron microscopy to examine it. The researchers repeatedly failed to find any evidence of decay. But one day, Ni just let the drops keep falling.
Electrified water droplets have created a hole in a Teflon coating, exposing the underlying copper (in dark brown), as seen in an electron microscope image. The biggest hole is several nanometers deep. Credit:
Courtesy of Zhongyuan Ni
“After thousands of drops, it looked like the drops were pinning onto the surface,” he says. After inspecting the copper under the microscope, Ni got his first glimpse of the destructive strength of electrically charged water drops.
After repeating the experiment several times with thousands rather than dozens of drops, the researchers found that the electrified water drops not only broke down the Teflon coating but also corroded the underlying copper. Up close, the copper sheets looked like scraped-up skin. But when the experiment was conducted with nonelectrified drops, neither the copper nor its Teflon coating was any worse for wear.
Even though the drops contained only around 0.2 to 2.0 nanocoulombs of charge—around one billionth of a standard unit—the impact of the charged drops was significant.
“We found that even with small amounts of charge, they can still cause damage,” Ni says. “It was quite surprising.”
Combating corrosion
That the electrified water drops can break down Teflon was also unexpected. A hardy, widely used metal coating, Teflon drastically reduces friction, prevents rust, and withstands high temperatures. If electrically charged raindrops can degrade it, they can do the same to other coatings, Ni says.
“The finding is important because it suggests that the electrical state of a water droplet, not just its chemical composition, acidity, or mechanical impact, can influence how corrosion begins,” says Guangwen Zhou, a professor of mechanical engineering at Binghamton University who was not involved in the study.
“This perspective could open up new approaches to designing protective coatings and materials that are more resistant to corrosion in environments where charged water droplets are present,” Zhou says by email.
Although Zhou thinks more research is needed on the impacts of electrically charged water drops under real-world conditions, he has some advice for those looking to safeguard cars, trains, buildings, and other outdoor infrastructure.
For instance, Zhou says, “increasing the dielectric strength and durability of coatings could potentially reduce this type of damage.” Another possibility is for material scientists to “design surfaces that reduce the charging of water droplets by controlling the surface chemistry, wettability, or electrical properties of the material over which the droplets move,” he says.
While Ni and his colleagues stress that electrically charged water drops are not the only corrosive forces (sunlight, wind, and other natural phenomena also play a role), their power should not be ignored.
Ni hopes his study’s findings will inspire more durable coatings and materials and, in doing so, extend the life of humankind’s most impressive creations, like the Eiffel Tower.
