Researchers have found that tiny electrical charges picked up by moving water droplets may be damaging cars and buildings like the Eiffel Tower. In experiments, thousands of charged droplets degraded Teflon and corroded the copper beneath it, while uncharged droplets caused no comparable damage. C&EN reports: 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.
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. “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. “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.”