Raindrops are tiny lightning bolts, and they’re corroding cars, study finds

The common idea is that rain wears away protective coatings on metal. Water carries salts and acids, the raindrop’s impact scrapes the surface, and oxygen completes the reaction. Paints, polymer films, and oxide layers are all designed to stop this process. But a new study shows that rain can do more than just scrape or dissolve.

The researchers began with a phenomenon called “slide electrification.” When a water drop slides over an insulating surface—such as a leaf, a painted wall, a windowpane, or a plastic panel—it takes charge from that surface and leaves an opposite charge behind. The voltages involved are large. Drops that have slid across a surface can carry up to 9,000 volts.

The question was: what happens when a charged drop lands on a new surface? To test this, the team released 35‑microliter drops—about the size of a real raindrop—containing a small amount of salt to mimic rainwater. The drops slid on a surface tilted at 50 degrees, picked up charge, rolled off the edge, and then fell five millimeters onto a copper plate that was coated with a 60‑nanometer film of Teflon, one of the most chemically resistant coatings available.

The researchers used four different surfaces to mimic real‑world conditions. One was a leaf from a Tradescantia spathacea plant that grew in an office. Another was a PVC foam board from a hardware store. The third was a sheet of transparent polystyrene sold as window glazing. The fourth was a fluorinated quartz surface that the scientists made in the lab. The drops that slid across the leaf carried about 0.2 nanocoulombs of charge; those that slid across the fluorinated quartz carried up to two nanocoulombs. A nanocoulomb in a raindrop is equivalent to several thousand volts.