Raindrops acquire tiny electrical charges up to 9,000 volts when sliding across surfaces, breaking through protective car coatings and accelerating corrosion, according to a recent study published in Nature by researchers from the Max Planck Institute for Polymer Research.
We usually think of a rainy afternoon as a natural car wash, but researchers have identified a hidden corrosive mechanism in everyday precipitation. While scientists have long blamed dissolved salts, acids, and atmospheric pollution for eating away at metal car bodies, a recent study published in Nature reveals that water drops act as microscopic lightning bolts. As raindrops slide across surfaces like plant leaves, window glass, and painted walls, they absorb static electricity through a process similar to tribocharging, which happens when we rub certain objects against each other like a balloon on our hair, accumulating voltages as high as 9,000 volts. Charged droplets are formed not only in natural phenomena such as clouds, thunderstorms, ocean waves, fountains, and waterfalls, but also naturally in industrial processes such as electrostatic spraying and inkjet printing.
Max Planck Institute Experiments Reveal How Charged Droplets Drill into Teflon
To understand the mechanics behind this electrical erosion, a research team led by John Yuen Nyi, Rüdiger Berger, and Hans-Jürgen Butt at the Max Planck Institute for Polymer Research, a world-leading basic research institution in polymer science and soft materials located in Mainz, Germany, simulated rainfall in a laboratory setting. The investigators prepared water that mimicked rainwater by adding a small amount of salt, creating 35-microliter droplets roughly the size of a large raindrop. They dropped these samples onto four distinct inclined surfaces positioned at a 50-degree angle: a Tradescantia spatacea plant leaf, a polyvinyl chloride (PVC) foam board, a transparent polystyrene sheet commercially available as window glass, and fluorine-coated quartz (also referred to as perfluoro octadecyltrichlorosilane, or PFOTS).
After sliding about 4 centimeters across these inclined surfaces, the droplets acquired electrical charges ranging from 0.2 nanocoulombs on the Tradescantia spatacea leaf up to 2 nanocoulombs on the fluorine-coated quartz. The research team explains that 1 nanocoulomb in an object the size of a raindrop is a powerful amount of electricity equivalent to several thousand volts. The team then allowed these charged droplets to fall from the object’s surface onto a copper plate coated with a 60-nanometer Teflon film positioned 5 millimeters below—a material reported as the most chemically resistant coating available commercially. When the experiment replicated a moderate afternoon rain with 3,000 drops, the results were definitive. Every single one of the four Teflon-coated copper plates was corroded. Examinations of the areas where the water droplets landed using an atomic force microscope revealed indentations several nanometers deep in some places, exceeding the thickness of the Teflon film and proving that the electrical discharge completely penetrated the coating to reach the underlying metal copper plate.
Conversely, water droplets that landed directly on the copper plate without sliding on the object surface beforehand, representing droplets without an electric charge, showed no damage whatsoever to the copper plate surface, even after being dropped 3,000 times. Furthermore, the research team used a high-speed camera to film the falling of water droplets. They confirmed that when a neutral droplet approached the copper plate, its base remained smooth and round until contact was made, whereas charged droplets exhibited completely different behavior. Their bases elongated into a sharp, angular shape called a Taylor cone as they approached the copper plate, which is the shape a liquid takes when the electrostatic force exceeds the surface tension, indicating that the electric field between the droplet and the metal has become strong enough to deform the water. The research team modeled the droplet as a conductive sphere floating on a conductive wall and calculated how the electric field strength changes with distance.
Rethinking Protective Coatings for Cars, Bridges, and Ships
These findings challenge assumptions about how water degrades industrial and consumer materials. The commonly accepted explanation for why rain causes corrosion is that water carries dissolved salts and acids to the car’s surface, the constant impact of raindrops wears down the protective coating, and oxygen then acts on it. Most corrosion-preventing measures, such as paints, polymer films, and oxide films, are based on this idea. However, the data indicates that an electrostatic spark can break through tough, heat-resistant, and water-resistant coatings, creating microscopic entry points for rust and decay.
The discovery could potentially change current tools used to preserve metal objects exposed to the elements, such as cars, bridges, and ships. The results of the study could help inform the design of protective coatings that are more resistant to electrically charged water drops. While the data suggests that a thicker protective layer could do the trick, the sources note that more research is needed on designing a coating specifically geared toward protecting metal against tiny sparks.
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