Raindrops can damage paint with microscopic “lightning”

Scientists have traditionally attributed this type of deterioration to two major processes. One is mechanical stress, in which repeated contact with water droplets creates friction that slowly erodes a coating or causes pieces of it to peel away. The other is chemical damage, especially when the water contains substances such as acids or salts.

A Hidden Electrical Source of Damage

Now, researchers have identified an additional mechanism that had largely escaped attention: the electrical charge carried by water droplets.

The work was led by Hans-Jürgen Butt, director at the Max Planck Institute for Polymer Research, in collaboration with researchers from the University of Bonn, South China University of Technology, MIT, and Johannes Gutenberg University Mainz.

"A few years ago, we investigated the physics behind how water droplets become charged as they slide across surfaces. This is a kind of 'friction electricity' in droplets and is physically much more complex than previously assumed," says Rüdiger Berger, group leader in the "Physics at Interfaces" department. "When such charged droplets strike a coating, they discharge locally and can puncture the layer in specific spots like a small flash of lightning -- with consequences for the coating's durability."

In other words, a droplet can pick up electrical charge while moving across a surface. When it later strikes a protective coating, that charge can suddenly discharge at the point of impact, potentially damaging the material.

Testing Thousands of Water Droplets

To test how much harm these tiny electrical events could cause, the researchers began with a surface evenly covered in Teflon, a material widely known for its use on frying pans.

They repeatedly dropped uncharged water droplets onto the Teflon. Even after 3,000 impacts, microscopic examination showed no visible changes in the coating.

The team then changed the experiment by allowing the droplets to roll over common materials before they hit the Teflon. These included a houseplant leaf, PVC, and polystyrene, such as that found in plastic windows.

As the water moved across those materials, the droplets accumulated electrical charge. The charged droplets were then allowed to fall onto the Teflon surface.

This time, the result was very different. After 3,000 impacts, microscopic examination revealed clear changes both in the protective surface and in the metal underneath it.

Different Surfaces Create Different Charges

How much electrical charge a droplet picked up depended strongly on the material it had traveled across.

"The charge a droplet acquires as it slides depends heavily on the specific surface -- we measured differences of up to a factor of ten," explains Zhongyuan Ni, the study's first author. "Regardless of this, we were able to detect changes in the coating in all experiments."

Despite those large differences in charge, every experiment involving charged droplets produced detectable changes in the coating.

Toward Longer Lasting Protective Coatings

The findings reveal an overlooked way that water can contribute to the gradual deterioration of protective surfaces. Along with friction and chemical reactions, tiny electrical discharges from charged droplets may also play a role in how coatings break down over time.

The researchers hope the discovery, published in the journal Nature, will help scientists develop more effective and longer-lasting coatings. Such materials could improve protection for cultural heritage sites, cars, major structures, and even ordinary garden fences.