Raindrops can cause corrosion through electricity, researchers from the Max Planck Institute for Polymer in Mainz have discovered. The study showed that sliding droplets on insulating surfaces generate enough charge to cause chemical changes and damage to materials, posing a new risk to protective coatings exposed to weather conditions.
Researchers used 35-microlitre water droplets containing a pinch of salt in their experiments. When these droplets slid down a surface inclined at 50 degrees, the friction generated an electric charge. The researchers found that the water droplets could have an electric charge of up to 9,000 volts, a surprisingly high level of voltage for droplets of that size.
To test the impact of this charge, the droplets were allowed to fall onto a copper plate coated with a thin layer of Teflon. Although the coating was intended to provide protection, the underlying copper showed signs of corrosion after 3,000 drops. The researchers identified that copper(II) oxide and basic copper(II) chloride had formed on the surface, confirmed by the application of Raman spectroscopy and X-ray diffraction.
The key factor in this process is the shape of the droplet. Due to electrostatic forces, droplets moving take on a shape similar to a Taylor cone. The researchers measured that a droplet carrying a charge of two nanocoulombs creates an electric field of 60 kilovolts per millimetre. This high local energy density is sufficient to penetrate or chemically alter protective layers, although thicker polymers, such as those 130 micrometres thick, showed greater resistance.
Comparisons showed that droplets falling directly onto a surface without sliding, i.e. without lateral movement, left the surface unharmed. However, dynamic sliding activates electrostatic forces that can lead to the degradation of the material. The researchers noted that polymers, such as polystyrene films, can undergo chemical restructuring under such conditions, creating rough patches that glow green under laser light, indicating the formation of double bonds in the carbon structure and a change in the polymer structure.
