Secret Power

Most people experienced watching a water droplet sliding on a car window, stopping, and then starting again, sometimes moving more quickly than before.

That’s due to the droplets building up an electrical charge 10 times greater than previously thought, according to a new study.

It showed that when a droplet gets stuck on a small bump or a rough surface, pressure builds up until the droplet can “jump” or “slip” past the obstacle, creating an irreversible charge.

Before, scientists believed this only occurred when the liquid moved from a wet surface to a dry one, Science Daily explained.

“We have (now) shown that charge can be created when the liquid first contacts the surface, when it goes from dry to wet, and is 10 times stronger than wet-to-dry charging,” said study author Peter Sherrell in a statement.

To arrive at this conclusion, researchers studied the charging effect with water and the material used in Teflon, polytetrafluoroethylene (PTFE), a type of plastic used in pipes and other fluid management materials. However, Teflon does not conduct electricity, so the charge generated cannot be easily or safely removed.

The team then measured the electrical charge and contact areas created by water droplets spreading and contracting on a flat surface of Teflon to simulate the movement on any kind of surface. Afterward, they used a special camera to capture frames of droplets sticking and slipping while simultaneously measuring the change in charge.

The first time water touched the surface, it created the biggest change in charge, from 0 to 4.1 nanocoulombs (nC), the researchers said. Then, as the water surface alternated between wet and dry, the charge oscillated between about 3.2 and 4.1 nC.

“To put things into perspective, the amount of electrical charge that water made by moving over the PTFE surface was more than a million times smaller than the static shock you might get from someone jumping next to you on a trampoline,” said the study’s first author, Shuaijia Chen.

This research didn’t show where the charge resides but demonstrated that it didn’t disappear. Instead, it is generated at the interface between the droplet and the surface and is likely retained by the droplet as it moves over the surface.

“Understanding how and why electric charge is generated during the flow of liquids over surfaces is important as we start to adopt the new renewable flammable fuels required for a transition to net zero,” said study author Joe Berry.

An electric shock inside a fuel container with flammable liquids could be dangerous, so it’s important to safely discharge any charge buildup on a solid surface once the liquid has moved away.

Currently, charge buildup in existing fuels is minimized by restricting flow, using additives, or other methods, but these may not be effective with newer fuels. This understanding could help develop coatings to reduce charge buildup in future fuels.