Evrim Yazgin
Cosmos science journalist
We’ve all watched water droplets dance across a car window, stopping and starting, sometimes with a rush. The droplets are building up an electrical charge, and physicists say it is 10 times greater than previously thought.
The research, published in Physical Review Letters, is based on observations of what happens when water droplets get stuck on a tiny bump or rough spot on a surface. They found that a force built up until the droplet “jumped or slipped” past the obstacle. This process leads to an irreversible charge build up which had never been noticed before.
Understanding this “stick-slip” system could be used to enhance surface design with controlled electrification and improve safety in energy-holding systems, such as hydrogen fuel.
“Previously, scientists have understood this phenomenon as occurring when the liquid leaves a surface, which goes from wet to dry,” says co-author Peter Sherrell from RMIT University in Melbourne, Australia.
“In this work we have 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.
“Importantly, this charge does not disappear. Our research did not pinpoint exactly where this charge resides but clearly shows that it is generated at the interface and is probably retained in 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,” says senior author Joe Berry, a fluid dynamics expert from the University of Melbourne.
“At present, with existing fuels, charge build-up is reduced by restricting flow, using additives or other measures, which may not be effective in newer fuels.”
The team investigated how water droplets build up charge on a material used in Teflon, polytetrafluoroethylene (PTFE).
The charge was tiny – for energy lovers among our readers – between 3.2 and 4.1nC.
“To put things into perspective, the amount of electrical charge that water made by moving over the PTFE surface was more than 1 million times smaller than the static shock you might get from someone jumping next to you on a trampoline,” says first author Shuaijia Chen, a PhD student at the University of Melbourne.
“That amount of charge may sound insignificant, but this discovery could lead to innovations that can enhance or inhibit the charge created in liquid-surface interactions in a range of real-world applications.”
The next step is for this droplet charge build-up to be tested on other liquids and surfaces.
