Water Droplets Generate Unexpectedly Strong Electrical Charge
Have you ever watched water droplets dance across a car window, hesitating, stopping, and then suddenly moving? Physicists have made a surprising discovery about this seemingly commonplace phenomenon: the electrical charge these droplets build up is far stronger than previously understood.
This research, published in Physical Review Letters, sheds light on the behavior of water droplets encountering tiny imperfections on a surface. The team found that as a droplet encounters an obstacle, an electrical force accumulates until the droplet suddenly “jumps or slips” past the obstruction. This “stick-slip” process creates an irreversible build-up of electrical charge that hadn’t been recognized before.
Discovering this can lead to innovations that can enhance or inhibit the charge created in liquid-surface interactions in a range of real-world applications.
“Previously, scientists have understood this phenomenon as occurring when the liquid leaves a surface, which goes from wet to dry,” explains 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.”
This new understanding of charge generation could have important implications. For example, understanding this “stick-slip” system may lead to improved surface designs with controlled electrification. This could enhance safety of energy-holding systems, like hydrogen fuel cells.
Joe Berry, a fluid dynamics expert from the University of Melbourne, and senior author on the study, spoke about the broader implications of this research. “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.”
The team focused their experiments on how water droplets accumulate charge on polytetrafluoroethylene (PTFE), the material used in Teflon. The electrical charge generated was relatively small—between 3.2 and 4.1 nanocoulombs (nC). To put it into perspective, explains Shuaijia Chen, a PhD student at the University of Melbourne and the study’s first author, this is “more than 1 million times smaller than the static shock you might get from someone jumping next to you on a trampoline.”
While the charge may seem tiny, the implications are significant. The team’s next step is to study this droplet charge build-up using other liquids and surfaces. This could lead to innovations that can enhance or inhibit the charge created in liquid-surface interactions in a range of real-world applications.
