Droplet discovery: Water creates 10 times more electrical charge than scientists thought

Researchers from RMIT University and the University of Melbourne have discovered that water generates an electrical charge up to 10 times greater than previously understood when it moves across a surface.

Speaking on the @AuManufacturing Conversations podcast recently, the researchers explained how they observed that when a water droplet becomes stuck on a tiny bump or rough spot, the force builds up until it “jumps or slips” past the obstacle, creating an irreversible electrical charge that had not been reported before.

The research, led by Dr Joe Berry from the University of Melbourne and Dr Peter Sherrell from RMIT, provides new understanding of this “stick-slip” motion of water over surfaces, potentially paving the way for surface design with controlled electrification.

“Most people would observe that rainwater drips down a window or a car windscreen in a haphazard way, but would be unaware that it generates a tiny bit of electrical charge,” Sherrell told the podcast. His research at RMIT specialises in capturing and using ambient energy from the environment.

“Previously, scientists have understood this phenomenon as occurring when the liquid leaves a surface, which goes from wet to dry. 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,” Sherrell explained.

Berry, a fluid dynamics expert from the Department of Chemical Engineering at the University of Melbourne, highlighted the safety implications of their discovery during the interview. “An electric shock inside a fuel container with flammable liquids could be dangerous, so charge build-up on a solid surface needs to be safely discharged after a liquid has moved on.”

“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,” Berry added.

On the podcast, the researchers described their “deceptively simple” experimental setup. The team investigated this charging effect using water and polytetrafluoroethylene (PTFE), the material used in Teflon. They measured the electrical charge and contact areas created by water droplets spreading and contracting on a flat plate of PTFE – effectively simulating the movement of droplets over the surface.

The researchers described how PhD student Shuaijia Chen, the first author of the research, conducted over 500 experiments to test the reproducibility of their findings. Chen noted that the first time water touched the surface created the biggest change in charge, from 0 to 4.1 nanocoulombs (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,” Sherrell explained in the interview.

Although the amount of charge may seem insignificant, both researchers were enthusiastic about how this discovery could lead to innovations that enhance or inhibit charge created in liquid-surface interactions across various applications.

During the podcast, both researchers emphasised that this discovery came from curiosity and persistence, with Sherrell noting they pursued the research without specific funding initially, collaborating with masters students to build their experiment on a “shoestring budget.”

The researchers plan to study the stick-slip phenomenon with other types of liquids and surfaces. They aim to investigate where stick-slip motion can affect safety design of fluid handling systems, such as those used to store and transport ammonia and hydrogen, as well as methods to recover electricity and speed up charging from liquid motion in energy storage devices.

The research was published in the journal Physical Review Letters.

Picture: The team behind the water charging experiment: Dr Joe Berry, Dr Peter Sherrell and PhD scholar Shuaijia Chen (left to right) in a lab at RMIT University. (Credit Peter Clarke, RMIT University)