If a water droplet flowing over a surface gets stuck, and then unsticks itself, it generates an electric charge. The discoverers of this so-called depinning phenomenon are researchers at RMIT University and the University of Melbourne, both in Australia, and they say that boosting it could make energy-harvesting devices more efficient.
The newly-observed charging mechanism is conceptually similar to slide electrification, which occurs when a liquid leaves a surface – that is, when the surface goes from wet to dry. However, the idea that the opposite process can also generate a charge is new, says Peter Sherrell, who co-led the study. “We have found that going from dry to wet matters as well and may even be (in some cases) more important,” says Sherrell, an interdisciplinary research fellow at RMIT. “Our results show how something as simple as water moving on a surface still shows basic phenomena that have not been understood yet.”
Co-team leader Joe Berry, a fluid dynamics expert at Melbourne, notes that the charging mechanism only occurs when the water droplet gets temporarily stuck on the surface. “This suggests that we could design surfaces with specific structure and/or chemistry to control this charging,” he says. “We could reduce this charge for applications where it is a problem – for example in fuel handling – or, conversely, enhance it for applications where it is a benefit. These include increasing the speed of chemical reactions on catalyst surfaces to make next-generation batteries more efficient.”
More than 500 experiments
To observe depinning, the researchers built an experimental apparatus that enabled them to control the sticking and slipping motion of a water droplet on a Teflon surface while measuring the corresponding change in electrical charge. They also controlled the size of the droplet, making it big enough to wet the surface all at once, or smaller to de-wet it. This allowed them to distinguish between multiple mechanisms at play as they sequentially wetted and dried the same region of the surface.
Their study, which is published in Physical Review Letters, is based on more than 500 wetting and de-wetting experiments performed by PhD student Shuaijia Chen, Sherrell says. These experiments showed that the largest change in charge – from 0 to 4.1 nanocoulombs (nC) – occurred the first time the water contacted the surface. The amount of charge then oscillated between about 3.2 and 4.1 nC as the system alternated between wet and dry phases. “Importantly, this charge does not disappear,” Sherrell says. “It is likely generated at the interface and probably retained in the droplet as it moves over the surface.”
The motivation for the experiment came when Berry asked Sherrell a deceptively simple question: was it possible to harvest electricity from raindrops? To find out, they decided to supervise a semester-long research project for a master’s student in the chemical engineering degree programme at Melbourne. “The project grew from there, first with two more research project students [before] Chen then took over to build the final experimental platform and take the measurements,” Berry recalls.
The main challenge, he adds, was that they did not initially understand the phenomenon they were measuring. “Another obstacle was to design the exact protocol required to repeatedly produce the charging effect we observed,” he says.
Potential applications
Understanding how and why electric charge is generated as liquids flow during over surfaces is important, Berry says, especially with new, flammable types of renewable fuels such as hydrogen and ammonia seen as part of the transition to net zero. “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,” he explains. “This knowledge may help us to engineer coatings that could mitigate charge in new fuels.”
The RMIT/Melbourne researchers now plan to investigate the stick-slip phenomenon with other types of liquids and surfaces and are keen to partner with industries to target applications that can make a real-world impact. “At this stage, we have simply reported that this phenomenon occurs,” Sherrell says. “We now want to show that we can control when and where these charging events happen – either to maximize them or eliminate them. We are still a long way off from using our discovery for chemical and energy applications – but it’s a big step in the right direction.”
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