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Static electricity is so commonplace that it can come across as simple. Rub a balloon against your head, and the transfer of charges will make your hair stand on end. Shuffle your feet on a carpet, and the charge imbalance you produce can shock an innocent passer-by.
So it might come as a surprise that static electricity — which arises from what researchers in the field call the triboelectric effect — has left scientists racking their brains for centuries. Some of the basics are clear. Materials transfer charges when they’re rubbed or otherwise come into contact with each other: one becomes more positively charged and the other more negatively charged. Opposite charges attract, whereas identical charges repel, and ta-da, you have a primary-school science experiment.
But most everything else in this field remains baffling. Is it the electrons, ions, or bits of material that transfer the charge? Why do some materials charge positively and others negatively? What happens when two samples of the same material come into contact? For instance, when “rubbing a balloon on a balloon”, says experimental physicist Scott Waitukaitis at the Institute of Science and Technology Austria in Klosterneuburg. A big part of the problem is that experiments tend to misbehave, with the same procedures producing different results.
Now, researchers are picking apart some of the puzzles that have long plagued the field. With sophisticated laboratory set-ups that carefully control for compounding factors, Waitukaitis and his team have found that the charging of some materials has a strange tendency to hinge on their past interactions. This week in Nature, Waitukaitis and his colleagues report that carbon-carrying surface molecules can have a role in guiding which way charge is exchanged.
These discoveries “are the best work in a really long time” in the field, says Daniel Lacks, a chemical engineer who has studied triboelectricity at Case Western Reserve University in Cleveland, Ohio. Other teams are investigating how surface area and velocity during impact might govern charge transfer, and how the breaking of chemical bonds contributes.
The influx of research seems to be driven by a desire to scrutinize the fundamental physics at play, says Laurence Marks, a materials scientist at Northwestern University in Evanston, Illinois. A better understanding of the science of static electricity could lead to improved devices that use it to power remote sensors or wearable technologies without batteries, for example. It could also help to prevent the electrical discharges that can cause industrial explosions.
It’s becoming increasingly clear that static electricity is far from a simple phenomenon that abides by one clear-cut set of rules, researchers say. Instead, each exchange of charges could be shaped by several factors that vary with the circumstances. Some of these factors are now known, and others are still waiting to be uncovered.
Ancient observations
The history of static electricity dates back to at least the ancient Greek period. Triboelectric includes the Greek words for ‘rubbing’ and ‘amber’, because, after amber is rubbed against fur, it attracts light objects such as feathers. At the end of the sixteenth century, English physicist William Gilbert identified other materials that had the same attractive power, including glass, diamonds and sapphires, and distinguished this type of electrical pull from that of magnetism. In the centuries that followed, scientists learnt that lightning was an electrostatic discharge, a supersized version of the benign zap that comes from shuffling feet across a carpet, and invented early electrostatic generators — forerunners of the Van de Graaff generators that wow students in science museums.
By the mid-eighteenth century, researchers had also begun documenting which materials became negatively charged and which positively, producing lists called triboelectric series. These rank materials from the most likely to charge positively to the most likely to charge negatively, with rabbit fur listed close to the top and silicon near the bottom, for instance.
There was a lull in efforts to understand the phenomenon for part of the twentieth century before interest resurged around the turn of the twenty-first century. Marks attributes this renewed interest at least in part to the invention of the triboelectric nanogenerator. This device relies on the triboelectric effect to convert mechanical energy into electricity. It attracted researchers who were interested in fresh ways to power small technologies. “In the last ten years, the field has literally exploded,” says Giulio Fatti, a mechanical engineer at Imperial College London.
Even with the attention boost, however, the fundamentals of triboelectricity have remained elusive. There are some generally accepted ideas, says Marks. A material has a specific potential for a charged particle to escape that depends on the material’s surface and composition. This potential is called the material’s work function and, so far, it applies best to metallic materials, Waitukaitis says. A sample also needs to be able to trap the charged particles, so they are kept in place when the materials separate after the exchange. But physicists are still pinning down the exact mechanisms behind these phenomena.
Other details of the contact seem to matter, too. But what matters most, under which circumstances, and for what material,s remains unclear. Whether triboelectricity can be explained by existing physics or whether it demands its own model has been an open question, says Marks.
Looking to the past
Waitukaitis and his team were investigating how samples of the same material can exchange a charge when they encountered the inconsistent results that have long frustrated researchers in the field. Triboelectric series are difficult to reproduce. Teams have obtained variable results concerning which materials become more positively or negatively charged, and, even, different findings with the same samples.
Waitukaitis tasked his then-PhD student, Juan Carlos Sobarzo, with attempting to form a series using samples of the same silicone-based polymer. But Sobarzo couldn’t obtain any consistent results. In one experiment, sample A would become negatively charged when interacting with sample B. In the next, it would become positively charged.
“For a very long time, we thought we were doing something wrong,” Waitukaitis says. “We thought there was some variable we weren’t controlling.”
Even when the team carefully controlled for humidity — because researchers thought that water on a material’s surface could affect how it charges — the results remained befuddling.
Then, Sobarzo dug up a set of samples that had already been through many experiments, and tested how they interacted with fresh ones. Quickly, the researchers noticed that the samples that had been through more contact tended to become negatively charged. In further experiments, they kept track of how many contacts each sample had already undergone.
“That’s when things started to make sense. The samples that had more touches in their history were always charging negatively,” Waitukaitis says. “What looked like chaos was an indication of the samples evolving.”
The researchers suspect this evolution has to do with how the sample’s surface deforms with each contact.
In the current paper, Waitukaitis, working with Galien Grosjean, an applied physicist at the Autonomous University of Barcelona, Spain, and their colleagues, looked deeper into how charge is exchanged between two seemingly identical materials. This time, they worked with oxides — materials, such as sand, that are made up of atoms bonded to oxygen — and used several technologies, including a device that levitates samples to keep their charge from changing. They also used a high-speed camera to measure the samples’ charge precisely.
Before the experiment, the scientists thought that water on the materials’ surface might affect the charge exchange. But samples stored in either a humid or dry environment did not seem to be affected noticeably. Then, the researchers baked the materials and found that the baked samples tended to become charged negatively after contact, and the unbaked ones positively.
After exploring the materials’ interfaces, the researchers realized that the baking process changed the results by getting rid of the carbon-carrying molecules on the materials’ surface. These types of molecule, such as the carbon-rich greenhouse gas methane, are commonly picked up from the air. They “slowly but surely get on every surface,” Grosjean says. The findings suggest that the material is more likely to become positively charged after contact if it has a greater number of carbonaceous molecules on its surface.
Waitukaitis says the team did a double-take after discovering that it was the carbon-carrying molecules at play. “You hardly ever hear people talk about those molecules in the static-electricity field,” he says.
These results provide first steps towards understanding which factors influence charge transfer the most. So far, the contact-history findings seem to pertain only to polymer materials such as plastics, whereas the latest results apply just to oxides.
Still, the work indicates that there is no one-size-fits-all answer to how materials charge. “The idea of a permanent triboelectric ordering among different materials is a mirage,” says Waitukaitis.
That such small factors could be so impactful isn’t necessarily a new idea, says Lacks. “But what is totally new are these really systematic experiments to prove that a particular contaminant is playing a governing, controlling role,” he adds. The field has “moved away from the hand-waving to a more scientific proof.”
Zapping forward
Other groups are doing their own disentangling. Researchers in South Korea, for example, reported that they could control the charge transfer by manipulating a material’s internal electric field. “This was meaningful because triboelectricity had long been considered largely uncontrollable,” says study co-author Sang-Woo Kim, who studies triboelectric energy harvesting at Yonsei University in Seoul. The findings, Marks says, fit with existing electromagnetic principles, suggesting that triboelectrification doesn’t need a fresh set of rules. And a team in Germany has found that as the impact velocity between two colliding metals increases, so does the impact surface area, which can affect charge transfer. The link between impact velocity and charge transfer had been up for debate.
Fatti and his collaborators have studied triboelectricity and the breaking of chemical bonds, finding that a metal can break the chemical bonds on a polymer’s surface when the two materials interact. This instability creates the right chemical conditions for electrons to be exchanged to re-stabilize the bond. The findings, reported last January, could help researchers to create better-performing triboelectric nanogenerators, they say.
Further research might also help to prevent the electrical discharges that cause damage or ignite explosions — at industrial factories, for instance. Other applications include controlling the charge held in materials through 3D printing to create a temporary electric equivalent of a permanent magnet and assessing the damage that the Moon’s prolific dust could do to future lunar base camps.
Marks says that since he started working in the field in 2018, he’s found that more physicists and chemists are applying “hard-core analysis” to static electricity, performing painstakingly careful measurements.
Waitukaitis agrees that more labs are “getting careful” with experiments. “Then those labs share the techniques that helped them with other labs,” he says. It’s still a small, tight-knit group of scientists with one dedicated conference a year — although he’s been trying to spread his enthusiasm for triboelectricity at larger physics meetings.
Now that groups are beginning to identify the parameters that matter most for some charge transfers, Waitukaitis hopes that physicists’ understanding of the phenomenon will be rounded out. “I’m not sure we’re making things simpler,” he adds. “But we’re doing what is necessary to make sense of this.”
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