Solving the 250-year-old puzzle of how static electricity works

When you rub a balloon on your hair to make it float and cling, you might not think of it as one of the deepest – and strangest – mysteries of science. When you reach out to open a door and your finger sparks against the doorknob, that doesn’t seem like an enigma that defies explanation. Nor does it when socks stick to the inside of a shirt on the way out of the dryer.

Static electricity – or, as scientists prefer to call it, contact electrification – was our species’ first introduction to the electromagnetic force. The word “electricity” was coined in the 1600s and derives from the Greek elektron, or amber. As early as the 6th century BC, the ancients knew that amber rubbed against an animal pelt “animates” and begins to attract dust. Our species has had literal millennia to come to grips with contact electrification, so it is perhaps shocking that we still have little idea how it works.

“Everything is unknown,” says physicist Scott Waitukaitis at the Institute of Science and Technology Austria. Different researchers report different findings from the same experiments. Seemingly solid results fail to replicate. Scientists struggle even to figure out which materials become positive or negative on contact with other materials. “We can’t even get the experiments to just behave,” Waitukaitis adds.

That last part, at least, is starting to change. Waitukaitis and his colleagues have recently discovered a hidden rule that helps explain static electricity’s unruliness: the way an object exchanges charge when touched isn’t an unchanging, intrinsic property, but rather a product of its history. Samples “remember” previous contacts. That progress is being made in a field notoriously devoid of it means some physicists are labelling the development a “big deal”, one that should lead others to rethink their experiments into the phenomenon. Others are noting the wild reality that the systematic workings of this ostensibly simple curiosity – something that most people have encountered, yet few think much about – are only now starting to be illuminated.

Untangling static electricity

Contact electrification is what it says on the tin: electrification via contact. It happens when two objects touch and somehow swap electrical charge. Even when both start out neutral, with their charges balanced, one ends up positive and the other negative. “There’s no way to predict based on material properties which is positive and which is negative. The only way you’ll know is by trying,” says engineer Daniel Lacks at Case Western Reserve University in Ohio.

How the charge transfer occurs isn’t known either. In fact, scientists still don’t agree on what actually gets transferred – in metals, it is electrons, but other materials might exchange charged atoms or small molecules, or even entire chunks of material.

“Nobody can find a thorough explanation,” says Waitukaitis’s colleague Juan-Carlos Sobarzo over the indignant hissing of the humidifier in the chamber where he performs his experiments.

When Sobarzo joined Waitukaitis’s lab, he didn’t think he would discover an explanation for the disorder that plagues experiments on static electricity. Instead, he planned to test one particular hypothesis: we know that humidity can influence static charging, so Waitukaitis had a hunch that water molecules on the surfaces of materials facilitated charge-swapping. But before they could test anything, the duo needed a way to perform experiments that could be replicated. That might sound easy – tapping two objects together isn’t exactly rocket science. But in the study of contact electrification, replication is a tall order.

“There are too many things that have been proven to have an effect on how things exchange charge,” says Sobarzo. The humidifier’s hissing surges in protest as Sobarzo sticks his hands through the ports on the front of the box and picks up a sample, a fingernail-sized square of clear silicon polymer called polydimethylsiloxane (PDMS).

If this were a real experiment, Sobarzo wouldn’t touch the sample at all. His hands could transfer charge to the PDMS and ruin the experiment – indeed, almost anything could ruin the experiment. The setup is so sensitive that Sobarzo and Waitukaitis originally wanted to avoid putting their hands in the box altogether. The researchers started off manipulating samples through rubber gloves mounted on the outside, but the gloves were too staticky, so they removed them. Then, even Sobarzo’s arm hair started smuggling in static. Installing anti-static flaps on the ports mostly solved that problem. But that was just the beginning.

The air temperature and humidity inside the chamber had to be continuously monitored and adjusted. The air inside was filtered to remove dust, which can carry charge. Before experiments, the samples were fanned with ionised air to neutralise any excess charge on their surfaces. Instead of ordering ready-made PDMS, Sobarzo whipped it up himself to control any properties that could influence the results. During experiments, even touch was standardised: a motorised rig tapped the PDMS samples together inside a closed copper tube, while a pressure sensor ensured that each contact was equally forceful; an electrometer hooked up to the tube measured the samples’ charge.

Static charging is sensitive to phenomena across an enormous range of scales in space and time, from individual atoms on surfaces to sparks we can observe with the naked eye, says physicist Yaroslav Sobolev at the Center for Algorithmic and Robotized Synthesis in South Korea. Predicting anything at the human scale feels impossible, he says, since a single molecule out of place can influence a material’s large-scale behaviour. The situation recalls the butterfly effect of chaos theory: tiny tweaks to a system compound over time or space to produce dramatically divergent outcomes later. “A tiny effect can make a very big difference,” says Lacks.

Loops of charge

So, for Sobarzo and Waitukaitis, getting their “Frankenstein” setup to work took years of trial and error. “It was very frustrating for a long time,” says Waitukaitis. And once the system was finally working, the pair immediately discovered something that completely derailed their plans to study water molecules: a few of the PDMS samples inexplicably kept ending up negatively charged.

Scientists have long paid attention to the tendency of certain materials to charge negatively or positively after touching other materials. For instance, glass usually becomes positively charged on contact with paper, and humans usually charge negatively after touching carpet. Observations like this are often organised into lists called triboelectric series, triboelectricity being another word for static electricity. The first such list was published in 1757. “But not much progress has really been made since the 1700s because it’s a really, really difficult problem,” says Lacks. Triboelectric series produced by different labs often disagree.

Materials might begin with a tendency to end up positive or negative after a contact, only to switch after a while. Sometimes, they can even produce illogical loops: A would become negative on touching B, B would become negative on touching C, but A would become positive on touching C. Such series loop back on themselves like an impossible staircase in an M.C. Escher print.

Given their handful of stubbornly negative PDMS samples, “we wanted to know if identical materials, when they are touched together, would create a triboelectric series”, says Waitukaitis. “And, well, it led us down the rabbit hole.”

Sobarzo got to work in the lab. On the first try, “we got a series, a perfect series”, he says. The samples charged relative to each other in a neat chain, with no unruly loops. “We were super excited. Like: oh my God, we found this! Nobody has done this before!”

That excitement was short-lived. Waitukaitis was stunned – and cautious. He feared a fluke and asked Sobarzo to repeat the experiment. True to triboelectric form, the samples refused to behave. The series was scrambled. But Sobarzo was convinced that his first result hadn’t been mere chance. He went back to the lab, intent on re-creating his first series with a fresh batch of PDMS. At first, it didn’t work. Frustrated, he tried again with the same set of samples. It didn’t work. He tried again and again and again until, finally, after about a week of stubborn determination, he had his perfect series again. And this time, he knew why.

Somehow, Sobarzo realised, his simple silicon squares had “remembered” their contact history. Sobarzo observed that, at the beginning, the samples charged randomly. When he tried to arrange them into series, he got knotted loops instead of neat chains. But after a few days and several hundred contacts, the initially random behaviour grew more regular. A perfect triboelectric series emerged.

In follow-up experiments, Sobarzo and Waitukaitis confirmed that the mere act of contact changes how PDMS charges – in general, samples are more likely to become negatively charged after more contacts.

“It didn’t confirm what I had already thought. It surprised me a great deal that there would be this self-organisation,” says Troy Shinbrot at Rutgers University in New Jersey, who studies static.

But what is contact actually changing? How can blocks of polymer remember being touched?

“We probably spent a year on that. We tried every fancy, expensive, surface science tool we could get our hands on. And every one of them said the surfaces before and after contact were identical,” says Waitukaitis. But finally, after a thorough analysis using atomic force microscopy, the pair realised that contacted samples were smoother at small scales than fresh ones – like saws that had blunted their teeth.

Because researchers still don’t know how contact electrification happens, it is hard to say why a smoother surface would cause a sample to become more negatively charged. But Waitukaitis is now pretty sure it isn’t the presence of water molecules. Instead, he and Sobarzo suspect a phenomenon called flexoelectricity: charging by bending. Bending a surface squeezes charges together in some places and spreads them out in others. Pressing a material over a rougher surface bends it more as it accommodates the peaks and valleys of the textured interface. Since PDMS samples smooth out with contacts, perhaps their tendency to charge negatively is a hint that flexoelectricity could be involved in contact electrification.

Shinbrot and Lacks think the new data could also be consistent with other charging mechanisms, though. It is possible that the phenomenon we call contact electrification is actually several different processes working in tandem, none of them yet understood, and they could be different in other materials than in PDMS. Key questions in contact electrification – which charges are transferred and how – are still open. But now, researchers have a better shot at answering them.

The hidden complexity of electrification

Waitukaitis and Sobarzo’s results revealed a hidden order underlying the chaos of triboelectric series: contact history matters. So it isn’t surprising that so many experiments haven’t been replicated. It isn’t possible to repeat exactly the same measurement with exactly the same sample, since the mere act of contact changes how a sample charges the next time it is touched. Tracking contacts in future experiments could help resolve some of the confusion that has long plagued the field.

Contact electrification is so entrancing because it shouldn’t be – something this mundane shouldn’t be so mysterious. But familiarity often conceals complexity. The phenomenon is hard to understand for many of the same reasons that scientists struggle to explain and predict weather, the economy, consciousness and life.

Shinbrot points out that contact electrification is an example of self-organised order in defiance of the usual natural drive to entropy, the thermodynamic measure of disorder. The second law of thermodynamics holds that, overall, entropy can’t decrease. And it is usually taken for granted that touching, shaking, heating or otherwise jostling up a system will make it less orderly, jumbling any organisation of particles or, indeed, charges. The exact opposite happens in tribocharging. “You take two identical materials, you rub them together, one becomes more positive. You rub it more and it becomes even more positive. So, what happens to entropy?” asks Shinbrot.

This isn’t breaking the second law of thermodynamics – entropy can decrease locally if it increases somewhere else. On the other hand, we generally think of this kind of mysterious, self-organised order as distinctive to life and other complex systems like economies and climate. For scientists hoping to understand everything about our universe, the fact that even dead silicon squares can be so complex is perhaps a sobering thought.

Twenty years before he devised the most important equation of quantum mechanics, Erwin Schrödinger wrote his PhD thesis on contact electrification. He never returned to the subject. Quantum mechanics is famously hard – but perhaps not as hard as static charging.

“People who are serious about figuring stuff out start with contact electrification. And after some time, they just run away,” says Sobolev.

But Waitukaitis isn’t running. If anything, this project has only drawn him further into the mystery. “It’s so juicy. You can have large hadron colliders and you can have quantum computers, but you can’t understand why a balloon rubbed on hair makes it stick,” he says. “The deeper we get, the more mysterious it becomes, the harder it becomes. And that makes you want it more.”