For centuries, static electricity has been the subject of intrigue and scientific investigation. Now, researchers from the Waitukaitis group at the Institute of Science and Technology Austria (ISTA) have uncovered a vital clue to this enduring mystery: the contact history of materials controls how they exchange charge. The groundbreaking findings, now published in Nature, explain the prevailing unpredictability of contact electrification, unveiling order from what has long been considered chaos.
From a tiny electric jolt when touching a doorknob to styrofoam peanuts that cling to a mischievous cat’s fur — the well-known and seemingly simple phenomenon of static electricity has puzzled people since antiquity. How could this ubiquitous effect, frequently demonstrated to bedazzled children by rubbing a balloon on their hair, still not be completely understood by scientists?
Static electricity goes by multiple names, but scientists prefer to call it ‘contact electrification’. As opposed to what the name ‘static electricity’ might imply, the essence of the effect is not static but includes movement, as some charge is transferred whenever two electrically neutral materials touch. “There is no escaping contact electrification; everyone experiences it. That’s why it might come off as a surprise to us that we don’t understand how exactly it happens,” says Scott Waitukaitis, Assistant Professor at the Institute of Science and Technology Austria (ISTA) who led this work together with ISTA PhD student Juan Carlos Sobarzo. Now, the team has uncovered a key piece of the puzzle that had remained unknown for centuries: “We tested different parameters that might affect contact electrification, but none of them could soundly explain our results. That’s where we stopped to think: what if it’s contact itself that’s affecting the charging behavior? The word ‘contact’ is already in the name, yet it has been widely overlooked,” says Sobarzo.
“A total mess”
Despite the phenomenon’s ubiquity, understanding how different materials undergo contact electrification has eluded physicists and chemists for centuries. While scientists were able to describe the mechanism for metals in the 1950s, electrical insulators have proven trickier to understand — although they are the materials that exchange charge the most. Historically, several studies have suggested that insulators could be ordered based on the sign of charge they exchange, from the most positive to the most negative. For instance, if glass charges positively to ceramic and ceramic does the same to wood, then glass (usually) charges positively to wood. Thus, glass, ceramic, and wood would form a so-called “triboelectric series.”
The problem with these triboelectric series, according to Waitukaitis, is that different researchers get different orderings, and sometimes even the same researcher does not get the same order twice when they redo their own experiment. “Understanding how insulating materials exchanged charge seemed like a total mess for a very long time: The experiments are wildly unpredictable and can sometimes seem completely random,” he says. In light of this “total mess,” physicists and material scientists could not seem to agree on any model to explain the mechanism. To make matters worse, they had to cope with the troubling fact that even identical materials, such as two balloons, exchanged charge. After all, the materials are supposed to be the same, so what determines where the charge goes?
Order arising from chaos
Waitukaitis and Sobarzo wagered that this “same-material” contact electrification could hold the keys to understanding the effect more broadly. By working with “identical” materials, they reduced the number of free variables to a minimum — they just had to find the one thing that made samples different. For their material, they chose polydimethylsiloxane (PDMS) — a clear silicone-based polymer out of which they made plastic-like blocks.
At this point, the leading hypothesis for why identical materials exchanged charge was random variations of surface properties. Frustratingly, the team’s initial results also reflected randomness and unpredictability. Not yet suspecting the samples’ contact history to play a role, they had been testing various conditions, sometimes using the same samples, completely unaware that these were evolving with each additional contact. Exploring where to take the research, they thought of testing whether identical PDMS samples would order into a triboelectric series. “I took a set of samples I had at hand — back then I would reuse them for multiple experiments — and to my disbelief, I saw that they ordered in a series on the first try,” says Sobarzo. Excited by this unexpected result, the team attempted to repeat the experiment with fresh samples but were quickly disappointed to see random results again. “At this point, we could’ve thrown in the towel,” says Sobarzo. “However, I decided to try again with this same set of samples the next day. The results looked better, so I kept trying until at the fifth try the samples ordered into a perfect series.” Sobarzo had just stumbled upon the answer to why the old samples worked on the first try. Repeated contact somehow allowed the samples to evolve. “As soon as we started keeping track of the samples’ contact history, the randomness and chaos actually made perfect sense,” says Waitukaitis. Indeed, the team found that the samples started behaving predictably after around 200 contacts and that the more ‘contacted’ sample consistently charged negatively to the one with the lower contact history. The researchers even showed that the PDMS samples could reliably form a ‘pre-designed’ triboelectric series if the number of contacts and the order of experiments were controlled.
A smoother surface
The idea that a sample’s contact history could control the way it charges had never been proposed. With this, the team explains why so many contact electrification experiments appear random and uncontrollable. Yet a question remains — how does the act of contact change the samples? So, the team pushed further, using various surface-sensitive techniques on samples before and after contact. Among all the parameters they investigated, only one provided any hint at all: they spotted discrete changes in the materials’ surface roughness at the nanoscopic scale. More concretely, they showed that contacts smoothed the tiniest bumps on a material’s surface. How this causes contact electrification the team does not know, but as it is the only change they could detect, it is highly suggestive. “We managed to reveal a big clue to an elusive mechanism that is so fundamental to our understanding of electricity and electrostatics and yet kept scientists puzzled for so long,” says Sobarzo. Waitukaitis concludes, barely able to hide his excitement, “We showed that the science of static electricity is not so hopeless anymore.”
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