New experiments on static electricity cast doubt on previous studies in the field
Static electricity is an everyday phenomenon, but it remains poorly understood. Researchers at the Institute of Science and Technology Austria (ISTA) have now shed new light on it by capturing an “image” of charge distributions as charge transfers from one surface to another. Their conclusions challenge longstanding interpretations of previous experiments and enhance our understanding of how charge behaves on insulating surfaces.
Static electricity is also known as contact electrification because it occurs when charge is transferred from one object to another by touch. The most common laboratory example involves rubbing a balloon on someone’s head to make their hair stand on end. However, static electricity is also associated with many other activities, including coffee grinding, pollen transport and perhaps even the formation of rocky planets.
One of the most useful ways of studying contact electrification is to move a metal tip slowly over the surface of a sample without touching it, recording a voltage all the while. These so-called scanning Kelvin methods produce an “image” of voltages created by the transferred charge. At the macroscale, around 100 μm to 10 cm, the main method is termed scanning Kelvin probe microscopy (SKPM). At the nanoscale, around 10 nm to 100 μm, a related but distinct variant known as Kelvin probe force microscopy (KPFM) is used instead.
In previous fundamental physics studies using these techniques, the main challenges have been to make sense of the stationary patterns of charge left behind after contact electrification, and to investigate how these patterns evolve over space and time. In the latest work, the ISTA team chose to ask a slightly different question: when are the dynamics of charge transfer too fast for measured stationary patterns to yield meaningful information?
Mapping the charge on the contact-electrified surface of a polymer film
To find out, ISTA PhD student Felix Pertl built a special setup that could measure a sample’s surface charge with KPFM; transfer it below a linear actuator so that it could exchange charge when it contacted another material; and then transfer it underneath the KPFM again to image the resulting change in the surface charge.
“In a typical set-up, the sample transfer, moving the AFM to the right place and reinitiation and recalibration of the KPFM parameters can easily take as long as tens of minutes,” Pertl explains. “In our system, this happens in as little as around 30 s. As all aspects of the system are completely automated, we can repeat this process, and quickly, many times.”

This speed-up is important because static electricity dissipates relatively rapidly. In fact, the researchers found that the transferred charge disappeared from the sample’s surface quicker than the time required for most KPFM scans. Their data also revealed that the deposited charge was, in effect, uniformly distributed across the surface and that its dissipation depended on the material’s electrical conductivity. Additional mathematical modelling and subsequent experiments confirmed that the more insulating a material is, the slower it dissipates charge.
Surface heterogeneity likely not a feature of static electricity
Pertl says that these results call into question the validity of some previous static electricity studies that used KPFM to study charge transfer. “The most influential paper in our field to date reported surface charge heterogeneity using KPFM,” he tells Physics World. At first, the ISTA team’s goal was to understand the origin of this heterogeneity. But when their own experiments showed an essentially homogenous distribution of surface charge, the researchers had to change tack.
“The biggest challenge in our work was realizing – and then accepting – that we could not reproduce the results from this previous study,” Pertl says. “Convincing both my principal investigator and myself that our data revealed a very different physical mechanism required patience, persistence and trust in our experimental approach.”
The discrepancy, he adds, implies that the surface heterogeneity previously observed was likely not a feature of static electricity, as was claimed. Instead, he says, it was probably “an artefact of the inability to image the charge before it had left the sample surface”.
A historical precedent
Studies of contact electrification studies go back a long way. Philippe Molinié of France’s GeePs Laboratory, who was not involved in this work, notes that the first experiments were performed by the English scientist William Gilbert clear back in the sixteenth century. As well as coining the term “electricity” (from the Greek “elektra”, meaning amber), Gilbert was also the first to establish that magnets maintain their electrical attraction over time, while the forces produced by contact-charged insulators slowly decrease.
“Four centuries later, many mysteries remain unsolved in the contact electrification phenomenon,” Molinié observes. He adds that the surfaces of insulating materials are highly complex and usually strongly disordered, which affects their ability to transfer charge at the molecular scale. “The dynamics of the charge neutralization, as Pertl and colleagues underline, is also part of the process and is much more complex than could be described by a simple resistance-capacitor model,” Molinié says.
Although the ISTA team studied these phenomena with sophisticated Kelvin probe microscopy rather than the rudimentary tools available to Gilbert, it is, Molinié says, “striking that the competition between charge transfer and charge screening that comes from the conductivity of an insulator, first observed by Gilbert, is still at the very heart of the scientific interrogations that this interesting new work addresses.”
“A more critical interpretation”
The Austrian researchers, who detail their work in Phys. Rev. Lett., say they hope their experiments will “encourage a more critical interpretation” of KPFM data in the future, with a new focus on the role of sample grounding and bulk conductivity in shaping observed charge patterns. “We hope it inspires KPFM users to reconsider how they design and analyse experiments, which could lead to more accurate insights into charge behaviour in insulators,” Pertl says.
“We are now planning to deliberately engineer surface charge heterogeneity into our samples,” he reveals. “By tuning specific surface properties, we aim to control the sign and spatial distribution of charge on defined regions of these.”
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