New research finds important clues to the mystery of static electricity

From the faint electric current felt when touching a doorknob to the foam plastic particles clinging to a woolen sweater—static electricity phenomena are everywhere. Although "static electricity" is a commonly used term for us, scientists prefer to use "contact electrification" to describe this phenomenon. Different from what the term "static electricity" implies, this process is not static but involves the dynamic transfer of charges: when two electrically insulating materials come into contact, some charges will transfer from one material to the other. However, despite the commonness of contact electrification, scientists still haven't fully uncovered its essence, especially the problem of how different materials exchange charges during the contact process. A newly published study in the journal *Nature* provides a breakthrough answer: the contact history of materials determines how they exchange charges. This discovery not only provides a reasonable explanation for the unpredictability of static electricity phenomena but also reveals the hidden rules behind their seemingly chaotic nature. **Chaos within Disorder** In 1757, when physicist Johan Wilcke was studying static electricity phenomena, he found that after glass came into contact with paper, the glass acquired a positive charge. And when paper came into contact with sulfur, the paper also carried a positive charge. Therefore, he inferred that the glass would be positively charged after coming into contact with sulfur. This discovery laid the foundation for the "triboelectric series", that is, materials are ranked according to their tendency to gain or lose charges. For example, in Wilcke's experiment, glass, paper, and sulfur formed a triboelectric series. However, this theory has a long-standing unsolved problem: different researchers often obtain different material rankings in different experiments, and even the same experimenter may get completely different results when repeating the experiment. The inconsistency of the results makes it difficult for the triboelectric series to become a unified theory. What's even more confusing is that even for the same materials, such as two balloons, they will still exchange charges after contact—does this mean that there are some unknown variables within the materials? What exactly determines the direction in which the charges go? **The Emergence of Patterns** In the new study, the researchers realized that the key to solving this puzzle might lie in studying the contact electrification between "the same materials". They chose a transparent silicon-based polymer, polydimethylsiloxane (PDMS), as the experimental object and used it to create a batch of standardized samples to minimize variables as much as possible and thus determine the core factors affecting charge exchange. The researchers initially speculated that the randomness of charge exchange between the same materials might be caused by the tiny random differences on the material surface. However, the experimental results still showed unpredictable characteristics and did not reveal clear rules, which puzzled the research team. Then, the researchers tried to conduct multiple experiments with the same group of PDMS samples. Surprisingly, after repeated experiments, they found that the samples began to show a triboelectric ranking! But when they replaced them with a new group of samples, the results returned to a randomly disordered state. This phenomenon made them realize that perhaps it was the repeated contacts that led to the evolution of the material's behavior. Further research shows that after about 200 contacts, the samples exhibit predictable behavior—the samples with more contact times tend to be negatively charged after contact compared to the samples with fewer contact times. What's even more exciting is that the researchers even found that under strict control of the contact times and the experimental sequence, the PDMS samples can be "pre-programmed" into a stable triboelectric series. This means that by manipulating the contact history of materials, we may be able to accurately predict and control their static electricity behavior. **Contact History Shapes Static Electricity** This discovery for the first time proposes a new perspective: the contact history of materials determines how it gets charged. This concept has never been proposed before, and it is precisely this factor that explains why the results of many contact electrification experiments show randomness. However, there is still a key question that has not been answered—what exactly changes in the material at the microscale to make it exhibit different charge transfer behaviors? In order to explore this, the researchers conducted a variety of high-precision surface characterization analyses on the samples. Finally, they found the only significant change: the nanoscale roughness of the material surface had changed. Specifically, repeated contacts will make the material surface smoother, and the tiny protrusions are flattened. Although it is still unclear how this change directly affects charge transfer, it is the only variable that the researchers can detect, so it becomes a key clue to understanding this phenomenon.