Lone of the side effects of the recent Covid-19 pandemic is that the handshake and, even more, the daily kiss, which had completely deserted our human relationships, only seem to recolonize them extremely timidly, if at all … However, these brief contacts, which most of the time went unnoticed, were the moment of discreet and complex interactions and exchanges (beyond the banal aerial pathogens), with sometimes incomprehensible consequences. Surprisingly, this subtle physics also concerns inert and insulating materials, because whatever insulator they are, they exchange charges by contact (but not with anyone), in an interaction that literally goes as far as lightning strikes ( but not anyhow).
This is precisely the subject of a recent article by Nature Physics, in which a corner of the veil has been brilliantly lifted on an ancient enigma of triboelectricity. The problem concerns the simplest contact electrification experiment: take two surfaces made of easily charge-exchanging insulators (in the article, Plexiglas and a silicone elastomer) whose cleanliness and flatness have been particularly cared for. It is then a question of bringing them into contact, of separating them, and of using any means whatsoever to visualize the distribution of the charges on each surface.
Small flashes near the detachment front
In a simple world, one would expect to recover two uniformly charged surfaces, with the more electrophilic material (silicone) becoming negatively charged. However, since the first experiments around 1940, whatever the experimental devices and materials used, researchers have systematically observed that the charges are distributed on each surface in a capricious way, often forming complicated patterns called “mosaics”, with alternating positively and negatively charged areas on the same surface. Thus, on silicone, despite a total negative charge, there are large areas of positive polarity.
For lack of a more satisfactory explanation, various microscopic residual inhomogeneities have been invoked to explain these puzzling patterns. The researchers focused this time on the role of small flashes that occur near the detachment front. A phenomenon already known, easy to see even by peeling off an adhesive in the dark (it has been shown that X-rays are even emitted if the experiment is carried out in a moderate vacuum!), but which no one had thought of linking to the mosaic structure of the deposited charges…
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