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Who hasn’t rubbed a balloon on their hair only to see it stick to a wall afterward? This childhood experiment illustrates triboelectricity, a phenomenon in which simple contact or friction causes an electrical charge transfer between two materials. Although familiar in everyday life, its mechanisms are still poorly understood. Even more surprisingly, this process is not limited to solids. It can also occur when a water droplet slides on a hydrophobic surface—a unique case of “liquid triboelectrification.”
A team of researchers [1] has now shed light on this phenomenon using an unprecedented electrostatic mapping technique, capable of tracking charges left in a droplet’s wake over both time and space. The results are striking: rather than remaining confined to the initial trail, these charges spread rapidly across the entire surface. Their lateral mobility is so high that it even exceeds that of ions in aqueous solution, contrary to the expected slowdown due to contact with the solid.
Simulations conducted alongside these experiments reveal that these charges correspond to hydrated ions trapped at the interface. In other words, each ion remains surrounded by a thin shell of water molecules that partially insulates it from the substrate. In this unique state, dubbed an “ionic puddle,” the ion literally glides over the surface, limited only by friction between its hydration shell and the substrate. This interfacial friction, much lower than anticipated, explains the extraordinary diffusion speed observed.
Another surprise: the propagation of charges does not depend on their density. Ions move independently of one another, without repelling each other, indicating that the process is entirely governed by interaction with the surface. Furthermore, by adjusting the water’s pH, it is possible to change both the sign and magnitude of the deposited charge, confirming the direct involvement of the ionic species in the droplet.
These findings reveal a new state of matter, called the “ionic puddle,” where hydrated ions behave like nearly free particles despite being trapped on the surface. They provide fresh insight into triboelectrification between solids, which may originate from the transfer of nanometric charged water films. In the long term, understanding the dynamics of these “ionic puddles” could inspire advances in nanofluidics, ionic electronics, or even the design of energy harvesters based on droplet motion.
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Reference
Z. Benrahla, T. Saide, L. Burnaz, E. Verneuil, S. Gravelle, & J. Comtet, Giant mobility of surface-trapped ionic charges following liquid tribocharging, Proc. Natl. Acad. Sci. U.S.A. 122 (37) e2505841122, https://doi.org/10.1073/pnas.2505841122
(2025).
Contact
Paul Turpault – Scientific Communication, ESPCI Paris - PSL
Jean Comtet – Researcher, co-author of the study
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