Ionic puddles: And the ions slided at interfaces
Researchers have recently demonstrated that ions, charged particles most commonly found in solution, can also become confined to the surface of a solid in a hydrated state, forming ‘ionic puddles’ that allow them to explore the surface at surprising speeds.
References :
Giant mobility of surface-trapped ionic charges following liquid tribocharging, Zouhir Benrahla, Tristan Saide, Louis Burnaz, Emilie Verneuil, Simon Gravelle, Jean Comtet, Proceedings of the National Academy of Sciences 122 (37) e2505841122 – Published: 9 September 2025.
DOI : 10.1073/pnas.2505841122
Open Archives : arXiv
Triboelectricity encompasses all phenomena whereby friction or contact between solid objects causes the deposition or separation of electrical charges on their surfaces. Although these phenomena can be observed in everyday life (for example, when rubbing a balloon against hair, a set of manifestations commonly referred to as ‘static electricity’), their mechanisms remain poorly understood.
Surprisingly, this phenomenon occurs not only when two solids rub together, but also between a liquid and a solid, particularly when a drop of water slides over a hydrophobic surface. This phenomenon of ‘liquid triboelectricity’ has been attracting growing interest in recent years, as it strongly influences the sliding dynamics of drops and could lead to applications in energy generation. While measuring the charge acquired by the droplets is relatively easy, the nature and dynamics of the opposite-sign charges deposited on the surface remain poorly understood to date.
The present study was carried out in the following CNRS laboratories:
Sciences et Ingénierie de la Matière Molle (SIMM, CNRS/ESPCI Paris - PSL/Sorbonne Université)
Laboratoire interdisciplinaire de physique de Grenoble (LIPhy, CNRS/Université Grenoble Alpes)
New experiments conducted by researchers, combining spatiotemporal mapping of the charge profile on the surface of the solid and molecular dynamics simulations, have revealed the very particular nature and dynamics of these charges. By modulating the ions present in the droplets, the researchers first confirmed that the charging mechanism originates from ions initially adsorbed at the solid/liquid interface, which become trapped on the surface of the solid. Surprisingly, they then observed that these surface charges, initially confined to the wake of the droplet, diffuse and spread across the entire surface at a remarkably high speed, which can exceed that of the same ions in aqueous phase. This surface diffusion process does not depend on the density of deposited ions, as the process is dominated by interactions between the charges and the substrate, while the charges interact very little with each other due to their relatively low charge density. This hypothesis was confirmed by molecular dynamics simulations of hydrated ions adsorbed on model hydrophobic surfaces, which showed that in this very particular state, ionic mobility is limited mainly by interfacial friction between the solvation shell of the adsorbed ion and the solid surface.
These results highlight a new state of matter, known as an ‘ionic puddle’, formed by hydrated ions trapped on solid surfaces. The very specific nature of these charged puddles raises many questions related to ionic interactions and dynamics at the molecular level. They also shed new light on the understanding of triboelectrification of solids, which is thought to originate from the transfer of charged nanometric water films, similar to those highlighted here.
This work has been published in the Proceedings of the National Academy of Sciences.
