Rub two insulators together and they exchange electrical charge: the triboelectric effect powers party tricks with balloons, but also volcanic lightning, clinging powders and dust explosions in industry. Why apparently identical particles end up with wildly different charges has resisted explanation for decades. A study led by Nicolás Mujica of the Universidad de Chile's Physics Department, with colleagues at the Universidad de O'Higgins, the Pontificia Universidad Católica and the Institute of Science and Technology Austria, isolates one culprit: size.
The experiments, run in the laboratories of the Faculty of Physical and Mathematical Sciences in Santiago and at the Universidad de O'Higgins, used large numbers of oxide insulating particles with identical chemical composition and similar surfaces but carefully controlled sizes. The result, published in Physical Review Materials, is that size alone introduces a significant bias in how charge is distributed, with larger particles far more likely to acquire high charges, a relationship that grows with surface area. Charge distributions also contained many more extreme values than standard models predict.
The journal flagged the paper as an Editor's Suggestion and the American Physical Society highlighted it in Physics Magazine, recognition that the measurements add hard constraints to microscopic theories of charge generation that have been largely untestable. Practical stakes range from precise mixing of powdered medicines to managing static on planetary surfaces during space exploration and reducing explosion risk in industrial powder handling, all settings where knowing which particles carry the charge is half the battle.