Using acoustic traps to study contact electrification in granular materials

Oxide insulators are among the most abundant solid materials in the universe. In granular form—such as in volcanic plumes, sandstorms, and protoplanetary disks—they undergo vast numbers of collisions. With each collision, charge separation can occur through the triboelectric effect, although the underlying mechanism remains largely unknown. Initially insignificant, this effect can accumulate and generate substantial amounts of electrostatic potential energy, influencing cohesion and often leading to sparks or even lightning. Using acoustic traps, we isolate a single grain, induce controlled collisions with a substrate, and precisely measure the resulting electric charge. Our results demonstrate that charging is controlled by a mixture of adsorbed species naturally recruited from the atmosphere. Tiny variations in these adsorbates, linked to the sample's history, are responsible for symmetry breaking between otherwise identical grains. Furthermore, we observe this behavior consistently across oxide samples made of different materials, which may provide insight into their respective positions in the triboelectric series.