Investigating the origins of static charge in granular systems of silica and other oxides using acoustic traps

From rubbing a balloon on one's hair to the dramatic display of volcanic lightning, the triboelectric effect is a widespread phenomenon where contact between objects leads to an exchange of electric charge. Despite its ubiquity, our understanding of the underlying physics remains largely phenomenological. Among the many open questions, one is particularly relevant to earth science and astrophysics: why do objects made of the same material continually exchange electric charge? This effect is especially pronounced in systems involving grains or powders, where frequent collisions can result in a significant buildup of electrostatic potential energy. Such processes can influence the dispersal range of aerosols in the atmosphere, determine whether protoplanetary dust will coalesce, and even trigger thunderstorms during volcanic eruptions or forest fires. Using acoustic levitation, we isolate individual grains and conduct controlled collisions with a substrate, measuring the charge by observing the grain's behavior in electric fields. This method can accurately resolve individual collision events, allowing us to investigate various proposed charging mechanisms and explore in detail what causes the breaking of symmetry between positively and negatively charging samples. We determine that slight variations in surface composition due to molecules recruited from the atmosphere can lead to drastic changes in the charging behavior.