This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: When two objects touch and then separate, they can swap tiny amounts of electric charge, even if they are made from exactly the same material. This 'triboelectric effect' is the source of everyday static electricity, but it is still poorly understood in large samples of powder, where vast numbers of particles are constantly colliding with each other.
Through new research published in Physical Review E, Tom O'Hara and colleagues at the University of Bristol have investigated the effect using acoustic levitation: using sound waves to trigger repeated collisions between pairs of particles. Their experiment shows for the first time how charge builds up on repeatedly colliding particles, before leveling off at a set limit. Charge exchange through collisions plays a role in systems including windblown dust, clouds of volcanic ash and the powders handled in many industries.
But because these systems involve so many collisions, it has been extremely difficult to follow how charge evolves on any single particle. As a result, researchers have long debated whether the charge on colliding particles keeps on growing or eventually approaches some maximum value. To address this question, O'Hara's team built on earlier advances in trapping small objects in midair using carefully shaped sound waves.
Their new platform, named "MultiLev," offers far more control, allowing the team to bring two tiny polystyrene balls together, make them collide, then pull them apart again. To measure the charge on each particle, the researchers placed a pair of "Faraday cups" on either side of the collision point. These devices detect the charge on an object placed inside them.
Crucially, the cups were designed to let sound waves pass straight through, so each particle could be returned to its own cup after every collision and have its charge measured, without disrupting the levitation. After each individual collision, the team found that fairly random amounts of charge were transferred between the particles. But as the collisions mounted up, a clear pattern emerged: the particles gained steadily growing opposite charges, before this growth leveled off toward a maximum, rather than continuing indefinitely.
This behavior closely matches the charging of a capacitor. For now, the precise process behind the charge exchange remains unclear. In future studies, O'Hara's team hopes that stronger acoustic traps could allow them to levitate denser materials and to move particles farther apart between collisions for cleaner measurements.
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