Physicists have debunked a three-decades-old explanation for how grains of the same material rub together to generate static electricity — an effect seen, for example, in volcanic ash clouds.

One theory posited that because larger grains hold more trapped, high-energy electrons, they redistribute the electrons to smaller grains when two touch, creating static electricity. Heinrich Jaeger from the University of Chicago, Illinois, and his colleagues measured the surface density of trapped electrons around different-sized grains of zirconium dioxide silicate, as well as the grains' charge. The authors found that there are far too few trapped electrons to account for the observed static build-up when the grains are mixed.

Instead, other charged particles, such as ions from water films or from the surrounding atmosphere, could accumulate on the grains' surface and be responsible for the effect, the team suggests.

Phys. Rev. Lett. 112, 218001 (2014)