CM In the literature: partially resolved

Why identical materials charge and what sets the sign

In plain words

Two pieces of the same material can charge each other on contact, which seems to break symmetry. Recent experiments show that contact history decides which piece charges positive, but the microscopic reason is not known.

Precise statement

Identical insulators (polymers, glass, grains of one material) acquire net charge on contact, and same-material grains charge by size (small grains usually negative). Sobarzo et al. (Nature 2025) found that repeated contacts order identical PDMS samples into a reproducible triboelectric series, in which samples with more prior contacts charge negative relative to those with fewer; surface analysis found only nanoscale changes of morphology. Determine the microscopic surface property that changes with contact history and sets the sign, and predict the sign for a given pair from measurable surface quantities. An answer is the identified property with tested predictive accuracy.

What would settle it

Blind prediction of charging sign for many identical-material pairs from a measured surface property, above chance with stated accuracy.

Status in the literature

Unverified note

A 2025 Nature study showed that contact history orders identical samples; the microscopic quantity behind it remains unidentified.