Contact electrification of insulators

5 problems

• Which species carry charge between contacting insulators?

  Contested

  When two insulators touch, charge moves between them, even though insulators have no free electrons. Experiments support electrons, ions such as hydroxide from adsorbed water, or torn-off bits of material, depending on the system.

• Origin of nanoscale bipolar charge mosaics on contacted surfaces

  Open

  Maps made with a sharp electrical probe show that after contact a surface is covered with patches of both positive and negative charge, about one ten-thousandth of a centimeter across, though the total charge is small. Why charge forms such a mosaic and what sets the patch size is unexplained.

• How humidity and contact history control charge transfer

  Open

  The same pair of materials can charge strongly in dry air and weakly, or with reversed sign, in humid air, and repeated contacts change the result. A quantitative account of these effects is missing.

• Why identical materials charge and what sets the sign

  Partially resolved

  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.

• Physical origin of the triboelectric series

  Open

  Materials can be ranked in a list, the triboelectric series, in which each material charges negative against those above it. The list is only partly reproducible and no material property is known that predicts it.