  The triboelectric effect, charge transfer during sliding, is well established
but the thermodynamic driver is not well understood. We hypothesize here that
flexoelectric potential differences induced by inhomogeneous strains at
nanoscale asperities drive tribocharge separation. Modelling single asperity
elastic contacts suggests that nanoscale flexoelectric potential differences of
$\pm$1-10 V or larger arise during indentation and pull-off. This hypothesis
agrees with several experimental observations, including bipolar charging
during stick-slip, inhomogeneous tribocharge patterns, charging between similar
materials, and surface charge density measurements.
