  Charge separation at moving three-phase contact lines is observed in nature
as well as technological processes. Despite the growing number of experimental
investigations in recent years, the physical mechanism behind the charging
remains obscure. Here we identify the origin of charge separation as the
dewetting of the bound surface charge within the electric double layer by the
receding contact line. This charge depends strongly on the local electric
double layer structure close to the contact line, which is affected by the
gas-liquid interface and the internal flow of the liquid. We summarize the
charge separation mechanism in an analytical model that captures parametric
dependencies in agreement with our experiments and numerical simulations.
Charge separation increases with increasing contact angle and decreases with
increasing dewetting velocity. Our findings reveal the universal mechanism of
charge separation at receding contact lines, relevant to many dynamic wetting
scenarios, and provide a theoretical foundation for both fundamental questions,
like contact angle hysteresis, and practical applications.
