  Despite the unsettled mechanism of electricity generation from the continuous
flow of liquids on a surface, the charge-discharge theory has been widely
accepted for alternating current (AC) generation from a moving droplet. It has
been recently extended to rationalize direct current (DC) generation across a
droplet moving between two different materials. By designing a reconfigurable
contact between a metal wire and a water droplet moving on graphene, we show
that the charge-discharge theory cannot explain the reversal of current when
water-metal interfaces switch from dynamic to static. All experiments can be
described after we distinguish a dynamic from a static interface and generalize
the photovoltaic-like effect to all dynamic junctions: excited electrons and
holes in a moving interface will be separated and swept under the built-in
electrical field, leading to a DC response. This generalized theory will lead
to an understanding and the design of efficient electricity generation based on
interfacial charge transfer.
