  The finding of droplet-based electricity generator (DEG), based on the moving
boundary of electrical double layer, has triggered great research enthusiasm,
and a breakthrough in instantaneous electric power density was achieved
recently. However, the dynamic mechanism for such droplet-based electricity
generators remains elusive, impeding optimization of the DEG for practical
applications. Through comprehensive experiments, we developed a dynamic model
of surface charge density that can explain the underlying mechanism for the
DEGs. The spreading droplet in touch with the top electrode can be equivalently
regarded as an additional part of the top plate of the DEG capacitor, and the
change of droplet area causes the change of surface charge density of the
plates, driving electrons to migrate between the two plates. The insight of the
dynamic mechanism paves a way for optimal design and practical applications of
DEGs
