Figures 2-4 show the dynamic distribution of surface
charge at different voltage with cycles of 20, 500 and 2000 ms,
respectively. In Figure 2a, the first discharge corresponding
with eight charge spots was observed near the peak point of
applied voltage. This phenomenon, named simultaneous
discharge, had been exhaustively investigated in our previous
research [21]. At the decreasing part of voltage, the positive
charges gradually decayed. After a voltage zero-crossing point,
the negative discharges tended to take place at the sites where
positive surface charges had existed. But the decaying of
negative charge was not obvious.