In the present study, the dc electric field was fixed at 18 V and supplied by typical dc voltage equipment (PR18-1.2 V, Kenwood Co. Ltd., Japan). The gap between the electrodes was 125 μm for both setups (Fig. 2(a) and (b)), the gap was carefully adjusted, the polyimide film had a thickness of 125 μm, and 144 V/mm was applied to the dispersion at the initial stage.