A pixel in an electrowetting display (EWD) can be viewed as a confined water/oil two-phase microfluidic system that can be manipulated by applying an electric field. The phenomenon of charge trapping in the protective dielectric and conductivity of the oil phase reduce the effective electric field that is required to keep the three-phase contact line (TCL) in place. This probably leads to an oil-backflow effect which deteriorates the electro-optical performance of EWD devices. In order to investigate charge trapping and conduction effects on the device electro-optical response, an EWD device was studied, which was fabricated with a black oil, aiming for a high-contrast ratio and color-filter display. For comparison, we also prepared a device containing a purple oil, which had a lower electrical conductivity. As anticipated, the black-oil device showed faster backflow than the purple-oil device. A simple model was proposed to explain the role of oil conductivity in the backflow effect. In addition, the rebound and reopening effects were also observed after the voltage was switched to zero. The above observations were strongly dependent on polarity. By combining observations of the polarity dependence of the oil conductivity and assuming that negative charges trap more strongly in the dielectric than positive charges, our experimental results on rebound and reopening can be explained. In the AC optical response, the pixel closing speed decreased in time for intermediate frequencies. This is likely related to the phenomenon of charge trapping. It was also found that the periodic driving method could not suppress the backflow effect when the driving frequency was above ~10 kHz. Our findings confirm the significance of the above charge-related effects of EWD devices, which need to be investigated further for better understanding in order to properly design/use materials and driving schemes to suppress them.
Keywords: charge conduction in oil; charge trapping in fluoropolymer; electrowetting display device; oil backflow.