Oil Conductivity, Electric-Field-Induced Interfacial Charge Effects, and Their Influence on the Electro-Optical Response of Electrowetting Display Devices

Abstract

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.

High Dielectric Nonpolar Interfacial Layers Derived from Nanocellulose for Electrowetting Devices

“…Teflon AF1600X (AF), a random copolymer of 2,2‐bis(trifluoromethyl)‐4,5‐difluoro‐1,3‐dioxole, and tetrafluoroethylene at a solid content of 4.2 wt% in a perfluorinated solvent, [ 54 ] or other innovated composites such as fluorinated ZrO 2 mixed with AF1600, [ 55 ] are most often adapted as a coating material to prepare the nonpolar (hydrophobic) and insulating layer on a conductive ITO glass substrate. As reported, an 800 nm thick AF layer has a water contact angle of 118°, under the applied voltage of 0–80 V, it gradually decreased to 70°.…”

A Separated Reset Waveform Design for Suppressing Oil Backflow in Active Matrix Electrowetting Displays

“…The oil is pushed into the corners of a pixel. At this time, the pixel presents the color of the reflective panel, as shown in Figure 1 D. The proportion of oil and water on the surface of the dielectric layer is determined by the balance of the three-phase contact line [ 31 ]. The three-phase contact line is formed by the interfacial force among the oil, water, and the dielectric layer, and it can be modified when a voltage is applied to the system.…”

Dynamic Adaptive Display System for Electrowetting Displays Based on Alternating Current and Direct Current

“…Technologies such as liquid crystal display (LCD), organic light-emitting diode (OLED), and electrophoretic paper display (EPD) provide more convenience for information interaction [ 4 , 5 ]. Compared with LCD, EWD has a higher contrast ratio in strong ambient light, and it does not need to increase power consumption to adjust brightness as LCD does [ 6 , 7 , 8 ]. The reflective display technology can further replace paper reading and contribute to low-carbon environmental protection.…”