A Novel Bistable Pixel Structure Model for Electrowetting Displays Based on Asymmetric Electric Field

As the most potential new reflective display technology, electrowetting display (EWD) has the advantages of simple structure, fast response, high contrast and rich colors. However, the EWD is based on two fluid system, so the conventional pixel structure is difficult to achieve bistable display control. In this paper, a novel pixel structure model with upper and lower symmetry for electrowetting display devices (EWDs) was proposed. And based on an asymmetric electric field, the two fluids could be controlled to flow vertically to achieve bistable display. The model includes four independently driven electrodes, two partition strips and two oil tanks, and the distance between the upper and lower substrates is low, which can realize the stable storage of oil in the upper and lower tanks. Firstly, based on COMSOL Multiphysics simulation environment, the oil motion control in a pixel model was successfully realized by coupling the phase field and electrostatic field of laminar two-phase flow. By using the designed voltage driving sequence, the free switching between on-state and off-state could be realized to achieve bistable display control. Then, the bistable pixel structure was optimized to reduce the switching time between the upper and lower substrates. It was found that by reducing the pixel height, not only the time of oil switching between the upper and lower substrates could be reduced, but also a third steady state of pixels could be formed by using the partition strips. After the electric field was removed, there was also an energy barrier to blocking the flow of oil, giving the model the ability to form a third steady state. Finally, an accelerated driving method was proposed for this model, which could significantly improve the oil spreading speed by applying a lower voltage to the electrode where the oil is located during the spreading process. Compared with the natural spreading of oil, the whole spreading time was shortened by 33.3%.