Liquid behavior inside a reflective display pixel based on electrowetting

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Abstract
This article deals with the behavior of fluids inside a reflective display based on electrowetting. The advantage of using electrowetting as a principle for a reflective display has been demonstrated [R. A. Hayes and B. J. Feenstra, Nature (London) 425, 383 (2003)]. The principle is based on the controlled two-dimensional movement of an oil/water interface across a hydrophobic fluoropolymer insulator. The main objective of this article is to show experimentally the influence of the oil film curvature on the kinetics of the optical switch. For this we explore the electrowetting behavior and the fluidic motion as a function of several parameters, including addressing voltage, colored oil film thickness, oil type, and device size. The electro-optic characteristics and the switching dynamics of a single electrowetting pixel are studied. The results indicate that the competition between capillary forces and electrostatic forces governs the voltage driven oil contraction while capillary forces only drive the oil relaxation upon voltage removal. Consequently, a major parameter that controls the electrowetting behavior is the curvature of the oil/water interface. When increasing the oil film thickness or decreasing the device size, the oil film curvature increases. Hence, the capillary forces become stronger and the voltage required to achieve a particular oil contraction increases. With increasing curvature of the spherical oil cap, oil film relaxation, which is only capillary driven, is more rapid. The oil viscosity also plays a role in the speed of the oil movement. The reduction of the oil viscosity leads to an increase in the extent and speed of the oil/water interface movement. A linear relationship between the pixel capacitance and the resulting pixel white area percentage is found experimentally and is reconciled with an electrical model.