Liquid behavior inside a reflective display pixel based on electrowetting

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.

Principles of Microfluidic Actuation by Modulation of Surface Stresses

“…As discussed in section 4.1, contact angle gradients can be used to drive droplets with speeds on the order of 3-25 cm/s, and also be used to split (Figure 6 f -h), merge, or mix droplets with very low power consumption (Cho et al 2003, Pollack et al 2000. The rapid change in droplet shapes induced by electrowetting has been utilized for tunable optical lenses (Berge & Peseux 2000), switchable mirrors (Lea 1986), and optical displays (Roques-Carmes et al 2004). Electrowetting in combination 440 DARHUBER TROIAN with superhydrophobic surfaces allows for even greater variability in contact angle (Krupenkin et al 2004).…”

Modeling the Fluid Dynamics of Electrowetting on Dielectric (EWOD)

“…This level set function is used to define the domain of the droplet at each instant of time, allowing the pressure and velocity fields to be computed from finite difference approximations to (13), (14), (15), (16), (17). We combine these methods in a third order Runge-Kutta time-stepping algorithm which specifies an order to the computation of the pressure field, velocity field, and level set update (see Fig.…”

“…3) Update Velocity Field: The fluid velocity components, , obey two first order time differential equations given by (16) and (17). The pressure gradient provides a forcing term in the equations, which causes a velocity field to develop.…”

“…Applications for these devices range from microfluid transport [9], mixing [10], dispensing [11], and "lab-on-a-chip" devices that automate functions like sensing and testing of biological samples [12], [13] to tunable optical fiber devices [14], [15] and reflective displays [16].…”

A Mixed Finite Element Method for EWOD That Directly Computes the Position of the Moving Interface

“…Thus, the droplet can move, split, and merge in a controlled fashion. Several applications use EWOD as a main driving force: mass spectrometry [44,25], 'lab-on-a-chip' [33,18,20,32], particle separation/concentration control [10,41], auto-focus cell phone lenses [5], and colored oil pixels for laptops and video-speed smart paper [19,30,31].…”