Robert A. Hayes

Video-speed electronic paper based on electrowetting

In recent years, a number of different technologies have been proposed for use in reflective displays. One of the most appealing applications of a reflective display is electronic paper, which combines the desirable viewing characteristics of conventional printed paper with the ability to manipulate the displayed information electronically. Electronic paper based on the electrophoretic motion of particles inside small capsules has been demonstrated and commercialized; but the response speed of such a system is rather slow, limited by the velocity of the particles. Recently, we have demonstrated that electrowetting is an attractive technology for the rapid manipulation of liquids on a micrometre scale. Here we show that electrowetting can also be used to form the basis of a reflective display that is significantly faster than electrophoretic displays, so that video content can be displayed. Our display principle utilizes the voltage-controlled movement of a coloured oil film adjacent to a white substrate. The reflectivity and contrast of our system approach those of paper. In addition, we demonstrate a colour concept, which is intrinsically four times brighter than reflective liquid-crystal displays and twice as bright as other emerging technologies. The principle of microfluidic motion at low voltages is applicable in a wide range of electro-optic devices.

Amorphous fluoropolymers as insulators for reversible low-voltage electrowetting

Amorphous fluoropolymers have been used as a hydrophobic “top coat” of insulating materials for electrowetting, to confer the reversibility required for practical applications. The electrical properties of such fluoropolymers (in particular breakdown voltages of less than 20 V/μm) are often implicated in poor electrowetting performance. However, we have recently found that the intrinsic electrical properties, including the breakdown voltage, of appropriately prepared fluoropolymer coatings are sufficient to allow the amorphous fluoropolymer to function both as the insulator and hydrophobic surface for electrowetting. A typical electrowetting material system is then reduced to the basic material components of electrode, insulator, and conducting liquid. This simplicity facilitates both the fabrication of electrowetting devices, the soluble fluoropolymer insulator being directly wet coated onto the electrode materials, as well as the study of charging mechanisms. Reversible electrowetting on insulators thin...

Liquid behavior inside a reflective display pixel based on electrowetting

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.

A video-speed reflective display based on electrowetting: principle and properties

Electrowetting is presented as a novel principle for a reflective display. By contracting a colored oil film electrically, an optical switch is obtained with many attractive properties that make it very suitable for use as a reflective display, for instance, as electronic paper. Firstly, it has the high reflectivity (>40%) and contrast ratio (15) required for a paper-like optical appearance. In addition, the principle shows a video-rate response time (<10 msec) and has a clear route toward a high-brightness color display. 1 Finally, the electro-optical response is independent of cell-gap thickness, which will be very beneficial when moving toward a flexible display.

A physical model describing the electro-optic behavior of switchable optical elements based on electrowetting

Recently, we have demonstrated reflective display pixels based on the voltage-controlled two-dimensional movement of an oil/water interface across a hydrophobic fluoropolymer insulator [R. A. Hayes and B. J. Feenstra, Nature (London) 425, 383 (2003)]. Here a physical model is reported that describes the basic electro-optic behavior of these electrowetting-based optical elements. The model extends the classical electrowetting theory developed for a free water droplet on an insulator surface to the controlled two-dimensional movement of a confined oil/water interface. The model takes into account the spatial extent of the oil layer (<5mm), where the oil film is well approximated by a spherical cap. The calculated results are in very good agreement with experimental data without employing fitting parameters. The model can be used to predict and optimize the electro-optic behavior of devices as a function of a range of well-defined parameters including the oil film thickness, the thickness of the insulating l...

52.1: A High Brightness Colour 160 PPI Reflective Display Technology Based on Electrowetting

Electrowetting can be used to provide the pixel engine for a reflective display. By contracting a colored oil film electrically, an optical switch with a high reflectivity (>40%) and contrast (<15) is obtained. In addition to the attractive optical properties, the principle exhibits a video-rate response speed (∼10 ms) and has a clear route toward a high-brightness color display [1]. Here we additionally show that the display principle is readily scalable to a pixel size of 160 μm.

Electrowetting-Based Displays: Bringing Microfluidics Alive On-Screen

We present the use of electrowetting as a technology for very bright and energy-efficient displays. The intrinsic nature of the system as an optical switch provides an excellent starting point upon which a wide variety of display configurations can be based. Here we focus our discussion on reflective displays, in which the main advantages of the technology are best utilized. The properties of the reflective display will be discussed, illustrating the combination of a paper-like optical performance and video-speed switching.

Electrowetting-based optics

Electrowetting is electrostatic manipulation of liquids. It can be used to displace and deform volumes of polar liquids. A very promising application area is optics. The surface of a volume of liquid can be used as a tunable lens and displacement of the liquid can change the refraction, diffraction or transmission of light when passing through the liquid. In this paper we describe a selection of various tunable optical components that make use of electrowetting, ranging from refractive and diffractive lenses to diaphragms and displays.