Tunable liquid iris actuated using electrowetting effect Cheng-Chian Yu, a Jeng-Rong Ho, b, * and J.-W. John Cheng a a National Chung Cheng University, Department of Mechanical Engineering, 168 University Road, Chiayi 62102, Taiwan b National Central University, Department of Mechanical Engineering, 300 Jhongda Road, Jhongli City 32001, Taiwan Abstract. A configuration for a tunable liquid iris, which consists simply of two immiscible liquids and two flat indium tin oxide (ITO) glass substrates, is proposed. The two immiscible liquids are transparent salt solution and opaque oil, respectively. The top ITO electrode was precoated with a 2-μm-thick polydimethylsiloxane film as the dielectric layer, while the surface of the bottom electrode was specially treated using ultraviolet irradiation to define specific hydrophilic regions. The iris aperture’s diameter could easily be regulated by varying the direct current bias voltages between the two electrodes. Results show that the aperture diameter can be continuously varied from 1.5 mm at the voltage-off state to 3.5 mm at a bias of 350 V. This liquid iris takes the advantages of low fabrication cost, fast response time, low-power consumption, and easy reversibility without the need of any mechanical movable parts. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. [DOI: 10.1117/1.OE.53.5.057106] Keywords: tunable liquid iris; electrowetting; aperture stop; immiscible liquids. Paper 131553 received Oct. 9, 2013; revised manuscript received Feb. 28, 2014; accepted for publication Mar. 21, 2014; published online May 16, 2014. 1 Introduction In an optical system, the aperture is the opening that deter- mines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture stop, an important element in most optical designs, functions much like the iris (pupil) of the eye and controls the effective diameter of the lens opening. The size of the aperture stop affects the depth of field and optical aberrations of the image. Nowadays, most cameras use a type of adjustable aperture known as an iris diaphragm, and often referred to simply as an iris. The iris diaphragm is usually placed in a system of lenses, and the size of the aperture modulates the amount of light that passes through the lens. The center of the diaphragm’s aperture coincides with the optical axis of the optical system. Common iris diaphragms are adjusted by movable blades, simulating the iris of the eye. Iris diaphragms have been developed for about 150 years. 1 With the market blooming of consuming electronics, the need of tunable lens has been increasing. As the adaptive liquid lens develops, many new types of the liquid iris have been presented. According to their driving mechanism, liquid irises can be classified into three major types: the fluidic-pressure driven type, 2–4 the dielectric-force driven type, 5–7 and the electrowetting effect driven type. 8–11 The fluidic-pressure driven type has a relatively larger tunable range. But an extra pumping system is needed to produce the pressure required and to regulate the fluid flow. It is, thus, relatively difficult to control lens deformation precisely. Moreover, the operating time for this type is relatively long that restricts its applicability in today’s state-of-the-art opti- cal devices. The dielectric-force driven type, however, has the merits of accurate regulation and quick response time. Usually, it needs two immiscible dielectric liquids with dif- ferent dielectric constants, and the larger difference in dielec- tric constants, the better tunability of the iris. Nevertheless, to obtain a pair of liquids with appropriate dielectric constants is difficult that limits its development. Comparing with the first two types, the liquid iris driven by the electrowetting effect is an effective approach in regard of the iris’ control- ling. The literature shows that a large range of variation in iris diameter could be achieved in a reasonable operating time. However, the configuration of the reported devices is still complex and required sophisticated fabrication process. In this study, we propose a novel type of tunable liquid iris that is configured like a liquid iris but driven by the electro- wetting effect. It has very simple structure and exhibits faster response time than the exiting liquid irises based on the electrowetting effect. In this electrowetting liquid iris, the two liquids employed in the chamber are immiscible and one of them is conductive but the other is nonconductive. Therefore, the types of liquids suitable for electrowetting iris are more than those for dielectric liquid iris. The fea- sibility of liquid motion driven by the electrowetting effect has been well demonstrated in a variety of areas, such as electrowetting displays, electrowetting light valves, and electrowetting liquid lens. 12–15 2 Design and Fabrication Figure 1 shows the schematics for illustrating the fabrication steps and the operation principles of the electrowetting liquid iris. This iris consists of two immiscible liquids and two indium tin oxide (ITO) glass substrates as electrodes. The two liquids are opaque oil and transparent salt solution. The surface of the top ITO electrode was coated with a 2-μm-thick polydimethylsiloxane (PDMS) film (Sylgard 184, Dow Corning, Midland, Michigan) as the dielectric layer. The two liquids were then sandwiched by the two ITO substrates. Processes for fabricating liquid iris were described as follows: 1. The two ITO glass substrates were thoroughly cleaned before proceeding the fabrication steps. The substrates were cleaned sonicatedly in deionized water, acetone, and ethanol for 5 min. When the solvents were *Address all correspondence to: Jeng-Rong Ho, E-mail: jrho@ncu.edu.tw Optical Engineering 057106-1 May 2014 • Vol. 53(5) Optical Engineering 53(5), 057106 (May 2014)