Light actuation of liquid by optoelectrowetting

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Abstract

Optical actuation of liquid droplets has been experimentally demonstrated for the first time using a novel optoelectrowetting (OEW) principle. The optoelectrowetting surface is realized by integrating a photoconductive material underneath a two-dimensional array of electrowetting electrodes. Contact angle change as large as 30° has been achieved when illuminated by a light beam with an intensity of 65 mW/cm2. A micro-liter droplet of deionized water has been successfully transported by a 4 mW laser beam across a 1cm×1 cm OEW surface. The droplet speed is measured to be 7 mm/s. Light actuation enables complex microfluidic functions to be performed on a single chip without encountering the wiring bottleneck of two-dimensional array of electrowetting electrodes.

Introduction

Surface tension is a dominant force for liquid handling and actuation in microscale. Several mechanisms have been proposed to control surface tension, including thermocapillary [1], electrowetting [2], [3], and light-induced surface tension change [4]. Among them, the electrowetting mechanism has received increasing interests because of its fast switching response and low power consumption. The surface tension between the solid–liquid interface is modified by external electric field, which reduces the contact angle. Examples of electrowetting-based microfluidic systems include optical switches [5], digital microfluidic circuits [6], and liquid lenses with variable focal length [7].

Transport of liquid in droplet forms offers many advantages. It eliminates the need for pumps and valves, has no moving parts, and is free of leak and unwanted mixing. For Lab-on-a-chip applications, several fluidic functions, such as liquid injection, transportation, mixing, and separation, need to be integrated on a single chip. This has been achieved recently by Cho et al. [6]. For a general purpose fluidic chip that is capable of manipulating multiple droplets simultaneously requires a two-dimensional array of electrodes to control the local surface tension. However, this results in a large number of electrodes that presents a challenge for both control and packaging of such chips. The problem becomes even more severe as the droplet size scales down. Though the number of electrodes can in principle be reduced by integrating electronic decoders on the chip, similar to the memory access circuits, this will significantly increase the cost of the chip.

In this paper, we report on a novel mechanism for light actuation of liquid droplets. This is realized by integrating a photoconductive material underneath the electrowetting electrodes. We called this mechanism “optoelectrowetting (OEW)”. We have successfully fabricated a prototype chip with 1cm×1 cm area. A micro-liter droplet has been successfully transported to any location on the chip. This approach completely eliminates the wiring bottleneck of conventional electrowetting schemes. This concept is extendable to nano-liter or smaller droplets by scaling down the gap spacing and the electrode size.

Section snippets

General concept

Fig. 1(a) shows the general electrowetting mechanism. A droplet of polarizable liquid is placed on a substrate with an insulating layer between the liquid and the electrode. When an external voltage is applied, the surface tension at the solid–liquid interface is modified and the contact angle changes. The voltage dependence of the contact angle, θ(VA), is described by Eq. (1)cos[θ(VA)]=cos[θ(0)]+12εLVVA2where VA, d, ε, and γLV are applied voltage, thickness of insulating layer, dielectric

Photoconductivity measurement

To effectively change the contact angle by light, the photoconductor needs to satisfy the following criteria:

  • (1)

    Low dark conductivity: This ensures the voltage will mainly drop across the photoconductor in the absence of light.

  • (2)

    Short carrier recombination lifetime: This enables fast switching of contact angles and high speed actuation of liquids.

  • (3)

    Visible light response: Low cost visible light sources (either diode lasers or light-emitting diodes) are readily available.

Amorphous silicon satisfies all

Experimental demonstration of liquid transport by light

Fig. 9 shows the photograph and microscope picture of the OEW device. The chip area is 1cm×1 cm, and the area of each Al electrode is 50μm×100 μm. Over 20,000 electrodes have been fabricated on the OEW surface. To demonstrate light actuation, a water droplet with a diameter of 2 mm is sandwiched between a Teflon-coated ITO glass and an OEW surface with a gap spacing of 0.5 mm. A 4 mW laser at 532 nm wavelength is used to drag the liquid droplet. The droplet is successfully moved across the 1cm×1 cm

Conclusion

A novel optoelectrowetting mechanism has been proposed to move liquid droplets by light. This mechanism combines electrowetting with light response of photoconductors. Light actuation enables a large number of electrowetting electrodes to be addressed without wiring bottlenecks. Experimentally, over 20,000 electrodes have been integrated on a 1cm×1 cm area and only a single electrical bias is needed for the entire device. Amorphous silicon is chosen to be the photoconductive material because of

Acknowledgements

The authors would like to thank Sagi Mathai and Jui-Che Tsai for assisting our measurement, Pamela Peterson for valuable discussions of fabrication process, and Professor Chih-Ming Ho for valuable discussions. This project is supported in part by DARPA Optoelectronics Center through Center for Chips with Heterogeniously Integrated Photonics (CHIPS) under contract #MDA972-00-1-0019.

Pei Yu Chiou received BS degree in mechanical engineering from National Taiwan University, Taiwan, in 1998. He is currently a doctoral student in UCLA electrical engineering with major in MEMS. His research interest is in optical MEMS and Bio-MEMS. He is doing research about optical actuation of microfluid.

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    By integrating with the plasmonic-enhanced OEW device, the LOS platform enables on-chip sample processing without any bulky and complex mechanical components like pumps, tubes, and microchannels. Furthermore, the LOS platform can be advantageously offered for droplet-based microfluidic sample preparation by eliminating complex wiring, patterning, and interconnection issues of the electrodes typically encountered by conventional electrowetting and OEW devices [19,23,25,26,48]. For on-chip LAMP assays, the LOS platform was equipped with a transparent ITO heater below the OEW device, as shown in Fig. 1(a).

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Pei Yu Chiou received BS degree in mechanical engineering from National Taiwan University, Taiwan, in 1998. He is currently a doctoral student in UCLA electrical engineering with major in MEMS. His research interest is in optical MEMS and Bio-MEMS. He is doing research about optical actuation of microfluid.

Hyejin Moon received BS and MS degrees in chemical engineering from Sogang University, Seoul, South Korea, in 1995 and 1997, respectively. She is currently a doctoral student in UCLA MEMS program. Her major research interest is in microactuators using surface tension.

Hiroshi Toshiyoshi received ME and PhD degrees in electrical engineering from the University of Tokyo, Tokyo, Japan, in 1993 and 1996, respectively. Since 1996, he has been a PhD lecturer with the Institute of Industrial Science, the University of Tokyo. Since 1999, he has been a visiting assistant professor at University of California, Los Angeles, for his sabbatical year. His research interest is MEMS for free-space optics.

Chang-Jin Kim received the PhD degree in mechanical engineering from the University of California, Berkeley, in 1991 with a study on MEMS. He received the BS degree from Seoul National University and MS from Iowa State University with the Graduate Research Excellence Award. He joined the faculty at UCLA in Mechanical and Aerospace Engineering Department in 1993 after post-doctoral work at UC Berkeley and the University of Tokyo. His research is in MEMS, especially the issues related to mechanical engineering, including design and fabrication of microstructures, microactuators and systems, and use of surface tension in microscale. He has established a formal MEMS PhD major field in his department and is active in various MEMS professional courses. Professor Kim is the recipient of the 1995 TRW Outstanding Young Teacher Award and the 1997 NSF CAREER Award. Professor Kim served as Chairman of the Micromechanical Systems Panel of the ASME DSC Division in 1996 and co-organized the Symposium on Micromechanical Systems between 1994 and 1996 for the ASME International Mechanical Engineering Congress and Exposition (IMECE). Professor Kim also organized the 1996 ASME Satellite Broadcast Program on MEMS. He served as General Co-Chairman of the 6th IEEE International Conference on Emerging Technologies and Factory Automation (ETFA’97) and served in the Technical Program Committees of the IEEE MEMS Workshop, International Conference on Solid-State Sensors and Actuators (Transducers), and the SPIE Symposium on micromachining and microfabrication. Currently, he is serving in the Executive Committee of ASME MEMS Subdivision as a subject editor for the IEEE/ASME Journal of MEMS.

Ming C. Wu received his BS degree in Electrical Engineering from National Taiwan University in 1983, and the MS and PhD degrees in Electrical Engineering and Computer Sciences from the University of California, Berkeley in 1985 and 1988, respectively. From 1988 to 1992, he was a Member of Technical Staff at AT&T Bell Laboratories, Murray Hill. In 1993, he joined the Faculty of Electrical Engineering, Department of UCLA, where he is currently Professor. He is also Director of UCLA’s Nanoelectronics Research Center, and Vice Chair for Industrial Relations. His current research interests include Micro Electro Mechanical Systems (MEMS), Optical MEMS (or MOEMS), biophotonics, microwave photonics, and high speed optoelectronics. Dr. Wu was the founding Co-Chair for IEEE LEOS Summer Topical Meeting on Optical MEMS in 1996. The meeting has now evolved into IEEE LEOS International Conference on Optical MEMS that are hosted in Europe, Asia, and US. Dr. Wu has also served in program committees of many other conferences, including optical fiber communications (OFC), conference on lasers and electro-optics (CLEO), IEEE Conference on Micro Electro Mechanical Systems (MEMS), LEOS Annual Meetings (LEOS), International Electron Device Meeting (IEDM), Device Research Conference (DRC), International Solid-State Circuit Conference (ISSCC), and Microwave Photonics (MWP) Conferences. Dr. Wu has published over 340 papers, contributed 4 book chapters, and holds 10 US patents. He is a David and Lucile Packard Foundation Fellow (1992–1997), and an IEEE Fellow.

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