OPTICAL ACTUATION OF MICROFLUIDICS BASED ON OPTO-ELECTROWETTING Pei Yu Chiou and Ming C. Wu Department of Electrical Engineering, University of California at Los Angeles Los Angeles, CA 90095-1594, USA Hyejin Moon and Chang-Jin Kim Department of Mechanical and Aerospace Engineering, University of California at Los Angeles Los Angeles, CA 90095-1594,USA Hiroshi Toshiyoshi Institute of Industrial Science, University of Tokyo, Japan ABSTRACT 1 Optical actuation of liquid droplets has been experimentally demonstrated for the first time using a novel opto-electrowetting principle. The opto-electrowetting (OEW) surface is realized by integrating a photoconductive material underneath a two- dimensional array of electrowetting electrodes. Contact angle change as large as 40° has been achieved when illuminated by a light beam with an intensity of 65 mW/cm 2 . A micro-liter droplet of deionized water has been successfully dragged by a 4-mW laser beam across a 1cm x 1cm OEW surface. The droplet speed is measured to be 7 mm/sec. 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], and light-induced surface tension change [3]. Among them, the electrowetting mechanism has received increasing interests because of its fast switching response and low power consumption. The surface tension between the liquid-solid interface is modified by external electric field, which reduces the contact angle. Examples of electrowetting-based microfluidic systems include optical switches [4], digital microfluidic circuits [5] and liquid lenses with variable focal length [6]. 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 by Cho, et al., recently [5]. 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 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 address decoders on the chip, similar to the memory access circuits, this will significantly increase the cost of the chip. Travel support has been generously provided by the Transducers Research Foundation and by the DARPA MEMS and DARPA BioFlips programs. 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 “opto-electrowetting”. We have successfully fabricated a prototype chip with 1 cm x 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 droplets (or smaller) and simultaneously manipulation of multiple droplets. PRINCIPLE OF OPTO-ELECTROWETTING Figure 1(a) shows the general electrowetting mechanism. A droplet of polarizable and conductive 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 is described by Eq. (1): 2 2 1 )] 0 ( cos[ )] ( cos[ A LV A V d V γ ε θ θ + = (1) (a) V conductive liquid insulation layer electrode (b) conductive liquid insulation layer electrode photoconductor AC Figure.1 (a) Conventional electrowetting under DC bias, and (b) opto-electrowetting with an integrated photoconductor under AC bias. θ θ 0-9640024-4-2/hh2002/$20©2002TRF DOI 10.31438/trf.hh2002.67 269 Solid-State Sensors, Actuators, and Microsystems Workshop Hilton Head Island, South Carolina, June 2-6, 2002