JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 9, NO. 2, JUNE 2000 171 Surface-Tension-Driven Microactuation Based on Continuous Electrowetting Junghoon Lee and Chang-Jin (CJ) Kim Abstract—This paper describes the first microelectromechan- ical systems (MEMS) demonstration device that adopts surface tension as the driving force. A liquid-metal droplet can be driven in an electrolyte-filled capillary by locally modifying the surface ten- sion with electric potential. We explore this so-called continuous electrowetting phenomenon for MEMS and present crucial design and fabrication technology that reduce the surface-tension-driving principle, inherently powerful in microscale, into practice. The key issues that are identified and investigated include the problem of material compatibility, electrode polarization, and electrolysis, as well as the micromachining process. Based on the results from the initial test devices and the design concept for a long-range move- ment of the liquid-metal droplet, we demonstrate a liquid micro- motor, an electrolyte and liquid-metal droplets rotating along a microchannel loop. Smooth and wear-free rotation of the liquid system is shown at a speed of 40 mm/s (or 420 r/min along a 2-mm loop) with a driving voltage of only 2.8 V and little power consumption (10–100 W). [494] Index Terms—Continuous electrowetting, liquid metal, mi- crofluidics, micromotor, surface tension. I. INTRODUCTION T HE ability to control surface tension promises a new pow- erful actuation mechanism for microelectromechanical systems (MEMS) because of the advantageous scaling effect of the surface tension in microscale. Being a force proportional to the length of contact between two immiscible media (e.g., liquid and solid), the surface-tension force linearly decreases with the size. In contrast, almost all other familiar types of forces scale down with higher powers of length scale. Pressure force, for example, scales down with the second power of the size of an object, while inertia forces (e.g., weight) scale down with the third power. Trimmer systematically compared various types of forces available for microactuation [1]. According to his paper, magnetic force scales down with the second to fourth power, and electrostatic force scales down with the first to third power of size, depending on the mode of operation. Surface tension, a negligibly weak force in macroscale (i.e., in a meter scale), gains relative importance in smaller scale (e.g., millimeter), and eventually becomes the dominant force in microscale (e.g., micrometers). This dominance has been frequently encountered ever since the conception of miniatur- Manuscript received October 1, 1999; revised January 11, 2000. The work of J. Lee was supported by the Iljoo Academic and Cultural Research Foundation. Subject Editor, O. Tabata. The authors are with 38-137 Engineering IV, Mechanical and Aerospace En- gineering Department, University of California at Los Angeles, Los Angeles, CA 90095-1597 USA (e-mail: cjkim@seas.ucla.edu). Publisher Item Identifier S 1057-7157(00)05024-1. ized machines [2]. One good example is stiction problems of micromechanical elements in surface-micromachined devices [3], [4]. While surface tension has been understood as an important force in microscale, there have been limited efforts to utilize it for MEMS, instead of avoiding it. A few examples of utilization of surface tension are the bubble check valve that increases ejec- tion frequency of a droplet from the inkjet head by several folds [5] and the passive valve to stop liquid flow using an abrupt ge- ometry change in the channel [6]. Both are examples of passive usage of surface tension in blocking flows. Active examples in- clude the valveless bubble pump [7] and the optical switch using a moving silicone oil drop [8]. In the valveless bubble pump, a train of vapor bubbles is propelled along a microchannel by tem- perature difference, creating a pumping action. However, sur- face tension was not the major driving force for the motion in the scale tested. In the optical switch with a moving silicone oil droplet, the silicone oil drop was driven by thermally in- duced surface-tension difference (thermocapillary effect). The device, however, seemed to require considerable power to pro- duce enough surface-tension difference to move the oil drop. Since the rate of change of surface tension by temperature vari- ation is rather small for most of the materials, a rather steep tem- perature gradient (i.e., high power input) was needed to obtain a motion by the thermocapillary effect. The electrical method of changing the surface tension is be- lieved to be the most feasible one for effectively creating a local surface-tension variation. The corresponding principle has been known as electrocapillary for more than a century in electro- chemistry [9]. Its main purpose was to understand the adsorption characteristics of ions in the interface between two immiscible conductive liquids (e.g., mercury and electrolyte). Typical ex- periments showed the surface tension can easily vary by 50% with a potential difference of less than 1 V across the interface [10]. Beni et al. demonstrated the motion of mercury drop in a glass capillary applying the electrocapillary [11], [12]. The principle for the motion was called continuous electrowetting (CEW), and they showed the feasibility of an optical switch using the principle [13]. However, the CEW mechanism has yet to be realized in MEMS. Matsumoto et al. attempted to use electrowetting for MEMS years ago [16]. Although some pos- sible schematic configuration for MEMS was proposed in the research, the testing was limited to a primary level setup. No working microdevice has been reported until [19] and [20]. This paper describes a systematic approach to design and fab- ricate MEMS that would demonstrate micractuation by electri- cally controlled surface tension. We use the principle of CEW 1057–7157/00$10.00 © 2000 IEEE