Chang-Hsien Tai 1 Ruey-Jen Yang 2 Min-Zhong Huang 2 Chia-Wei Liu 3 Chien-Hsiung Tsai 1 Lung-Ming Fu 4 1 Department of Vehicle Engineering, National Pingtung University of Science and Technology, Pingtung, Taiwan 2 Department of Engineering Science, National Cheng Kung University, Tainan, Taiwan 3 Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan 4 Department of Materials Engineering, National Pingtung University of Science and Technology, Pingtung, Taiwan Received December 9, 2005 Revised March 30, 2006 Accepted April 6, 2006 Research Article Micromixer utilizing electrokinetic instability-induced shedding effect This paper presents a T-shaped micromixer featuring 457 parallelogram barriers (PBs) within the mixing channel. The presented device obtains a rapid mixing of two sample fluids with conductivity ratio of 10:1 (sample concentration:running buffer concentra- tion) by means of the electrokinetic instability-induced shedding effects which are produced when a direct current (DC) electric field of an appropriate intensity is applied. The presented device uses a single high-voltage power source to simultaneously drive and mix the sample fluids. The effectiveness of the mixer is characterized experimen- tally as a function of the applied electrical field intensity and the extent to which the PBs obstruct the mixing channel. The experimental results indicate that the mixing performance reaches 91% at a cross-section located 2.3 mm downstream of the T-junction when the barriers obstruct 4/5 of the channel width and an electrical field of 300 V/cm is applied. The micromixing method presented in this study provides a sim- ple low-cost solution to mixing problems in lab-on-a-chip systems. Keywords: Electrokinetic instability / Microfluidic mixer / 457 Parallelogram barriers DOI 10.1002/elps.200500900 1 Introduction Microfluidic devices have many key advantages over conventional large-scale analytical techniques, including shorter detection times, a higher resolution, reduced sample consumption, ease of portability, and dis- posability [1–7]. Therefore, these devices are used to perform a wide range of clinical analyses nowadays, and are widely employed throughout the food and chemical industries, etc. The small flow channels which character- ize these microfluidic systems increase the surface to volume ratio. This is advantageous in many applications due to a higher throughput (achieved by way of paralleli- zation), shorter analysis times, and improved perfor- mance and reliability, etc. However, the Reynolds number of the liquid flow in microfluidic devices tends to be very small (typically less than 10). At such low Reynolds num- bers, the flow is essentially laminar and hence turbulent mixing does not occur. Therefore, the homogenization of two different solutions within the microchannel is achieved by diffusion mechanisms alone and therefore mixing takes place very slowly. Many microfluidic devices attempt to overcome this limitation by designing longer mixing channels in order to extend the sample retention time (and hence the opportunity for diffusion). However, the poor mixing efficiency of diffusion-based microfluidic devices limits their practicality for real world applications. Consequently, improving the mixing performance of microfluidic structures is an important step in realizing micro-total analysis systems (m–TASs). The micromixer is a crucial element in many microfluidic systems (or lab-on-a-chip devices), and its character- istics determine the overall quality of the reaction which can be achieved. Therefore, developing a thorough understanding of the mechanisms governing electro- kinetic manipulations in general, and those associated with discrete micromixers in particular, is essential if microfluidic systems designs are to be optimized. As mentioned above, microfluidic mixing systems are gen- erally limited to the low Reynolds number regime, and hence species mixing is dominated by diffusion mechan- Correspondence: Professor Lung-Ming Fu, Department of Materials Engineering, National Pingtung University of Science and Technol- ogy, 1 Hseuh Fu Road, Nei Pu, Pingtung, 91201, Taiwan E-mail: loudyfu@mail.npust.edu.tw Fax: 1886-8-7740522 Abbreviations: DC, drect current; DI, deionized; EKI, electrokinetic instability; OM, optical microscopy; PB, parallelogram barriers; PR, positive photoresist 4982 Electrophoresis 2006, 27, 4982–4990 Additional corresponding author: Chien-Hsiung Tsai; E-mail: chtsai@mail.npust.edu.tw  2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.electrophoresis-journal.com