Gwo-Bin Lee 1 Lung-Ming Fu 2 Che-Hsin Lin 3 Chia-Yen Lee 1 Ruey-Jen Yang 1 1 Department of Engineering Science, National Cheng Kung University, Tainan, Taiwan 2 Graduate Institute of Materials Engineering, National Pingtung University of Science and Technology, Pingtung, Taiwan 3 Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-sen University, Kaohsiung, Taiwan Dispersion control in microfluidic chips by localized zeta potential variation using the field effect A new technique to minimize the effects of turn-induced dispersion within U-shaped separation channels by using the field effect within a capacitor to vary the zeta poten- tial along the channel walls in the vicinity of the microchannel is described. The effects of the separation channel geometry, the fluid velocity profile, and the use of the field effect to control the zeta potential on the band distribution in the detection area are extensively discussed. The results for a U-shaped separation channel indi- cate that varying the zeta potential by controlling the field effect significantly reduces the band dispersion induced by the 907 turns within the channel. Finally, it is shown that the application of the proposed localized zeta potential variation method also results in a correction of the band tilting phenomenon and a reduction in the race- track effect. Keywords: Field-effect flow control / Microfluidics / Miniaturization / Racetrack effect DOI 10.1002/elps.200305880 1 Introduction Capillary electrophoresis (CE) [1–5] is often used in appli- cations requiring the separation of biological or chemical particles, e.g. in the separation of DNA for genetic engi- neering purposes. The CE technique exploits the differing mobility characteristics of charged molecular species when subjected to an externally applied electric field. Electroosmotic flow is a bulk flow of solution resulting from movement induced in the double layer at the capil- lary surface upon application of the electric field [6–9]. Under the conditions often employed for CE, electroos- motic flow is from the positive to the negative electrode. Negatively charged species will undergo electrophoretic migration against the electroosmotic flow. In both cases, however, the overall direction of charged species in the electric field will be a sum of both electrophoretic and electroosmotic velocities. The injection and separation channels of a microfluidic chip are generally designed within a compact area of the microfabricated chip. Such microfluidic chips often use electrokinetic manipulation techniques to handle and analyze the fluid, and by cou- pling the chips with other analytical techniques, such as on-line detection [10], it is possible to provide a complete chemical analysis system which satisfies the lab-on-a- chip concept, e.g., filtration-concentration-CE [11], PCR- CE [12, 13], dielectrophoresis (DEP)-CE [14], and CE-col- lection [15]. Typically, the separation channel on a microfluidic chip is designed in the form of a straight line. However, this con- figuration occupies a larger chip area, and so the use of serpentine channels has been developed in order to achieve longer separation lengths within a more compact area. Recent studies addressing the design of serpentine microchannels have concerned themselves with increas- ing the separation efficiency, decreasing the manufactur- ing cost, and facilitating product miniaturization. For example, Jacobson et al. [16] carried out theoretical and experimental investigations of band traverses in U-shap- ed turn channels. Their study presented a theoretical res- olution analysis of chip-based separations, and linked the turn-induced band-broadening effect directly to the angle of the turn and to the width of the separation channel. Culbertson et al. [17] demonstrated that constant-radius corners increase the dispersion of a sample flowing in an electrokinetic microchannel, and suggested that this reduces the benefits provided by the additional separa- tion length. The stretching of the analytical band, which occurs as it traverses a turn, is commonly referred to as the “racetrack effect”. Paegel et al. [18] conducted an experimental study into the effects of the taper ratio in folded, tapered-turn separation microchannels. Their re- sults indicated that a taper ratio (defined as the ratio of the width in the separation channel to the width of the channel in a turn) of n = 4:1 was successful in correcting Correspondence: Prof. Ruey-Jen Yang, Department of Engi- neering Science, National Cheng Kung University, Tainan, 701, Taiwan E-mail: rjyang@mail.ncku.edu.tw Fax: 1886-6-276-6549 Abbreviations: APD, avalanche photodiode; EDL, electrical double-layer Electrophoresis 2004, 25, 1879–1887 1879  2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Miniaturization