High-efficiency electrokinetic micromixing through symmetric sequential injection and expansion Jeffrey T. Coleman, Jonathan McKechnie and David Sinton* Received 13th February 2006, Accepted 26th May 2006 First published as an Advance Article on the web 14th June 2006 DOI: 10.1039/b602085b Rapid electric field switching is an established microfluidic mixing strategy for electrokinetic flows. Many such microfluidic mixers are variations on the T- or Y-form channel geometry. In these configurations, rapid switching of the electric field can greatly improve initial mixing over that achieved with static-field mixing. Due to a fundamental lack of symmetry, however, these strategies produce lingering cross-channel concentration gradients which delay complete mixing of the fluid stream. In this paper, a field switching microfluidic mixing strategy which utilizes a symmetric sequential injection geometry with an expansion chamber to achieve high efficiency microfluidic mixing is demonstrated experimentally. A three-inlet injector sequentially interlaces two dissimilar incoming solutions. Downstream of the injector, the sequence enters an expansion chamber resulting in a dramatic (two orders of magnitude) decrease in Peclet number and rapid axial diffusive mixing. The outlet concentration may be accurately varied over the full spectrum by tuning the duty cycle of the field switching waveform. The chips are designed with input from a previous numerical study, manufactured in poly(dimethylsiloxane) using soft-lithography based microfabrication, and tested using fluorescence microscopy. In the context of on-chip chemical processing for analytical operations, the demonstrated mixing strategy has several features: high mixing efficiency (99%), compact axial length (2.3 mm), steady outflow velocity, and readily variable outlet concentration (0.15 , c* , 0.95). Introduction Flexible and effective microfluidic mixing, typically of reagents and sample, is central to many on-chip analytical processes. 1–3 More specifically, on-chip microfluidic mixing devices have applications in microfluidic chemical synthesis experiments, 4 immunoassays, 5 and protein analysis, 6,7 and can offer benefits to other on-chip processes which demand fast, highly-uniform mixing of analytes and reagents such as in the study of chemical reaction kinetics. 8,9 Microfluidic mixing is typically limited to diffusion due to the laminar nature of microflows. To date, research in the area of fluid mixing in microchannels has focused on two, rather different, objectives. One goal has been to minimize the axial broadening and dissipation of discrete samples during transport and/or separation in the streamwise direction, while a second common objective has been to maximize cross-stream mixing of two adjacent laminar streams. Strategies to increase stream–stream mixing have focused on increasing both concentration gradients and the interfacial area available for diffusion. 10 Mixing strategies are commonly divided into two categories: active and passive. Active mixers utilize external forces such as modulating pressures 11–13 or oscillating electric fields 14–17 whereas passive mixers 18–23 utilize cross-stream diffusion and, in some cases, strategic surface patches or geometries to introduce chaotic advection within the flow. The most basic passive mixer is the T-form mixer, in which two adjacent laminar streams mix via cross-stream diffusion, 18,19 and rela- tively long mixing channels are required to attain high mixing efficiencies. A number of strategies have been developed to increase the mixing rate in T-form mixers. Recent studies have found that the mixing rate can be increased by promoting cross-stream transport of the fluid with strategic patterns of non-uniform surface charge along the channel walls. 20,21 Similar results for cross-stream transport have also been obtained using asymmetric grooves cut into the base of the microchannel. 22,23 Applying alternating driving forces at each fluid inlet is an active mixing technique that is attractive due to its relative simplicity. Deshmukh et al. 11 first utilized pulsatile micro- pumps to achieve increased interfacial area for mixing between two laminated microfluidic streams. Glasgow and Aubry 12 also applied pulsatile pressure-driven flow to improve mixing of a microfluidic stream with a perpendicularly connected inlet. Glasgow et al. 13 elaborated on their technique, achieving mixing efficiencies up to 84%. In the context of electrokinetic flows, mixing may be enhanced by varying the driving electrical potentials. These mixers require no moving parts, and no additional components other than external circuitry. Tang et al. 15 first achieved spatial composition modulation in the outlet of a T-form mixer by alternating the application of the potentials between inlet reservoirs. Glasgow et al. 16 demonstrated significantly enhanced microfluidic mixing using alternating voltages at the inlets of a T-form mixer. With 90u out of phase pulsing they predicted numerically that this Department of Mechanical Engineering, University of Victoria, PO Box 3055 STN CSC, Victoria, British Columbia, Canada V8W 3P6. E-mail: dsinton@me.uvic.ca; Fax: +1 (250) 721-6051; Tel: +1 (250) 721-8623 PAPER www.rsc.org/loc | Lab on a Chip This journal is ß The Royal Society of Chemistry 2006 Lab Chip, 2006, 6, 1033–1039 | 1033