Rapid electric field switching is an established microfluidic mixing strategy for electrokinetic flows. Many such microfluidic mixers are variations on the t- or y-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 suffer from lingering cross-channel concentration gradients which delay complete mixing of the fluid stream. Presented here is a field switching microfluidic mixing strategy which utilizes a symmetric sequential injector and an expansion chamber to achieve rapid and effectively complete microfluidic mixing. The three-inlet symmetric injector sequentially interlaces the two dissimilar incoming solutions. Just downstream of the injector, the sequence enters an expansion chamber and increased axial diffusion results in rapid mixing. The completely mixed solution is refocused into the outlet stream. The microfluidic chips are designed such that only the minimum number of independent fluid reservoirs is required. Chips are manufactured in polydimethylsiloxane using established soft-lithography based microfabrication methods. Fluorescence microscopy is employed to analyze, quantify and demonstrate the effectiveness of this mixing strategy, and determine a preferred operating frequency range. The microfluidic chip design is based on the findings of a recent numerical modelling based work that demonstrates the sequential injection micromixing concept.

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