Microfluidics promises to revolutionize chemical analysis, [1,2] synthesis, [3][4][5] and biotechnology [6] by combining processes such as mixing, separation, reaction, and detection in a single device. These systems function as "labs-on-a-chip", [7] creating a technology that is low-cost, high-throughput, miniaturized, and automated. Their decreased size imparts microfluidics with many benefits, but miniaturization results in a fundamental change in flow characteristics. Turbulent flow predominates at the macroscale, whereas fluids flow in a laminar fashion at the microscale, without the random mixing that is characteristic of turbulence. [8] These laminar flow conditions mean that multiple fluid streams tend to flow in parallel through microchannels, mixing only by diffusion across their interfaces. [9] Whereas laminar flow behavior has been exploited to good effect in microanalytical systems, many emerging applications of microfluidic devices require rapid and efficient mixing.
Miniaturizing the mixing process has been identified as a major hurdle in the performance and development of microfluidic devices. [10] In labs-on-a-chip, the purpose of mixing is generally to bring together solute species from two (or more) flows. In the laminar flow regime, mixing occurs slowly by diffusion of solute (and solvent) molecules across the flow boundary. For rapid mixing, laminar flow must be disrupted to give chaotic mixing, in which there is bulk transfer of fluid (solvent carrying solute) between the flows.
Micromixers can be either passive (static) or active devices. Passive micromixing strategies frequently rely on diffusion-controlled mixing, with multilamination or flowsplitting techniques to minimize the mixing equilibration time. [11] To increase the mixing rate beyond that limited by diffusion, passive mixers that induce lateral transport of fluid between streams have been devised. [12,13] Active micromixers use miniature stirrers or external fields to chaotically