An overview is given of current developments in micro mixing technology, where the emphasis is on liquid mixing in passive micro mixers. The mixers presented are differentiated by the hydrodynamic principle employed, and four important principles are discussed in some detail: hydrodynamic focusing, flow separation, chaotic advection and split-and-recombine flows. It is shown that these principles offer an excellent mixing performance in various dynamical regimes. Hydrodynamic focusing is a concept working very much independently of the Reynolds number of the flow. Flow separation offers a rich dynamical behavior over a Reynolds number scale of several hundred, with superior performance compared to purely diffusive mixing already found at low Reynolds numbers. For chaotic advection different implementations tailor-made for low and comparatively high Reynolds numbers exist, both leading to an exponential increase of the interface between two fluids. Split-and-recombine flows can only be realized in a close-to-ideal form in the low Reynolds number regime. Corresponding mixers can be equipped with comparatively wide channels, enabling a favorable ratio of throughput to pressure drop. The overview given in this article should enable a potential user of micro mixing technology to select the most favorable concept for the application envisaged, especially in the field of chemical process technology.

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