Enhanced Passive Micromixing via Asymmetric Height-Variable Micropillar Arrays at Low Reynolds Numbers

Efficient mixing in microfluidic systems is essential for many biological, chemical, and diagnostic applications, yet it remains challenging at low Reynolds numbers, where laminar flow and diffusion-limited transport dominate. Here, we investigate passive micromixing as a function of systematic micropillar height variation along the flow direction over a Reynolds number range of 0.1–10. Four micromixer configurations are examined, including a plain channel without micropillars and three micropillar arrays with uniform, half-square-wave, and backward sawtooth height profiles, implemented in a device comprising two mixing chambers. Mixing efficiency, efficiency ratio, and performance index are quantified experimentally and numerically in both chambers. All micropillar configurations outperform the plain design, demonstrating the effectiveness of three-dimensional geometric modulation for low-Re mixing. Among the tested designs, the asymmetric backward sawtooth configuration provides the most robust enhancement by promoting transverse flow components and sustained interfacial stretching. Mixing efficiency in the downstream chamber consistently exceeds that in the upstream chamber due to cumulative hydrodynamic development and stronger recirculation. The backward sawtooth design achieves a maximum mixing efficiency of approximately 98% at a Reynolds number of about 0.9, while efficiency ratios in the downstream chamber reach up to 1.6 relative to the plain configuration. At a Reynolds number of 0.1, the performance index of the uniform micropillar design is nearly four times higher than that of the plain channel. The close agreement between experimental and numerical results demonstrates that height-graded, asymmetric micropillar arrays offer an energy-efficient and scalable strategy for high-performance passive micromixing under diffusion-dominated conditions, with direct relevance to lab-on-a-chip systems.