Physics and dynamics of particle migration in zigzag inertial microchannels

Effective manipulation and separation of particles in inertial microfluidics is essential for a broad range of biomedical, clinical, and industrial applications. Among serpentine geometries, zigzag microchannels offer unique advantages, including high separation efficiency and purity, compact design, scalability, portability, and improved resolution for focusing small particles. However, the underlying physics governing particle migration in these structures remains poorly understood. This study aims to elucidate the mechanisms of particle migration in zigzag inertial microchannels by examining how Reynolds number and channel geometry, specifically height and width, affect the interplay between inertial lift and lateral drag forces. A combined numerical and experimental approach was employed to analyze particle migration under different flow and geometric conditions. Results show that reducing channel height enhances shear-gradient lift forces, enabling completion of the first migration stage, while the balance between rotation-induced lift and lateral drag determines the final focusing pattern. Increasing channel width, in contrast, causes only minor changes in hydraulic diameter in high aspect ratio channels (H ≪ W) and leaves the shear rate nearly constant across most of the cross section, resulting in weaker effects on particle focusing. At Re = 50, 3 μm particles achieve side focusing only at smaller heights, whereas 15 μm particles transition from double-stream to single-stream focusing as the force balance shifts. These findings establish a mechanistic framework for predicting particle migration and optimizing geometry for precise alignment. Such insights are directly relevant to applications such as flow cytometry and rare-cell analysis, where accurate focusing improves detection performance and efficiency.