A curvature-based annular contraction–expansion microchip for separation of small size distributed particles

The rapid and continuous separation of particles with small size differences is of great importance in industrial and biomedical applications. This study systematically investigates the effects of geometric parameter variations in a curved, periodic contraction–expansion inertial microfluidic device on particle focusing behavior, with the goal of achieving high-resolution separation of particles with relatively small size differences. By integrating fluorescence particle experiments with numerical simulations, key structural parameters of the chip were systematically adjusted to evaluate their influence on particle focusing positions and bandwidths, and the associated physical mechanisms were elucidated. The results demonstrate that tailored channel geometries can effectively modulate inertial focusing, thereby enabling efficient separation. Variations in curvature radius strongly influence particle trajectories by modifying secondary flow intensity and centrifugal effects within the channel. Changes in the number of contraction–expansion cycles alter flow asymmetry and progressively affect focusing position and width. Smaller particles (2 and 5 μm) exhibit higher sensitivity to structural parameter variations, resulting in pronounced focusing shifts toward the channel center and unstable bandwidths. In contrast, larger particles (10 μm) show lower sensitivity, maintaining relatively stable focusing positions biased toward the outer channel wall with more consistent bandwidths. Furthermore, adjusting the length and width of contraction and expansion regions regulates the balance between inertial lift and transverse drag forces, enabling tunable focusing performance. This work bridges the knowledge gap regarding geometry-induced focusing behavior in curved contraction–expansion microchannels and provides practical design guidelines for high-throughput, high-resolution particle separation.