Label-Free Particle Accumulation in Helical Microtubes with Secondary Flows

Curved microfluidic channels utilizing secondary flows offer a label-free strategy for sorting rare and heterogeneous targets, such as exosomes, T cells, and mesenchymal stem cells. However, a limited understanding of particle dynamics constrains sorting efficiency and precision. Here, we develop a three-dimensional microfluidic platform with helical microtubular channels that generates steady, tunable secondary flows across a broad range of Reynolds and Dean numbers to directly observe cross-sectional particle distributions. We identify five distinct dynamic patterns and classify their evolution from lift-dominated to drag-dominated regimes using a characteristic inertial force ratio (γ). Comparisons between rigid particles and deformable cells reveal an additional dependence on the capillary number (Ca) in determining the distribution behavior. We uncover a trade-off between focusing efficiency and diameter resolution and propose an off-axis collection strategy that leverages nonhorizontal equilibrium positions to enhance sorting performance. This work provides new insights into particle migration in secondary-flow microfluidics and support the design of high-precision, label-free separation systems for biological analysis.