Viscoelastic and Inertial Focusing of Asymmetric Cells in Spiral Channels

Explosive growth of microfluidics has triggered numerous advances in focusing, separating, ordering, and concentrating of cells Microfluidic systems capable of performing these functions are rapidly finding applications in clinical and biomedical fields. However, most microfluidic methods have been demonstrated using spherical particles in Newtonian fluids. Yet, fluids such as blood, saliva, and cytoplasm are non-Newtonian, and cells such as red blood cells and spermatozoa cells are asymmetrical in shape. These key differences can reduce effectiveness of the microfluidic separation methods. In this work, we use spiral inertial microfluidic devices to investigate migration dynamics and focusing evolution of beads in non-Newtonian, elasto-inertial flows. Coupling of Dean flow that arises from the spiral channel geometry with fluid elasticity yields complex migration behavior. The flow rate, device curvature, and medium viscosity were found to influence lateral migration of cells or particles within these channels. In addition, we used spermatozoa cells to investigate the mechanism of asymmetric shape effects on lateral migration in the spiral channel. The sperm cell migration direction and alignment were found to depend on the flow parameters, leading to differences in focusing equilibrium as compared with spherical particles. Ultimately, insights from this work offer a useful guide to microfluidic device design for improving efficiency of 3D focusing in cell sorting and cytometry applications.