Multiple red blood cell flows through microvascular bifurcations: cell free layer, cell trajectory, and hematocrit separation.

Multiple red blood cell (RBC) flows through a symmetric microvascular bifurcation model have been simulated with the two-dimensional immersed-boundary lattice-Boltzmann method. The cell free layer (CFL), the RBC separation process and trajectories, and the resulting hematocrit distributions in the daughter branches have been examined, and the effects of cell deformability, aggregation, and feeding hematocrit on the RBC separation have also been investigated. Our results show that the overall phase separation behavior is mainly related to the RBC distribution in the feeding flow (i.e., the CFL thickness). On the other hand, for individual RBCs, the hydrodynamic interaction plays a non-negligible role in determining their trajectories and destinations. A detailed examination of the flow and pressure fields in the bifurcation region indicates that the difference in flow pressure across the front and rear ends of a flowing RBC is the major driving force for the cell motion; while the shear stress on the back of a cell that has been pressed against the corner wall is responsible for the cell's slow sliding into a vessel branch. The results have also been compared with experimental studies, and reasonable agreement has been observed. The results and information from this study could be helpful for understanding the complex RBC separation process and its effects in microcirculation and relevant biomedical applications.