  Bifurcations and branches in the microcirculation dramatically affect blood
flow as they determine the spatiotemporal organization of red blood cells
(RBCs). Such changes in vessel geometries can further influence the formation
of a cell-free layer (CFL) close to the vessel walls. Biophysical cell
properties, such as their deformability, which is impaired in various diseases,
are often thought to impact blood flow and affect the distribution of flowing
RBCs. This study investigates the flow behavior of healthy and artificially
hardened RBCs in a bifurcating microfluidic T-junction. We determine the RBC
distribution across the channel width at multiple positions before and after
the bifurcation. Thus, we reveal distinct focusing profiles in the feeding
mother channel for rigid and healthy RBCs that dramatically impact the cell
organization in the successive daughter channels. Moreover, we experimentally
show how the characteristic asymmetric CFLs in the daughter vessels develop
along their flow direction. Complimentary numerical simulations indicate that
the buildup of the CFL is faster for healthy than for rigid RBCs. Our results
provide fundamental knowledge to understand the partitioning of rigid RBC as a
model of cells with pathologically impaired deformability in complex in vitro
networks.
