  Constrictions in blood vessels and microfluidic devices can dramatically
change the spatial distribution of passing cells or particles and are commonly
used in biomedical cell sorting applications. However, the three-dimensional
nature of cell focusing in the channel cross-section remains poorly
investigated. Here, we explore the cross-sectional distribution of living and
rigid red blood cells passing a constricted microfluidic channel by tracking
individual cells in multiple layers across the channel depth and across the
channel width. While cells are homogeneously distributed in the channel
cross-section pre-contraction, we observe a strong geometry-induced focusing
towards the four channel faces post-contraction. The magnitude of this
cross-sectional focusing effect increases with increasing Reynolds number for
both living and rigid red blood cells. We discuss how this non-uniform cell
distribution downstream of the contraction results in an apparent double-peaked
velocity profile in particle image velocimetry analysis and show that trapping
of red blood cells in the recirculation zones of the abrupt construction
depends on cell deformability.
