  Microfluidic technologies are commonly used for the manipulation of red blood
cell (RBC) suspensions and analyses of flow-mediated biomechanics. To enhance
the performance of microfluidic devices, understanding the dynamics of the
suspensions processed within is crucial. We report novel aspects of the
spatio-temporal dynamics of RBC suspensions flowing through a typical
microchannel at low Reynolds number. Through experiments with dilute RBC
suspensions, we find an off-centre two-peak (OCTP) profile of cells contrary to
the centralised distribution commonly reported for low-inertia flows. This is
reminiscent of the well-known "tubular pinch effect" which arises from inertial
effects. However, given the conditions of negligible inertia in our
experiments, an alternative explanation is needed for this OCTP profile. Our
massively-parallel simulations of RBC flow in real-size microfluidic dimensions
using the immersed-boundary-lattice-Boltzmann method (IB-LBM) confirm the
experimental findings and elucidate the underlying mechanism for the
counterintuitive RBC pattern. By analysing the RBC migration and cell-free
layer (CFL) development within a high-aspect-ratio channel, we show that such a
distribution is co-determined by the spatial decay of hydrodynamic lift and the
global deficiency of cell dispersion in dilute suspensions. We find a CFL
development length greater than 46 and 28 hydraulic diameters in the experiment
and simulation, respectively, exceeding typical lengths of microfluidic
designs. Our work highlights the key role of transient cell distribution in
dilute suspensions, which may negatively affect the reliability of experimental
results if not taken into account.
