Spatio-temporal dynamics of dilute red blood cell suspensions in a microchannel flow at low Reynolds number

Microfluidic technologies are commonly used for the manipulation of red blood cell (RBC) suspensions and analyses of flow-mediated biomechanics. To maximise the usability of microfluidic devices, understanding the dynamics of the suspensions processed within is crucial. We report novel aspects of the spatio-temporal dynamics of an RBC suspension 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 resembling the well-known "tubular pinch effect" which would arise from inertial effects. However, given the negligible inertia under our experimental condition, an alternative explanation is needed for this OCTP profile contrary to the centralised distribution commonly reported for low-inertia flows. 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.