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Blood is a complex fluid comprised of predominantly red blood cells (RBCs) suspended in a continuous, Newtonian phase, the plasma. Blood viscosity is highly dependent on the RBC concentration (haematocrit) and also displays shear thinning properties, as a result of RBC deformation and aggregation at high and low shear rates, respectively. However, these two phenomena also lead to uneven haematocrit distributions, which are exacerbated in microvascular bifurcations. In the present study, multifaceted experiments of human blood, perfused through bifurcating microchannels, are used to further elucidate the relationship between haematocrit, velocity and viscosity. A custom pressure based perfusion system was developed and was coupled with image acquisition for velocity measurement with μPIV and further processing. The acquired data was analysed in order to investigate the flow characteristics of human blood in two different idealised bifurcation geometries. The `cell-depleted layer' (CDL), a region of reduced haematocrit which occurs near the walls of the channel, and the continuous haematocrit distribution were experimentally measured. Analytical and numerical approaches were used to extract further information on the effect of flow rate, flow ratio and the presence of aggregation on microhaemodynamics. In the parent branch of the bifurcation, RBC aggregation was observed to increase the radial migration of RBCs away from the vessel wall. This enhanced the non-uniformity of the haematocrit downstream of the bifurcation and altered the relative velocity between the RBCs and the suspending medium. A skewed distribution of cells was observed downstream of the bifurcation, which resulted in skewed velocity profiles, which were also captured in the analytical and computational approaches. The geometry of the bifurcation was observed to influence the results and RBC aggregation quite significantly modified the haemodynamic characteristics even at high flow rates. |
