Inertial focusing of red blood cells suspended in square tube flows of blood plasma

Particles and biological cells suspended in laminar tube flows are known to migrate laterally due to the lift force generated by the effects of inertia, particle deformability, medium viscoelasticity, and so on. As a result, they are often observed to pass through specific locations in the downstream cross-section of the tube. In the present study, we investigated experimentally the lateral migration of red blood cells (RBCs) suspended in blood plasma flowing through square capillaries. A cross-section of the tube near the outlet was observed from the downstream side, and the position of each RBC center in the tube cross-section was detected to obtain the RBC distribution. RBCs were found to be focused around the tube centerline at low flow rates, due the effect of RBC deformability, whereas an increase in flow rates induced the RBC focusing off-center near four points located on the diagonal of the cross-section. Additional experiments using glutaraldehyde-hardened RBCs and various suspending media (Newtonian and viscoelastic fluids) indicated that the RBC focusing on the diagonal could be caused by a combined effect of deformability of RBCs and viscoelasticity of plasma as well as inertia.