Flow-induced fast and slow waltz of red blood cells, with rheological consequences.

Flow induced self-organization into crysalline-like structures have been reported in tank-treading dilute red blood cell suspensions in recent studies [Shen et al, PRL 2018]. Here, we report "waltz"-like synchronized tumbling / rotation of red blood cell (RBC) pairs, under moderate volume fraction (3 to 10 %) and confined simple shear flow, observed using a computational model with lattice Boltzmann fluid coupled with biconcave discs matching RBC elasticity. Without accounting for RBC-RBC attraction, hydrodynamic interactions (HI) between RBCs is the main contribution to the spontaneous formation of waltzing pairs. Furthermore, HI between RBC and the wall boundary result in cross-stream migration to the flow centerline with zero velocity, leading to localized waltzing RBC pairs exhibing crystalline-like order. We further examined how RBC doublet formation increases the tumbling rotation period and reduces the suspension intrinsic viscosity.