Red blood cell clustering in Poiseuille microcapillary flow

Red blood cell (RBC) flow-induced clustering, which is a physiopathologically-relevant process in microcirculation, is usually attributed to slower RBCs acting to create a train of trailing cells. Here, based on the first systematic investigation of collective RBC flow behavior in microcapillaries in vitro by high-speed video microscopy and 2D boundary integral numerical simulations, we show that RBC clustering is elicited by hydrodynamical interactions only, even if the effect of polydispersity is taken into account. Furthermore, lower applied pressure drops and longer residence times favor larger RBC clusters. The insight on the mechanism of RBC clustering provided by this work can be applied to further our understanding of RBC aggregability, which is a key parameter implicated in clotting and thrombus formation.