In dense suspensions, both rigid particles and deformable red blood cells (RBCs) exhibit a tendency to migrate away from the walls and towards the centerline of the vessel in which they flow. Here we experimentally investigate the situation in which both particles and RBCs flow through bifurcating vessels, which is particularly relevant for targeted drug delivery. In particular, we focus on the effects of RBC concentration (hematocrit) and particle volume fraction. Via high-speed imaging and Lagrangian tracking, we locate and track tens of thousands of microspheres as they flow through microfluidic bifurcating channels of arteriole size, in physiological ranges of hematocrit and shear rate. Our findings show that the margination behavior of the spherical particles is quantitatively controlled by the RBC-to-particle volume ratio, i.e., the ratio between the hematocrit and the particle volume fraction. As this increases, interactions with RBCs induce significant particle velocity fluctuations and enhance their diffusivity. While the deformable RBCs populate the center of the vessel, particles marginate and form layers adjacent to the sidewalls of the parent branch, lining the lateral sides of the bifurcation and child branches. Despite this tendency, a particle-depleted layer, of thickness comparable to the particle radius, remains at the wall. This behavior is opposite to that when suspensions are devoid of RBCs, where particles concentrate in the center of the parent branch and at the medial side of the child branches.

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• Blood flow
• Microfluidics
• Multiphase flows
• Particle-laden flows
• Physiological flows