Clusters of red blood cells in microcapillary flow: hydrodynamic versus macromolecule induced interaction.

We present experiments on RBCs that flow through micro-capillaries under physiological conditions. The strong flow-shape coupling of these deformable objects leads to a rich variety of cluster formation. We show that the RBC clusters form as a subtle imbrication between hydrodynamic interactions and adhesion forces because of plasma proteins, mimicked by the polymer dextran. Clusters form along the capillaries and macromolecule-induced adhesion contributes to their stability. However, at high yet physiological flow velocities, shear stresses overcome part of the adhesion forces, and cluster stabilization due to hydrodynamics becomes stronger. For the case of pure hydrodynamic interaction, cell-to-cell distances have a pronounced bimodal distribution. Our 2D-numerical simulations on vesicles capture the transition between adhesive and non-adhesive clusters at different flow velocities.