  We present experiments on RBCs that flow through microcapillaries under
physiological conditions. We show that the RBC clusters form as a subtle
imbrication between hydrodynamics interaction and adhesion forces because of
plasma proteins. Clusters form along the capillaries and macromolecule-induced
adhesion contribute 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 captures the transition
between adhesive and non-adhesive clusters at different flow velocities.
