  Driven or active suspensions can display fascinating collective behavior,
where coherent motions or structures arise on a scale much larger than that of
the constituent particles. Here, we report experiments and numerical
simulations revealing that red blood cells (RBCs) assemble into regular
patterns in a confined shear flow. The order is of pure hydrodynamic and
inertialess origin, and emerges from a subtle interplay between (i)
hydrodynamic repulsion by the bounding walls which drives deformable cells
towards the channel mid-plane and (ii) intercellular hydrodynamic interactions
which can be attractive or repulsive depending on cell-cell separation. Various
crystal-like structures arise depending on RBC concentration and confinement.
Hardened RBCs in experiments and rigid particles in simulations remain
disordered under the same conditions where deformable RBCs form regular
patterns, highlighting the intimate link between particle deformability and the
emergence of order. The difference in structuring ability of healthy
(deformable) and diseased (stiff) RBCs creates a flow signature potentially
exploitable for diagnosis of blood pathologies.
