  Plasma proteins such as fibrinogen induce the aggregation of red blood cells
(RBC) into rouleaux, which are responsible for the pronounced shear thinning
behavior of blood, control the erythro- cyte sedimentation rate (ESR) a common
hematological test and are involved in many situations of physiological
relevance such as structuration of blood in the microcirculation or clot
formation in pathological situations. Confocal microscopy is used to
characterize the shape of RBCs within rouleaux at equilibrium as a function of
macromolecular concentration, revealing the diversity of contact zone
morphology. Three different configurations that have only been partly predicted
before are identified, namely parachute, male-female and sigmoid shapes, and
quantitatively recovered by numerical simulations. A detailed experimental and
theoretical analysis of clusters of two cells shows that the deformation
increases nonlinearly with the interaction energy. Models indicate a forward
bifurcation in which the contacting membrane undergoes a buckling instability
from a flat to a de- formed contact zone at a critical value of the interaction
energy. These results are not only relevant for the understanding of the
morphology and stability of RBC aggregates, but also for a whole class of
interacting soft deformable objects such as vesicles, capsules or cells in
tissues.
