Red blood cells (RBCs) -- erythrocytes -- suspended in plasma tend to
aggregate and form rouleaux. During aggregation the first stage consists in the
formation of RBC doublets [Blood cells, molecules, and diseases 25, 339
(1999)]. While aggregates are normally dissociated by moderate flow stresses,
under some pathological conditions the aggregation becomes irreversible, which
leads to high blood viscosity and vessel occlusion. We perform here
two-dimensional simulations to study the doublet dynamics under shear flow in
different conditions and its impact on rheology. We sum up our results on the
dynamics of doublet in a rich phase diagram in the parameter space (flow
strength, adhesion energy) showing four different types of doublet
configurations and dynamics. We find that membrane tank-treading plays an
important role in doublet disaggregation, in agreement with experiments on
RBCs. A remarkable feature found here is that when a single cell performs
tumbling (by increasing vesicle internal viscosity) the doublet formed due to
adhesion (even very weak) remains stable even under a very strong shear rate.
It is seen in this regime that an increase of shear rate induces an adaptation
of the doublet conformation allowing the aggregate to resist cell-cell
detachment. We show that the normalized effective viscosity of doublet
suspension increases significantly with the adhesion energy, a fact which
should affect blood perfusion in microcirculation.Comment: 14page