  We investigate the shear thinning of red blood cell - dextran suspensions.
Microscopic images show that at low polymer concentration, aggregation
increases with increasing concentration until it reaches a maximum and then
decreases again to non-aggregation. This bell shape dependency is also deduced
from the rheological measurements, if the data are correctly normalized by the
viscosity of the suspending phase since a significant amount of polymers adsorb
to the cell surfaces. We find that the position of the maximum of this shear
rate dependent bell shape increases with increasing viscosity of the suspending
phase, which indicates a that the dynamic process of aggregation and
disaggregation is coupled via hydrodynamic interactions. This hydrodynamic
coupling can be suppressed by characterizing a suspension of 80% hematrocrit
which yields good agreement with the results from the microscopical images.
