Blood can be considered as a two phase, non-Newtonian fluid, and its flow characteristics, in particular in the microvasculature, are affected by this fact. To a large degree the complex mechanical nature of the fluid is due to red blood cell (RBC) aggregation phenomenon, which is found intense in a number of pathological conditions. The time and flow-dependent characteristics of the aggregated structures developed in blood affect its mechanical properties, and can be examined by various techniques. In this talk the influence of RBC aggregation on blood rheology at the macroscale will be discussed first. It will be shown that secondary aggregation characteristics may play a key role in explaining the behaviour of the mechanical properties of the fluid under various flow conditions. Secondly, the influence of the RBC aggregation phenomenon on the flow characteristics for microscale bifurcating geometries, resembling the microvasculature, will be discussed. It will be illustrated that the aggregation phenomenon, apart from the flow characteristics, affects the aggregate distribution in the bifurcation in a counterintuitive manner, i.e. the smaller aggregates appear in regions of lower shear, as opposed to the general understanding that aggregation increases at the specific shear conditions. Such behaviour is explained when considering the spatial distribution of aggregates in conjunction to the velocity field developed in the parent channel branches. Finally, an attempt to predict the local viscosity in the microchannels will be described based on existing constitutive equations for blood.
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