Key result
Smaller microtubes and porous media diminish blood viscosity while high shear rates induce dilatancy.
Flow geometry significantly alters blood viscoelasticity, with smaller spaces demonstrating the Fåhraeus-Lindqvist effect and high shear rates causing dilatancy regardless of red cell deformability or aggregation.
Microvascular flow models require geometry-specific rheology; leaves open human in vivo validation and clinical relevance.
The viscoelastic properties of blood are dominated by microstructures formed by red cells. The microstructures are of several types such as irregular aggregates, rouleaux, and layers of aligned cells. The dynamic deformability of the red cells, aggregation tendency, cell concentration, size of confining vessel and rate of flow are determining factors in the microstructure. Viscoelastic properties, viscosity and elasticity, relate to energy loss and storage in flowing blood while relaxation time and Weissenberg number play a role in assessing the importance of the elasticity relative to the viscosity. These effects are shown herein for flow in a large straight cylindrical tube, a small tube, and a porous medium. These cases approximate the geometries of the arterial system: large vessels, small vessels and vessels with many branches and bifurcations. In each case the viscosity, elasticity, relaxation time and Weissenberg number for normal human blood as well as blood with enhanced cell aggregation tendency and diminished cell deformability are given. In the smaller spaces of the microtubes and porous media, the diminished viscosity shows the possible influence of the Fåhraeus-Lindqvist effect and at high shear rates, the viscoelasticity of blood shows dilatancy. This is true for normal, aggregation enhanced and hardened cells.
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Thurston et al. (2006) studied Blood viscoelasticity. Flow geometry (large tube, small tube, porous medium) was evaluated on Viscoelastic properties (viscosity, elasticity, relaxation time, Weissenberg number). In smaller spaces of microtubes and porous media, diminished viscosity shows the possible influence of the Fåhraeus-Lindqvist effect, and at high shear rates, blood viscoelasticity shows dilatancy.
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