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Red Blood Cell Dynamics in Couette Flow

Red Blood Cell Dynamics

When blood flows in capillary tubes of less than 300 μm diameter its viscosity is less than might be expected and the percentage of red blood cells (haematocrit) is also reduced.  These observations are well known for Pouiseille flow – i.e. flow which is driven by pressure variation along a tube.  Such mechanisms in Couette flow – i.e. flow in a small gap between a moving boundary and a stationary boundary – are just about completely unknown.  An understanding of red blood cell dynamics in these circumstances is important as this affects the design of rotary blood pump.  If a red blood cells are exposed to high shear stresses its membrane will rupture and the cell contents are lost (a process known as haemolysis).  There is evidence (as yet scientifically untested) that red cells might be excluded from such small gaps in rotary blood pumps meaning that these cells cannot be lysed, but also that the viscosity of the blood in the bearings will be lower than might be expected and the bearing forces developed by blood hydrodynamic bearings will be weaker than expected.

In this research we aim to measure the extent of red cell exclusion as a function of gap and speed experimentally, to develop a description of the mechanical forces and processes at work and then to develop guidelines for the correct configuration of Couette gaps in rotary blood pumps.  The research approaches used include laser-based diagnostics of experimental blood flow, computational models both of particlulate flow and of continuum fluid flow and analytical models of particulate flow.

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