Flow behaviour of erythrocytes - I. Rotation and deformation in dilute suspensions

The behaviour of individual human red cells and rouleaux were studied under the microscope in suspensions of < 2% haematocrit: (i) in Poiseuille flow by tracking the particles in glass tubes, and (ii) in Couette flow by observing cells in the stationary layer between counter-rotating glass disks. At shear stresses $\ll $ 0.1 N m$^{-2}$, the rotations of red cells in plasma were in accord with theory applicable to rigid disks. At shear stresses > 0.1 N m$^{-2}$, especially in viscous isotonic Dextran solutions, the cells oriented themselves at a constant angle to the flow and their membrane appeared to rotate about the interior. Although this behaviour was analogous to that of liquid drops, the concavity was still present in the deformed erythrocytes, whose mean major diameters in plasma increased by 1.05 $\mu $m as the shear stress increased from 0 to 0.4 N m$^{-2}$. Deformation, through bending, was also observed with linear rouleaux of > 6 cells, here, at even the lowest shear stresses. By contrast, glutaraldehyde hardened cells and rouleaux rotated, without deforming, as rigid disks and rods respectively.