Shear deformation measurement of suspended particles: Application to erythrocytes

Laser diffraction produced by a single layer of erythrocytes in suspension is applied to assess the cell deformation in the erythrodeformeter. It is well known that erythrocytes become elliptically deformed when a fluid shear stress is applied to them. In the erythrodeformeter the red cell suspension fills the narrow gap between two parallel disks of glass, horizontally positioned, the lower of which rotates at a controlled speed while the upper is fixedly supported. A laser beam traversing the sample layer at r distance from the center of disks, displays a diffraction pattern ‘‘at infinity.’’ At no shear stress the cells have discoidal shapes and the diffraction pattern is circular. Shear‐induced elongation of cells leads to an elliptical diffraction pattern, its geometric characteristics being directly related to those of deformed cells. This deformation can be evaluated in terms of a deformation index calculated from the width and the length of the diffraction pattern ellipse. In the erythrodeformeter a similar index is evaluated from the light intensity traversing two slots of uniform width each one coincident with each of the two principal diameters of the pattern ellipse, the light intensity being measured by a photoelectric detection head.