Electrical sizing of particles in suspensions. 3. Rigid spheroids and red blood cells

The processes involved during the passage of a suspended particle through a small cylindrical orifice across which exists an electric field are investigated experimentally for an approximate prolate spheroid in the form of two tangent, rigid spheres (ragweed pollen particles) and for fresh, human red blood cells. Oscillograms of current pulses produced by both types of particles are presented and discussed in terms of particle shape and orientation and the effects of the hydrodynamic field. It is concluded that all the particles enter the orifice with their major axes aligned parallel to the orifice axis (electric field), but that during their passage some are rotated by the hydrodynamic field. Cells with their equatorial plane perpendicular to a radius of the orifice change their orientation with respect to the electric field as they are rotated, the others do not; only in the former case is there any deformation. It is shown that the bimodal or skewed size distributions can be explained on this basis, and that size (shape factor x volume) is actually a normally distributed variable (P > 95%). The average size of samples from 10 healthy adults was found to be 102.7 mu(3) with a coefficient of variation of 1.8%. For a volume of 87 mu(3), this corresponds to a shape factor of 1.18, an axial ratio (assuming a perfect oblate spheroid) of 0.26, and an equivalent major axis of 8.6 mu. The effect of high electric fields on red cell size distributions is mentioned.