Observation of aspherical particle rotation in Poiseuille flow via the resistance pulse technique. I. Application to human erythrocytes.

The resistance pulse detector (Coulter counter) has been widely applied to the problem of determining the volumes of insulating particles in electrolyte solutions. This technique is based on the simple relationship, DeltaR/R = fv/V, between the fractional resistance change DeltaR/R and the ratio of particle volume v to pore volume V. The proportionality constant f is a function of particle shape and orientation. Direct observation of the expected resistance anisotropies for aspherical particles is reported here. As predicted by simple hydrodynamic theory each individual resistance pulse has a periodically varying amplitude as it traverses a long pore in the shear field of Poiseuille flow. Characteristics of the particle motion allow improved volume distribution determinations by properly accounting for the shape factor. Application is made to normal human erythrocytes and a gaussian volume distribution with a coefficient of variation approximately 19% is found. The electrical shape effect for erythrocytes is consistent with an oblate ellipsoidal particle with a diameter-to-thickness ratio of 4. Analysis of the data indicates that the convergent entrance flow orients the cells so that they enter the pore with their axis of symmetry perpendicular to the pore axis.