Settling of fixed erythrocyte suspension droplets.

The fractionation of micron-size particles according to physical properties of size, density and surface characteristics by centrifugation and electrophoresis is hindered when the particles behave collectively rather than individually. The formation and sedimentation of droplets containing particles is an extreme example of collective behavior and a major problem for these separation methods when large quantities of particles need to be fractionated. In this paper, experiments that measured droplet sizes and settling rates for a variety of particles and droplets are described. Expressions are developed relating the particle concentration in a drop to measurable quantities of the fluids and particles. The number of particles in each droplet was then estimated along with the effective droplet density and certain trends are noted. Since a major application of this work is the purification of biological cells in the range of 10 microns, for which monodisperse inert particles are not available, red blood cells from different animals fixed in glutaraldehyde provided model particle groups with the necessary size range, visibility and stability for these fluid dynamical studies.