AS 5009.1:2003 Determination of particle size distribution by centrifugal liquid sedimentation methods
5.1 Upper size limit
The upper size limit, x St , U , by centrifugal sedimentation is largely dependent on the reliability of measurements taken in the first minute of the analysis. As in gravitational settling, the restriction on the Reynolds number limits the upper size. An additional complication is that the particles are no longer settling at a constant terminal velocity but are continually accelerating. For a reliable analysis, the accelerating force must be small in comparison to the centrifugal force, as this restricts the upper particle size which can be measured. Centrifugal sedimentation is typically used for sizing particles up to about 5 μm, but this can be extended by the use of a more viscous liquid and/or slower centrifuge speeds.
5.2 Lower size limit
The lower limit of size to which the centrifugal sedimentation methods can be applied is controlled by temperature variation, causing circulatory currents in the suspension, by flocculation of particles during the progress of sedimentation and by diffusion or Brownian motion of the very small particles. The random collisions of the molecules making up the liquid with a particle cause differences in the pressure on the particle from one part of the surface to another such that the particle is displaced (Brownian motion), see 5.2.1. Density currents are much more liable to occur in a centrifugal device than in a gravity system. All these factors render it advisable to shorten the sedimentation process as much as possible.
Note that charged particles in weak electrolytes have an electrical double layer associated with them. When these particles settle, the double layer is distorted with the result that an electrical field is set up which opposes motion. These electro-viscous effects can be reduced by the use of non-ionic liquids where possible.
5.3 Particle shape
At a low Reynolds number, the orientation of non-spherical particles is random so a single particle will have a range of settling velocities. As the Reynolds number increases, particles tend to orientate themselves to give maximum drag and will settle at the slowest of the range of velocities possible with random orientation (see reference [2] in the bibliography).