Comparing Laser Diffraction and Sedimentation

Sedimentation was one of the first analytical techniques developed for particle size analysis. It calculates the particle size by measuring the sedimentation rate of the particles within a liquid. Once this is know, the size can be calculated using the Stokes equation:

Where u st is the sedimentation rate, ρs and ρf are the density of the particle and the fluid respectively, n is the fluid viscosity and d is the particle diameter.

The above equation shows that the particle size reported by sedimentation will be dependant on the surface area of the particles being measured (d²). Particles with a large surface area compared to their volume will sediment slowly. As a general rule, as the aspect ratio of the particle increases, sedimentation will report a finer size compared to laser diffraction where the report size relates to the particle volume. This is particularly obvious when measuring plate-like particles such as micas and kaolin.

Another important factor in comparing sedimentation and laser diffraction will be the particle density. Porous particles will sediment more slowly than non-porous particles of a similar volume because of their low density. Laser diffraction would, however, always report a size that relates to the particle volume. Again, this will cause sedimentation to under-report the particle size compared to laser diffraction.