The effect of brownian motion on particle size analysis by sedimentation

The influence of Brownian motion on the results of particle size measurements obtained by gravity sedimentation methods has been investigated by mathematical simulation of the process. It is shown that the effects can be directly correlated with an effective mean Péclet number (which expresses the ratio of settling velocity to diffusive velocity) for the particular system. For very small Péclet numbers, diffusion effects dominate in all cases and sedimentation methods should not be used. For larger values of the Péclet number, the errors introduced by Brownian motion are most important for very narrow and very broad distributions. In general, the effects can be reduced by increasing the sedimentation height, but it must be remembered that this will lead to increased settling times and may increase errors due to other factors such as thermal convection.