MEASUREMENT OF THE AVERAGE VELOCITY OF SEDIMENTATION IN A DILUTE POLYDISPERSE SUSPENSION OF SPHERES

An X-ray attenuation technique is used to obtain the local concentration of spherical particles in a polydisperse suspension as a function of vertical position and time. From these experimental data, the average velocity of sedimentation in the homogeneous part of the suspension is derived by considering the variation with time of the total volume of particles located above a given fixed horizontal plane. Measurements have been performed in suspensions of particles which differ from each other in size with a total volume concentration in particles between 0.13% and 2.5%, and also in suspensions of particles which differ from each other both in size and in density, the total volume cooncentration being 2%. For the first kind of suspension, the experimental hindered settling factor is plotted versus the concentration and a linear regression analysis provides the slope with its 90% confidence limits: S(e) = -5.3 +/- 1.1. This experimental average coefficient of sedimentation is in good agreement with the theoretical average coefficient S(t) = 5.60 obtained from the results of Batchelor & Wen (1982). The second kind of suspension, for which permanent doublets of spheres may theoretically exist, is not in the range of validity of Batchelor & Wen's results. The experimental average coefficient of sedimentation for this case is found to be much larger than the prediction obtained by extrapolating Batchelor & Wen's results out of their range of validity. This increased velocity may be experimental evidence of the existence of permanent doublets.