Layer Formation of Dilute Colloidal Suspensions As a Result of a Concentration-Dependent Sedimentation Velocity: Some Simple Considerations and Suggestions for Further Experiments

Abstract.- In a number of experiments on sedimentation in colloidal suspensions, the formation of a vertically layered structure in the concentration profile has been observed. We argue that this may be interpreted as the formation of shock fronts in the concentration profile due to the concentration dependence of the sedimentation velocity, a mechanism first identified by Kynch. We propose several experiments to test this suggestion, and draw attention to the possibility of observing anomalous diffusion in a sedimenting suspension with reversible aggregation. For over a century[l], it has been known that suspensions often exhibit a stratified structure upon sedimentation. This stratification occurs in the form of the appearance of one or more horizontal bands, as sketched in fig. 1. Within each band, the suspension appears to be approximately homogeneous, but the concentration (volume fraction) of the suspension differs from one band to the other. Thus, a plot of the concentration us. height shows a staircaselike structure, with the dnterfacesn between the bands corresponding to the vertical positions where the concentration jumps. As discussed by Siano [2], this stratification has been observed in a number of different suspensions, including clays, oil emulsions, gold sols 131, membrane vesicles [41 and