The distribution of suspended particles under gravity

Abstract

The apparent stability and freedom from settling shown in practice by many colloidal suspensions has in recent years prompted renewed study of the distribution of the particles of colloidal systems under the influence of gravity, and results greatly at variance with earlier work have been obtained. The object of the present communication is (1) to show that the later conclusions are not justified by the experiments on which they are based, and (2) to describe experiments which favour a considerable extension of the applicability to colloidal solutions of a law similar to that which defines the behaviour of ordinary dilute solutions. Perrin, in 1909, showed that there was, within a depth of 0·01 cm. from the surface, a logarithmic decrease of concentration with height for suspensions of gamboge or mastic in gravitational equilibrium. In 1914 Costantin, working at lower depths than Perrin, and with a higher volume concentration of gamboge and correspondingly higher numerical concentration, obtained evidence of a repelling force between the particles at concentrations greater than 8 X 10 10 particles per cubic centimetre. Perrin showed how his equation could be modified to take this into account by assuming a law of the van der Waals’ type, which was found to agree with the observations up to the highest concentration investigated (6 X 10 11 particles per cubic centimetre). In the following year Westgren published the results of a series of measurements on selenium and gold sols with coarse particles, which were in complete harmony with Perrin’s law over a distance of 0·05 cm. and at concentrations up to 5 X 10 10 per cubic centimetre. These investigators were precluded from studying the distribution over greater ranges of depth solely by the restrictions of their experimental method, which demanded that for accurate counting the number of particles in the field of view should not be too large. For this reason also they were limited to moderately dilute sols and to rather large particles, that is, to specially prepared sols.