Hostname: page-component-7c8c6479df-24hb2 Total loading time: 0 Render date: 2024-03-29T01:51:18.550Z Has data issue: false hasContentIssue false

Sedimentation in a dilute polydisperse system of interacting spheres. Part 1. General theory

Published online by Cambridge University Press:  20 April 2006

G. K. Batchelor
Affiliation:
Department of Applied Mathematics and Theoretical Physics, University of Cambridge

Abstract

Small rigid spherical partials are settling under gravity through Newtonian fluid, and the volume fraction of the particles (ϕ) is small although sufficiently large for the effects of interactions between pairs of particles to be significant. Two neighbouring particles interact both hydrodynamically (with low-Reynolds-number flow about each particle) and through the exertion of a mutual force of molecular or electrical origin which is mainly repulsive; and they also diffuse relatively to each other by Brownian motion. The dispersion contains several species of particle which differ in radius and density.

The purpose of the paper is to derive formulae for the mean velocity of the particles of each species correct to order ϕ, that is, with allowance for the effect of pair interactions. The method devised for the calculation of the mean velocity in a monodisperse system (Batchelor 1972) is first generalized to give the mean additional velocity of a particle of species i due to the presence of a particle of species j in terms of the pair mobility functions and the probability distribution pii(r) for the relative position of an i and a j particle. The second step is to determine pij(r) from a differential equation of Fokker-Planck type representing the effects of relative motion of the two particles due to gravity, the interparticle force, and Brownian diffusion. The solution of this equation is investigated for a range of special conditions, including large values of the Péclet number (negligible effect of Brownian motion); small values of the Ptclet number; and extreme values of the ratio of the radii of the two spheres. There are found to be three different limits for pij(r) corresponding to different ways of approaching the state of equal sphere radii, equal sphere densities, and zero Brownian relative diffusivity.

Consideration of the effect of relative diffusion on the pair-distribution function shows the existence of an effective interactive force between the two particles and consequently a contribution to the mean velocity of the particles of each species. The direct contributions to the mean velocity of particles of one species due to Brownian diffusion and to the interparticle force are non-zero whenever the pair-distribution function is non-isotropic, that is, at all except large values of the Péclet number.

The forms taken by the expression for the mean velocity of the particles of one species in the various cases listed above are examined. Numerical values will be presented in Part 2.

Type
Research Article
Copyright
© 1982 Cambridge University Press

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Adler, P. M. 1981 Interaction of unequal spheres. I. Hydrodynamic interaction. Colloidal forces. J. Colloid Interface Sci. 84, 461474.Google Scholar
Batchelor, G. K. 1972 Sedimentation in a dilute dispersion of spheres. J. Fluid Mech. 52, 245268.Google Scholar
Batchelor, G. K. 1976 Brownian diffusion of particles with hydrodynamic interaction. J. Fluid Mech. 74, 129.Google Scholar
Batchelor, G. K. 1977 The effect of Brownian motion on the bulk stress in a suspension of spherical particles. J. Fluid Mech. 83, 97117.Google Scholar
Batchelor, G. K. 1982 Diffusion in a polydisperse system. J. Fluid Mech. (submitted).
Batchelor, G. K. & Green, J. T. 1972 The determination of the bulk stress in a suspension of spherical particles to order c2. J. Fluid Mech. 56, 401427.Google Scholar
Batchelor, G. K. & Wen, C.-S. 1982 Sedimentation in a dilute polydisperse system of interacting particles. Part 2. Numerical results. J. Fluid Mech. (in the press).
Dickinson, E. 1980 Sedimentation of interacting colloidal particles. J. Colloid Interface Sci. 73, 578581.Google Scholar
Feuillebois, F. 1980 Certain problemes d’écoulements mixtes fluide-particules solides. Doctoral dissertation presented to Paris VI University.
Haber, S. & Hetsroni, G. 1981 Sedimentation in a dilute dispersion of small drops of various sizes. J. Colloid Interface Sci. 79, 5675.Google Scholar
Jeffrey, D. J. 1982 Two unequal rigid spheres in low-Reynolds-number flow. Part 3. The mobility functions. J. Fluid Mech. (submitted).
Peterson, J. M. & Fixman, M. 1963 Viscosity of polymer solutions. J. Chem. Phys. 39, 25162523.Google Scholar
Reed, C. C. & Anderson, J. L. 1976 Analysis of sedimentation velocity in terms of binary particle interactions. Article in Colloid and Interface Science, vol. IV, ed. M. Kerker. Academic Press.
Wacholder, E. & Sather, N. F. 1974 The hydrodynamic interaction of two unequal spheres moving under gravity through quiescent viscous fluid. J. Fluid Mech. 65, 417437.Google Scholar