Statistical properties of a sedimenting suspension and the point particle approximation.

For a sedimenting suspension of hard sphere in a viscous liquid, bulk properties (such as average sedimentation speed), are sensitive to the statistics of the particle positions. Since hydrodynamic interaction of two spheres does not change their relative positions, determination of particle statistics must involve interactions of at least three particles. The results of interactions of three point particles to approximately determine the two particle distribution function are used. The results are valid only for interparticle distances much larger than a particle radius and much smaller than the average interparticle distance.