DENSITY PROFILES OF CONCENTRATED COLLOIDAL SUSPENSIONS IN SEDIMENTATION EQUILIBRIUM

Sedimentation equilibria in concentrated colloidal suspensions are stu died within the framework of density functional theory for inhomogeneo us fluids. The density profile in the gravitational field is calculate d exactly for the one-dimensional model of hard rods. The predictions of the local density approximation agree very well with coarse-grained density profiles derived from extensive Monte Carlo simulations of th ree-dimensional systems of hard spheres and charge-stabilized colloida l particles. The simulations show that the shape of the density profil es is very sensitive to the nature of the interactions between colloid al particles, and allow a direct test of a simple inversion procedure, which extracts the osmotic equation of state of colloidal suspensions from a measurement of their density profile. The method is sufficient ly sensitive to allow the crystal-fluid tie line to be located at high colloid concentrations. The local density approximation is generalize d to calculate density profiles of bidisperse hard sphere suspensions; the competition between gravity and entropy leads to nonmonotonic den sity profiles which are sensitive to the mass ratio, for a given size ratio. The density functional approach and Monte Carlo simulations are finally extended to study the effect of interparticle attractions on the density profiles, which exhibit a region of rapid variation corres ponding to the ''liquid-gas'' interface for sufficiently strong attrac tive interactions.