T. Biben et al., DENSITY PROFILES OF CONCENTRATED COLLOIDAL SUSPENSIONS IN SEDIMENTATION EQUILIBRIUM, The Journal of chemical physics, 98(9), 1993, pp. 7330-7344
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.