Competition between sedimentation and phase coexistence of colloidal dispersions undergravity

Abstract

After a brief review of the theory of sedimentation equilibria of colloidal systems, weconsider the specific case of binary systems of hard sphere colloids and non-interactingpolymer coils, the latter of vanishing buoyancy mass. The density profiles of the twocomponents are calculated within density functional theory and using Monte Carlosimulations. Under appropriate conditions the profiles exhibit discontinuities orsteeply varying regions associated with the interface separating colloid-rich andcolloid-poor phases. The position of the interface is shown to be very sensitive tothe strength of the gravitational field and, more surprisingly, to the total heightL of the suspension. Phase coexistence in the absence of gravity isshown to be entirely suppressed beyond a critical ratio of the heightL over the gravitational length of the colloids.