Extended sedimentation profiles in charged colloids: the gravitational length, entropy, andelectrostatics

Abstract

We have measured equilibrium sedimentation profiles in a colloidal model system withconfocal microscopy. By tuning the interactions, we have determined the gravitationallength in the limit of hard-sphere-like interactions, and using the same particles tested arecent theory (van Roij 2003 J. Phys.: Condens. Matter 15 S3569), which predicts asignificantly extended sedimentation profile in the case of charged colloids withlong-ranged repulsions, due to a spontaneously formed macroscopic electric field. For thehard-sphere-like system we find the gravitational length matches that expected. By tuningthe buoyancy of the colloidal particles we have shown that a mean field hydrostaticequilibrium description even appears to hold in the case where the colloid volume fractionchanges significantly on the length scale of the particle size. The extended sedimentationprofiles of the colloids with long-ranged repulsions are well described by theory.Surprisingly, the theory even seems to hold at concentrations where interactionsbetween the colloids, which are not modelled explicitly, play a considerable role.