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

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