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We have measured equilibrium sedimentation profiles in a colloidal modelsystem with confocal microscopy. By tuning the interactions, we have determinedthe gravitational length in the limit of hard-sphere-like interactions, andusing the same particles, tested a recent theory [R.van Roij, J. Phys. Cond.Mat. 15, S3569, (2003)], which predicts a significantly extended sedimentationprofile in the case of charged colloids with long-ranged repulsions, due to aspontaneously formed macroscopic electric field. For the hard-sphere-likesystem we find that the gravitational length matches that expected. By tuningthe buoyancy of the colloidal particles we have shown that a mean fieldhydrostatic equilibrium description even appears to hold in the case that thecolloid volume fraction changes significantly on the length scale of theparticle size. The extended sedimentation profiles of the colloids withlong-ranged repulsions are well-described by theory. Surprisingly, the theoryeven seems to hold at concentrations where interactions between the colloids,which are not modeled explicitly, play a considerable role

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