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

TL;DR

This study measures sedimentation profiles in charged colloids, confirming theoretical predictions of extended profiles due to electrostatic effects and demonstrating the applicability of mean field models even at high concentrations.

Contribution

The paper experimentally verifies a recent theory predicting extended sedimentation profiles in charged colloids caused by long-range electrostatic interactions.

Findings

Gravitational length matches hard-sphere expectations.

Extended profiles are well-described by the theory.

Mean field hydrostatic equilibrium applies even at high concentrations.

Abstract

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 [R.van Roij, J. Phys. Cond. Mat. 15, S3569, (2003)], 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 that 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 that 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 modeled explicitly, play a considerable role.