Denser fluids of charge-stabilized colloids form denser sediments

Granular matter, where solid-like elasticity emerges in the absence of crystalline order, has been actively studied over the last few decades, targeting fundamental physical understanding of granular packings and glasses, abundant in everyday life and technology. We employ charge-stabilized sub-micron particles in a solvent, known as colloids, to form granular packings through a well-controlled process, where initially homogeneous and thermodynamically equilibrated colloidal fluids form solid sediments, when subjected to an effective gravity in a centrifuge. We demonstrate that particles' volume fraction phi(j) in these sediments increases linearly with that in the initial fluid phi(0), setting an upper limit phi(RCP) approximate to 0.64 on both phi(j) and phi(0), where phi(RCP) coincides with the well-known, yet highly controversial, 'random close packing' density of spheres, providing new insight into the physics of granular packings. The observed phi(j)(phi(0)) dependence is similar to the one recently reported for colloidal hard spheres, sterically stabilized by surface-linked polymer combs (S. R. Liber, et al., Proc. Natl. Acad. Sci. U. S. A., 2013, 110, 5769-5773). However, the lower limit on sediment densities drops to phi(j) approximate to 0.49 in the present work, suggesting that sedimented charge-stabilized silica are able to overcome mutual electrostatic repulsions, forming gel-like structures stabilized by occasional van der Waals contacts. Finally, by introducing particle size polydispersity, which significantly modifies fluid structure and sedimentation dynamics, we almost completely diminish the phi(j)(phi(0)) dependence, bringing phi(j)(0) close to its value in frictionless systems.