Denser fluids of charge-stabilized colloids form denser sediments – Pilkhaz Nanikashvili, Alexander V Butenko, Shir R Liber, David Zitoun and Eli Sloutskin

congratulations to Pilkhaz Nanikashvili,   Alexander V Butenko,   Shir R Liber,   David Zitoun and   Eli Sloutskin 

to the article


Granular matter, where solid-like elasticity emerges in absence of crystalline order, has been actively studied in the last 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 φj in these sediments increases linearly with that in the initial fluid φ0, setting an upper limit φRCP≈0.64 on both φj and φ0, where φRCP coincides with the well-known, yet highly controversial, ‘random close packing’ density of spheres, providing a new insight into the physics of granular packings. The observed φj0) dependence is similar to the one recently reported for colloidal hard spheres, sterically-stabilized by surface-linked polymer combs. However, the lower limit on sediment densities drops to φj ≈ 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 φj0) dependence, bringing φj(0) close to its value in frictionless systems.