Observation of a shape-dependent density maximum in random packings and glasses ofcolloidal silica ellipsoids

Abstract

We have measured the random packing density of monodisperse colloidal silica ellipsoidswith a well-defined shape, gradually deviating from a sphere shape up to prolates withaspect ratios of about 5, to find for a colloidal system the first experimental observationfor the density maximum (at an aspect ratio near 1.6) previously found onlyin computer simulations of granular packings. Confocal microscopy of ellipsoidpackings, prepared by rapidly quenching ellipsoid fluids via ultra-centrifugation,demonstrates the absence of orientational order and yields pair correlation functionsvery much like those for random sphere packings. The density maximum, about12% above the Bernal random sphere packing density, also manifests itself as a maximum in thehydrodynamic friction that resists the swelling osmotic pressure of the ellipsoid packings.The existence of the density maximum is also predicted to strongly effect the dynamics ofcolloidal non-sphere glasses: slightly perturbing the sphere shape in a sphere glass willcause it to melt.