Contact numbers and radial distributions in suspensions of smooth and rough colloids

The contact number distribution for hard sphere suspensions is a measure of nearest neighbor interactions between colloidal particles. Many experimental studies in the literature uses the primary minima of the radial distribution function, g(r), to estimate the contact number distribution. Our experimental data show that this method overestimates the mean contact number, <z>. Instead, we investigate a different method to estimate the contact distance by extrapolating contact values of different volume fractions (f) to jamming point at random close packing (RCP). For a 3D suspension of smooth spheres, f_RCP ~ 0.64 with <z> ~ 6. For rough colloids, the added friction is expected to generate a reduced f_RCP and <z>. We hypothesize that these values of <z> at RCP could be extrapolated to better understand particle contacts at lower values of f. To test this hypothesis, we synthesized sterically stabilized, fluorescent poly(methyl methacrylate) colloids with smooth and rough surface morphologies. Confocal microscopy imaging and processing shows that the g(r) of the suspensions agree well with the Orstein-Zernicke equation for hard spheres. Depending on the stabilizer length and roughness, we found that the contact distance ranges between 1 to 1.1 times the particle diameter.