The effect of surface friction on the rheology of hard-sphere colloids

Abstract

Intuition plus experimental evidence indicate that an essential, but missing, ingredient from all previous theoretical and simulation models of experimental idealised hard-sphere monodisperse suspensions is surface friction. The modelling of real, i.e., rough, spheres requires at least one additional system parameter, to account for contact rotational friction, in thedimensional analysis leading to corresponding states scaling laws.

Idealised latex particles with variable surface roughness have been produced by a dispersion polymerisation in which the molecular weight of the polymeric steric surface stabilisers is varied. A degree of surface roughness of the dry particles is characterised experimentally using an argon adsorption technique. This differential adsorption technique also measures inhomogeneities in surface roughness, on a particular distance scale, “holes and crevies”, which arise from the uptake of the PVP steric stabiliser on the surface. This technique, however, does not discriminate interparticle roughness. Suspended particles show a solvation effect whereupon the higher the molecular weight of the steric stabiliser, the greater the surface friction.

Controlled stress rheological flow curves for three suspensions differing only in the extent of surface roughness are reported. The results show a substantial increase in yield stresses and apparent viscosities of dense suspensions with surface roughness.