Concentrated Colloidal Suspensions at Low Ionic Strength: A Hard-Sphere Model of Zero Shear Viscosity, Involving the Hard-Sphere Phase Transitions

The second electroviscous effect observed in charge-stabilized monodisperse colloidal suspensions as the ionic strength is lowered is tentatively predicted from a Hard-Sphere model of zero-shear viscosity η 0 which depends on an effective volume fraction, ϕ eff , directly related to the HS diameter d HS defined from Russel's HS approximation. The model involves the well-known HS phase transitions (ϕ 0 = 0.494 for "freezing" and ϕ D0 = 0.545 for "melting") through the following assumptions: i) as ϕ eff < ϕ 0 ( i.e. in the liquid phase) the maximum packing fraction ϕ m , involved in η 0 , remains close to ϕ RCP = 0.637, the random close packing, usually observed in experiments on rigid-sphere suspensions; ii) as ϕ 0 < ϕ eff < ϕ D0 ( i.e. in the coexisting phase domain) ϕ m decreases continuously from ϕ RCP to ϕ D0 . Model predictions are found in remarkable agreement with viscosity measurements (by C. Allain et al. , 1933) on a colloidal silica suspension (for which an almost complete knowledge of physico-chemical characteristics is available), especially the variations of the zero shear viscosity vs. the actual volume fraction of particles, at ionic strength varying from 0.001 to 0.1 Mol dm -3 NaCl.