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 monodis perse colloidal suspensions as the ionic strength is lowered is tentat ively predicted from a Hard-Sphere model of zero-shear viscosity eta0 which depends on an effective volume fraction, phi(eff), directly rela ted to the HS diameter d(HS) defined from Russel's HS approximation. T he model involves the well-known HS phase transitions (phi(O) = 0.494 for <<freezing>> and phi(DO) = 0.545 for <<melting>>) through the foll owing assumptions: i) as phi(eff) < phi(O) (i.e. in the liquid phase) the maximum packing fraction phi(m), involved in eta0, remains close t o phi(RCP) = 0.637, the random close packing, usually observed in expe riments on rigid-sphere suspensions; ii) as phi(O) < phi(eff) < phi(DO ) (i.e. in the coexisting phase domain) phi(m) decreases continuously from phi(RCP) to phi(DO). Model predictions are found in remarkable ag reement with viscosity measurements (by C. Allain et al., 1933) on a c olloidal silica suspension (for which an almost complete knowledge of physico-chemical characteristics is available), especially the variati ons of the zero shear viscosity vs. the actual volume fraction of part icles, at ionic strength varying from 0.001 to 0.1 Mol dm-3 NaCl.