Effects of polydispersity on osmotic properties of colloidal suspensions

Abstract

The effects of varying degrees of polydispersity on the osmotic equilibria of colloidal suspensions have been investigated by computer simulations for both two- and three-dimensional systems. The consequent effects on shear-flow properties have been examined (for 3D systems) by steady-state dynamical simulations. The model effective pair potential is of the colloidal soft-sphere form Phiij = ε(2rijσi + σj - 1)-n. Polydispersity was fixed by normal distributions of particle core diameters (σ); results for standard deviations of 30 and 50% in two dimensions and 5 and 10% in three dimensions are reported. At constant core volume fraction the effect of polydispersity on the fluid osmotic pressure is very slight in two dimensions but causes a large significant reduction of the osmotic pressure in three dimensions. In both cases, the effect of small degrees of polydispersity is to substantially increase the osmotic pressure and to destabilise the ordered phases at high volume fractions. The effects of polydispersity on shear flow are obtained as an extension of simulation and scaling methods for the monodisperse case. For the isokinetic shear-flow model flow curves, polydispersity increasingly reduces the shear-thinning effect at low shear rates and drastically enhances the (kinetic) shear-thickening effect at high shear rates. For real laboratory suspensions, scaling shows that the net effect on the critical shear-thickening disordering transition is to advance to lower shear rates the point at which it occurs. These effects have been seen experimentally and can be explained in terms of the destabilisation of ordered arrangements arising from geometric packing effects.