Rheology of weakly interacting colloidal particles at high concentration

Abstract

The paper describes an experimental study of the rheology of concentrated latex under conditions where a secondary miminum in the interparticle pair potential is expected. Particular attention is given to the Bingham stress σ_B derived from steady-shear-flow data, which is taken as a measure of the additional or extra stress arising from the interparticle attraction. The data are used to deduce a correlation describing the dependence of the Bingham stress on the strength of attraction, S, the latter being taken a proportional to the calculated depth of the secondary minimum, with the result σ_B=kf(ϕ)S^1.9d^–2, where k is a constant and d is the particle diameter. A weak attraction between colloidal particles can also be induced by the addition of non-adsorbing polymer. Data for systems of this type are analysed similarly and shown to support this dependence. Earlier data illustrating the effect of particle size and concentration on the storage modulus are also discussed. The dependence upon volume fraction is shown to be very similar to that of the Bingham stress and the variation of both properties is described well by an equation deduced from liquid-state theory on the assumption that the attractive forces do not greatly perturb the structure, this reads G_∞, σ_B∝(4ϕ²+ 2ϕ³–ϕ⁴)/(1 –ϕ)³. It is also shown, however, that for volume fractions ϕ > 0.2 or so this is virtually indistinguishable from the concentration dependence of the modulus observed for particulate gels formed by coagulation in the primary minimum. It would thus appear that the concentration dependence of the modulus is very insensitive to local structure and the strength of the attraction. In contrast, this is not the case for the particle-size dependence. The effect of attractive forces on the low-shear and high-shear viscosity coefficients is also described.