Equilibrium properties of reversibly flocculated dispersions

The structure of sonic flocculated dispersions can be changed reversibly by means of shearing. Often the changes are not instantaneous. The resulting shear-history effect gives rise to a complex but interesting rheological behaviour. Using non-aqueous suspensions of fumed silica, the rheological equilibrium properties of such systems are investigated. To change the floc structure, the water content of the particles is altered. As well as the steady-state shear viscosity, the equilibrium modulus and the yield stress are measured. Various techniques are compared. The effect of concentration on the equilibrium properties is used to test some structural models. The concentration dependence is best described by a power-law relation, the power being identical for modulus and yield stress. These results compare well with some theoretical predictions. Contrary to the assumptions used in the modelling, the yield stress is often dominated by kinetic phenomena. This shortcoming also shows up in the predictions for the critical strain.