This report covers the second year of the IFPRI Suspension Rheology Project 1985-88 at the K.U.Leuven. This project deals with the prediction,and manipulation of the rheological properties and the flow-induced structure in colloidal suspensions. It consists of two sub-projects. The first deals with colloidally stable systems, the second with weakly flocculated materials.
For the stable suspensions, the effect of particle size and that of stabilizer layer deformability is under investigation. Rheological measurements have been performed on suspensions, containing sterically stabilized PMMA particles of narrow size distribution and different particle sizes. This provides reference values for the projected measurements on systems with particle size distributions. In addition the data could be used to analyse the effect of stabilizer layer deformability on the rheology. It is concluded that this effect can be represented quite accurately by modifying two factors: the volume fraction at maximum packing and the factor expressing the shear stress dependence. The latter changes with concentration but it is shown that the shape of this curve hardly depends on softness. Quantitative results for the two parameters are given. Scaling relations are still under consideration.
Yielding was investigated, using silica suspensions. Their floe structure changes reversibly with shear. Yield stresses were measured with three different techniques. it is shown that, for some samples, identical results are obtained with the different techniques. However, in other cases deviations occur because of shear history effects during the measurement. Rational prediction of this phenomenon is not possible yet. For the silica under investigation yield stress and storage modulus change with concentration according to a power law. The data agree qualitatively with recent theoretical analyses.