EXECUTIVE SUMMARY
The long term objectives of the Suspension Rheology project have been formulated as: "TO be able to predict and manipulate the flow behaviour of disperse systems from an understanding of their chemistry and composition." A complete theoretical solution of this problem is not expected in the near future. The approach which has been taken here consists in developing suitable characterization techniques and applying them to specifically designed model systems. In this manner the contributions from the various parameters can be isolated and evaluated. Applying suitable scaling principles and data reduction schemes efficient strategies can then be developed to predict the flow behaviour of real systems on the basis of a small number of experiments.
Two specific problems have been addressed in the present project. They refer respectively to stable colloidal suspensions and reversibly flocculated systems. Sterically stabilized suspensions are used for the first case. The possible applications of hard sphere scaling have been investigated in the previous period. The work has now been extended to include a procedure to handle the softness of the stabilizer barrier in monodisperse systems. This includes experimental techniques to characterize the deformability of the stabilizer barrier. The results agree quite well with a first theoretical estimate. As a next step the problem of particle size distribution has been tackled. A first series of data has become available. They indicate that the various characteristics of the viscosity curves are affected differently by size distribution. The results suggest some trends which will require confirmation from further experiments.
The rheology of reversibly flocculated suspensions is still poorly understood. Experimental techniques for characterizing yielding and time-dependent behaviour (i.e. thixotropy) have been investigated. The suitability of constant stress devices for this purpose could be demonstrated. Yielding in weakly flocculated systems turns out to be a complex phenomenon. Structural rearrangements before the final yielding complicate the development of a theory. Some of the reversibly flocculated materials display a behaviour which resembles that of the strongly flocculated systems, which might provide a starting point for theories.
Measuring the concentration dependence of yielding and the equilibrium moduli is a suitable procedure to analyse structure in flocculated systems. Data on two systems with similar components but different particle interactions seem to be in good agreement with some of the theories. However, earlier data on more open floe structures obey a different concentration law. The discrepancy has not been solved yet.
No suitable data handling procedures for thixotropy are available. Data on two different systems indicate that the time scale of the stress transients can be related to the initial stress levels in step strain rate experiments. With a constant stress device a recovery time can be easily determined from step stress experiments with a final stress level below the yield stress. This leads to a suitable technique to characterize thixotropic recovery.
It can be concluded that:
1) for sterically stabilized suspensions procedures have been suggested to take into account the presence of a deformable stabilizer barrier and a start has been made with the analysis of the effect of particle size distribution; 2) for reversibly flocculated systems the suitability of some experimental characterization methods for yielding and thixotropy has been demonstrated, it is shown that the combined measurement of yielding and equilibrium modulus as a function of concentration provides insight in the floe structure.