This is the third annual report of the IFPRI SUSPENSION RHEOLOGY project 1991-1994 at K.U.Leuven(Belgium), As IFPRI decided to continue this project, dealing with the flow of reversibly flocculated suspensions, an annual report was re- quested at this stage rather than a final report. The project deals specifically with understanding and predicting the flow properties of suspensions which are flocculated at rest but can be deflocculated during flow.
During the two previous years rheological and dielectric measurements were performed on suitable dispersions to clarify the relation between flow and flow-induced microstructure in reversibly flocculated dispersions. During the third year the work focused on the rheological measurements, including the collection of data on the sys- tems which have been used earlier for dielectric measurements.
The rheological work has been divided in two parts. A first part deals with the stationary viscosities of well defined model systems. The results should be useful in evaluating theoretical approaches and scaling laws. Earlier, fragmentary, data have now been supplemented with data on two additional particle sizes, with volume fraction and temperature as parameters. The two sizes provide consistent results which, however, deviate from the earlier data as far as temperature effects are concerned. This discrepancy will be investigated further next year.
The second part of the rheological work is concerned with time-dependent phenomena or “thixotropy”, another important but complex feature of reversibly flocculated dispersions. The effect of temperature has been found to be a result of hydrodynamic effects and interparticle forces. Particle concentration increases the rate of both structural breakdown and recovery during flow. This underlines the effect of particle collisions on these rates. This result has been confirmed by a similar incrtzse in the rate of structural changes when the shear rate is increased. The stretched exponentional has been found to describe the data adequately. Some empirical data reduction schemes are suggested and will be investigated further in the coming year.