This is the second annual report of the IFPRI Suspension Rheology project 1991-1994 at the K.U.Leuven (Belgium). Whereas the two previous projects at K.U.Leuven dealt with the flow properties of stable colloidal dispersions, the present one concentrates on flocculated systems. In particular it is being attempted to generate some basic information on how to manipulate the complex rheology of reversibly flocculated dispersions. This latter term refers here to colloidal systems in which floes develop at rest, which however can be broken down reversibly during flow. Such systems occur frequently in materials processing operations.
During the first year a locally built dielectric device was used to probe the flow- induced changes in microstructure during flow and after stopping the flow, This led to the identification of a peculiar relaxational phenomenon. In addition a start was made with upgrading the instrument. This work has been continued and finished during the second year.
The application of the new set-up is demonstrated here for structure probing during flow and for following structural recovery after stopping the flow. Systematic measurements to study the relationship between structure and flow history are scheduled for the next year. In parallel with the dielectric approach, rheological experiments on reversibly flocculated dispersions are being performed. Work has been continued to develop suitable model systems, starting from sterically stabilized dispersions. By using suitable mixtures of suspending fluids and by adjusting the temperature as well, this has proved possible. Some preliminary rheological data are available and are being used to com- pare with the results of theoretical approaches based on square well potentials.
Thixotropy, time-dependent and reversible decrease of viscosity caused by flow, accompanies reversible flocculation. Non-aqueous dispersions of fumed silica have been formulated to study this phenomenon. Comparative transient measurements under constant stress and under constant shear rate are scheduled with these samples to elucidate the basic flow factors that govern the flow-induced structural changes.