Executive Summary
This is the first annual report of the IFPRI Suspension Rheology project 1991-1994 at the Katholieke Universiteit Leuven (Belgium). Whereas the previous projects here dealt with the flow properties of stable colloidal dispersions, the present concentrates on flocculated systems. In particular it will be attempted to generate some basic information on how to manipulate the rheology of “reversibly flocczllated systems”. The latter term refers here to colloidal dispersions in which the floes can be broken down reversibly by flow. Such systems occur frequently in materials processing operations.
During this first year an existing device for probing floe structure during flow with dielectric spectroscopy has been further developed. It is now being computerized, also the measuring range haa been extended to make the technique applicable to a wider range of materials. Meanwhile some exploratory dielectric work has been started on a particular dispersion, consisting of carbon black in mineral oil, to investigate the potential of the technique for less structurized systems than those used before. Time and frequency domain measurements have been combined to cover a larger part of the dielectric spectrum. In this manner a low frequency resonance between floe rotation and dielectric polarization has been identified. In addition a power law region could be detected at high frequencies. The effects of both shear rate and temperature have been studied. A general theory to calculate structural characteristics from the dielectric measurements is still lacking. A preliminary discussion of possible structural interpretations is given.
It is concluded that dielectric spectroscopy on flowing dispersions can be a quite sensitive techique to probe the changing floe structure during flow. The potential use of the technique developed here has been demonstrated. Possible experimen- tal and data handling prcr,edures are suggested and have been applied on a given dispersion. Awaiting a suitable model for the dielectric behaviour, the information is mainly qualitative but even that is extremely helpful while studying the rheological consequences of the variable structure.