Executive Summary
This is the final report of the 1994-1997 IFPRI Project on “Reversibly Flocculated Suspensions” at the K.U.Leuven, Belgium. It is an extension of an earlier project of the period 1991-1994. It aims at understanding the flow properties of collodial suspensions that are flocculated at rest but can be deflocculated during flow. This should provide support for predicting properties and formulating colloidal suspensions with controlled flow properties.
Two parts can be distinguished in this project. The first deals with the analysis of the steady state viscosities of weakly flocculated suspensions with controlled stability parameters. Reversibly flocculated suspensions often display a time-dependent vis-cosi ty, a phenomenon called “thixotropy” . This phenomenon is poorly understood, It is the purpose of the second part of this project to attempt to identify and possibly quantify the role of the major factors governing thixotropy.
For the first part suitable model suspensions have been developed by starting from sterically stabilized systems in which the stability is gradually reduced. This can be done by using media that are poorer solvents for the stabilizer molecules or by changing the temperature. Also particles of different sizes have been used. Measurements were performed during steady state shear flow and during oscillatory flow. As often in real systems the stability parameters of the present materials were not known. Viscosity measurements on dilute systems have been used to deduce these parameters. This turns out to be a possible route but the accuracy of the results is limited.
With the available stability parameters it was attempted to correlate the viscosity curves. Qualitative relations could be obtained but no quantitative predictions. It is however very difficult to obtain reliable steady state date on the systems under consideration. Lacking suitable predictions the possibility of using scaling and data reduction schemes was investigated. For systems with a yield stress, common in case of flocculation, such a scheme was found for superimposing data from different temperatures. Also a scaling for concentration was found, using the storage modulus as a scaling parameter for the stress. It turns out that this relation works because of a quite general relation between storage moduli and yield stress.
Flocculated suspensions become gel-like at relatively small particle concentrations. This gel point can easily be measured and can be related to material and structural parameters. Some general theories about gelation can be applied to flocculated suspensions. An interesting model from the literature, developed for suspensions, is applied to the data. It is based on a two-level structure, each with its own fractal structure. Further investigations have to verify the general usefulness of this approach or alternatively the need to develop more complex models for the microstructure in order to predict flow properties on the basis of colloidal parameters.
It turns out that there is no suitable general theoretical framework for thixotropy, the subject of the second part of the project. None of the existing models was found to describe adequately any detailed set of experimental results. Therefore it was decided to concentrate on detecting general patterns in real thixotropic systems. These could then be used to guide further theoretical work. Various suspensions of carbon black and fumed silica were used for this purpose. They were submitted to stepwise changes in shear rate of shear stress to analyse the resulting transient response. In this manner some general trends could be identified.
A stepwise decrease in shear rate has been used to probe the structural recovery or build-up of the samples. The resulting viscosity-time curves could all be fitted by two expressions. They provide means to express thixotropic experiments in a compact manner. The build-up rate changes with shear rate in such a manner that the curves nearly superpose when plotted versus strain. The initial rate of change of the viscosity is proportional to that viscosity. Breakdown experiments follow a somewhat different pattern, the viscosity change with time should be substituted by that with strain.
The rate characteristics derived from fitting the transients to a suitable equation, e.g. a stretched exponential, obey a general scaling procedure that has been empirically derived. This seems to be applicable over a wide range of materials. Detailed thixotropy models are still lacking. Comparing rheological and dielectric transient responses indicate the complexity of the underlying structural changes. These can be qualitatively explained but quantitative solutions are still lacking. This could be the subject of future work. As in the case of the steady state behaviour of weakly flocculated systems, it might be necessary to investigate first in more detail the complex nature of the flow-induced microstructure.