Suspension Rheology Project. Rheology of Reversibly Flocculated Suspensions

Publication Reference: 
Author Last Name: 
J Mewis, G Ourieva, J Schryvers
Report Type: 
ARR - Annual Report
Research Area: 
Wet Systems
Publication Year: 

Executive Summary

This is the second Annual Report of the 1994-1997 IFPRI Project on “Reversibly Flocculated Suspensions” at K.U.Leuven. It is an extension of a similar project dur- ing the period 1991-1994. The project aims at understanding and predicting the flow properties of colloidal suspensions that are flocculated at rest but can be deflocculated during flow.

Two parts can be distinguished in this project. The first deals with describing the viscosity curves of weakly flocculated suspensions with controlled colloidal stability parameters. Reversibly flocculated suspensions often display time-dependent viscosities or “thixotropy”. It is the purpose of the second parth of this project to attempt to identify and possibly quantify the role of the major factors governing thixotropy.

As far as the first part of the project is concerned, this second year was used to study suspensions based on two types of newly provided particles. They differ in particle size and in characteristics of the stabilizer layer. Firstly, stable suspensions of these particles have been studied in order to provide reference data with which to compare the data on reversibly flocculated systems with the same particles. Subsequently flocculation was very gradually induced by changing the medium as well as the temperature. Gelation temperatures were determined as a function of particle volume fraction, using dynamic moduli . These data were used to determine interaction parameters, which should be useful in relating and predicting viscosity functions. The rheologicaldata are being supplemented with small angle neutron scattering data on flowing systems. This provides information about the structure and the interaction parameters of the suspension. Preliminary results indicate the usefulness of this approach. More systematic data of this nature are being collected.

As for the second part, the available data have been extended in different ways. The scaling relations for sudden decreases in shear rate, i.e. for structure build-up, which had been suggested earlier have been confirmed by data on another system. In addition the kinetics of structure breakdown after a sudden increase in shear rate have been studied. It is shown that, not surprisingly, breakdown follows somewhat different rules than build-up.

Still, a strain scaling seems to hold here as well and the relation of the characteristic times with the initial stress is in line with that for build-up. Build-up was now also followed after stopping the flow by using dynamic moduli. It has been found that this aspect of behaviour can be quite different in different materials. Some dispersions develop a threedimensional particulate network nearly immediately after stopping the flow, others remain liquid-like while the moduli in- crease.

Finally, some thixotropic systems have been investigated by means of a dielectric technique. This should shed some light on the underlying structural behaviour. Transient dielectric data after stopping the flow are being generated. They are still under investigation.