Project ended 1991, report dated January 1992
EXECUTIVE SUMMARY
The long term aim of the IFPRI projects on suspension flow remains: “to be able to predict and manipulate the flow behaviour of suspensions. Within this framework, the general objective of this project is the development of a qualitative insight and possibly quantitative predictions for the rheological behaviour of stable colloidal suspensions.
The present project is the final stage of a series of IFPRI projects which dealt with the same objectives. The general approach has been to generate experimental results on well defined model systems in order to identify and possibly quantify the contributions from various phenomena to the rheology of stable colloidal auspen-sions. Specifically attention has been concentrated on three aspects, for which no satisfactory solution was found in the previous work:
- the effect of particle interaction forces (the repulsion forces which keep the particles from aggregating);
- the effect of particle size distribution;
- the onset of shear thickening, i.e. the limiting conditions beyond which flow becomes practically impossible.
Systematic measurements were performed on model colloidal systems in steady state and oscillatory shear flow. These systems consisted of a hard core on which a soft polymeric layer was grafted to keep the particles from aggregating. For monodisperse systems of spherical particles general expressions can be found for:
- the concentration dependence of the low and high shear Newtonian viscosities;
- the shape of the viscosity shear rate curve.
The concentration dependence uses the maximum packing as the only empirical parameter. This packing fraction depends on the deformability of the stabilizer layer. It has been shown that this deformability can be related to theinterparticle repulsion caused by the stabilizer layer. The repulsion potential can be derived from measurements of the storage moduli in oscillatory flow. From the potential an effective hard sphere radius can be calculated which can then be used to obtain the viscosities from the known results for suspensions of Brownian hard spheres.
It could also be shown that the shear thinning region shifts inversely proportional to the zero shear viscosity. The concentration dependence of the latter is closely related to that of the mobility and the diffusivity of the particles. For that reason it is also found to correlate with the relaxation times derived from oscillatory experiments. The various data reduction schemes and scaling principles which are now available make it possible to characterize a wide range of suspensions with a limited number of experiments, In addition the rheology can be related to colloidal properties of the systems under investigation.
Qualitative insight is provided for the possible effect of slight deviations from monodispersity. In order to understand large degrees of polydispersity, bimodal distributions containing large and small colloidal particles were prepared. A procedure is suggested to calculate the low and high shear Newtonian viscosities. It is based on the computation of an equivalent maximum packing which takes into account the polydispersity as well as absolute particle size effects caused by stabilizer deformability and Brownian motion. The procedure describes the available measurements well.
At high volume fractions the viscosity starts to rise at a given shear rate (shear thickening), limiting the application conditions for such materials. Two different kinds of shear thickening could be distinguished, respectively showing a gradual and a sudden increase in viscosity. From rheological and rheooptical measurements it is concluded that the sudden increase in viscosity is associated with the formation of large, hydrodynamic aggregates. Particle inertia, as expressed in the particle Reynolds number, does not provide a suitable scaling for shear thickening. The effect of volume fraction on the onset of this phenomenon is similar to that for the fluidity (inverse of viscosity). A reduction in particle size shifts the onset to higher shear rates.
In conclusion it can be said a consistent picture has been obtained of the rheology of polymerically stabilized colloidal suspensions. The available empirical relations and scaling principles make it possible to correlate various experiments and to predict viscosities of various materials from a small number of experiments. The correlation with colloidal properties provides the necessary insight in the contributions from various phenomena and provides a basis for a rational manipulation of the suspension rheology and for their formulation.