Executive Summary
This work deals with predicting the flow properties of polymerically (sterically) sta- bilized colloidal suspensions, In the second year of the present project work has been performed in the three sub-areas which are addressed in the project objectives:
- describing the effect of stabilizer layer deformability (or “softness”) on the rheological properties;
- elucidating the effect of bimodal distributions of particle size;
- predicting the onset of shear thickening.
The work on the stabiliser layer softness consisted mainly of evaluating scaling principles. Earlier work by Goddard suggested the use of a reduced volume fraction (dividing the effective volume fraction by the maximum packing) to describe soft particles. This procedure does not work very well for really soft particles but is shown here to give reasonable results for polymerically stabilized systems. A similar procedure is found to be useful to reduce the data for the characteristic shear stress that determines the flow conditions at which shear thinning occurs. It also could be shown that these conditions could be characterized more accurately by using shear rate rather than shear stress (i-e replacing the medium viscosity in the Peclet number by the low shear Newtonian viscosity rather than by the viscosity pertaining to the shear rate under consideration). This characteristic shear rate turns out to be quite independent of concentration and is closely related to the long time diffusion coefficient.
For bimodal distributions in particle size, systems are considered in which a relatively coarse fraction is mixed with small particles. A procedure is suggested to predict the limiting Newtonian viscosities. Its suitability is confirmed by comparing predicted and measured values for mixtures with various ratios of particles with 823 and 129 nm diameter.
The onset of shear thickening has been investigated for systems containing particles of a fixed particle size but changing the other parameters. In particular the effect of the medium viscosity has been studied by changing the nature of the me- dium as well as the temperature. Two types of shear thickening could be identified. The first one is characterized by a gradual increase in viscosity with shear rate while the flow remains smooth. In the second one a sudden change in viscosity can be observed together with the onset of erratic structural changes during flow. A particle Reynolds number does not provide an adequate scaling for the critical shear rate of shear thickening. A scaling related to a kind of Peclet number reduces the data much better.