Within the framework of the “SuspensionFlow”Project, the purpose of the present work is to predict the rheological properties of stable colloidal suspensions, in particular , polymerically (sterically) stabilized systems. This kind of stabilization can be used in ayueotis as well as in non-aqueous media.
At this stage three particular items of the said materials are under investigation. Firstly, we try to describe the “softness” effect, caused by the deformability of the stabilizer layer. From earlier work, data on three different particle sizes (i.e. degrees of softness) arc already available. They are now being supplemented with data on dispersions with intermediate softness.
This will allow an evaluation and eventually an improvement of the available theoretical approach and the available scaling methods. The experiments include viscosity measurements over a wide range of shear rates. The data support the validity of a theoretical approach used before. In addition the dynamic moduli are measured at various frequencies to characterize experimentally the particle interaction or the stabilizer layer softness. The second item concerns polydispersity. The earlier data have been taken on monodisperse systems. Some data on bimodal systems are available from the previous research period. These have now been supplemented with data on different particle sizes and different diameter ratios. A rule for estirnating the mixing ratio at which the viscosity reaches a minimum is suggested. Differences with the behaviour of bimodal systems of non-colloidal particles are, demonstrated and simple mixing rules for describing the non-Newtonian flow regime are shown to be inadequate.
Finally, the phenomenon of shear thickening or dilatancy (increase in viscosity with shear rate) has been investigated. On shear rate-controlled devices the sample often fractures at the dilatancy threshold. On a stress controlled device the shear rate suddenly drops at a critical value of the shear stress to remain constant at still higher shear stresses. In this latter regime the response is very irregular, indicating structural heterogeneities Near the critical condition liysteresis is observed as well as occasional jumps in shear. rate between the two extreme values. These are attributed to instabilities in microstructure. Scaling rations for the onset of dilatancy will be investigated.