The Rheological Properties of Concentrated Suspensions. III. Dynamic Properties and Their Correlations with Stationary Flow Properties

Abstract

Abstract The dynamic properties and stationary flow properties of suspensions of barium sulfate in polyisobutylene and in linseed oil have been measured by the use of an electrically-recording rotational rheometer. The concentration dependence, the influence of mixing conditions, the effects of the addition of water and of sodium stearate on the dynamic viscosity and the dynamic modulus, and the correlation of the dynamic properties with stationary flow properties have been examined experimentally. In suspensions of barium sulfate in linseed oil, the dynamic viscosity and dynamic modulus increase monotonously with an increasing concentration, and no critical concentration is observed up to the highest concentration. The apparent relaxation time shows a slight increase with an increase in the concentration of particles. In suspensions of barium sulfate in polyisobutylene, the dynamic viscosity and dynamic modulus increase abruptly above a critical concentration. The apparent relaxation time also increases remarkably above the critical concentration. The mean free energy of the activation for breaking links between particles and the mean number of links to be broken for the separation of two neighboring particles have been evaluated as 33∼34 kT and 1.49 respectively. The influence of additives on dynamic properties resembles that on stationary flow properties and can be explained qualitatively by assuming a van der Waals-London attraction between particles. However, in a viscoelastic medium, contractive forces arising from the elastic property of the medium seems to be altered by the addition of additives. For pseudo-plastic systems, the dynamic viscosity coincides with the apparent viscosity at lower frequencies. However, for plastic systems, the apparent viscosity may be higher or lower than the dynamic viscosity at lower frequencies, depending on the sensitivity of the network structures formed by particles and on the time necessary for the restoration of broken structures. The dynamic modulus at lower frequencies increases with an increase in the yield value, while the former increases proportionally to the latter at higher concentrations also. The non-linearity of dynamic properties has been observed as a feature of their amplitude dependence. The change in the dynamic modulus is more remarkable than that in the dynamic viscosity. The experimental results described above at least qualitatively agree with the theoretical predictions based on a network model.