A new criterion is introduced for the onset of shear thickening in a concentrated dispersion. The criterion follows from the assumption that shear thickening occurs when the shear forces overrule the interparticle forces. A force balance at small interparticle distances is used to predict the dependence of this critical shear rate on the volume fraction. It is deduced that the critical shear rate is proportional to the interparticle distance, has a linear dependence on the magnitude of the stabilizing force, an inverse linear dependence on the dispersion medium viscosity, and an inverse linear dependence on the particle radius. The model is confirmed experimentally with viscosity measurements on various electrostatically stabilized dispersions. The validity of the model is also checked with data obtained from the literature. Experiments indicate that polydisperse dispersions exhibit pronounced shear thickening but with a less dramatic increase in viscosity than monodisperse dispersions.

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SynopsisThe viscosity of concentrated shear thickening dispersions was measured as a function of shear rate, Couette cylinder size, and time. The level of the low shear rate viscosity, which was found to be independent of system size and time, could bc predicted by the equation of Frankel and Acrivos. At shear rates above the critical shear rate for shear thickening in highly concentrated (#> 0.57) dispersions of monodispcrse particles strong viscosity instabilities were detected, together with a dependence on cylinder size. The instabilities are attributed to reversible order-disorder transitions, e.g., from strings to clusters. This dependence on cylinder size is due to wall slip, slipping planes in the dispersion, and even plug flow in the gap. With less concentrated or polydisperse dispersions the effects are much less severe but there is thixotropy, probably due to a reordering of the dispersion.

SynopsisA new, theoretically more satisfactory, definition for relative viscosities of suspensions in generalized Newtonian media is presented, which according to the viscosities of suspensions and pure liquids should be compared to equal aver--aged squared strain rates 3 in the liquid phases. value of 2.5 over the whole range of n. According to the other definitions, KE is 2.5 only for n = 1; with n decreasing to zero, KE decreases linearly to 0.75 (n, under equal strain rate) or rises exponentially to m (7,. under equal strain stress). The values of KE, as deduced from experiments, obey the theoretical interrelations derived. There are strong indications that the value of n has no influence on the inhomogeneity in rate of strain in dilute suspensions.

The zeta-potential of monodisperse polystyrene latices as a function of ionic strength is reported. Latices exhibit anomalous electrokinetic behavior:  instead of the expected decrease in mobility with increasing electrolyte concentration, a maximum occurs. In this paper it is shown that both in electrophoresis of dilute dispersions and in electroomosis of concentrated plugs, the zeta-potential as a function of ionic strength passes through approximately the same maximum at about the same electrolyte concentration. After the correction for the influence of conductance by ions in the diffuse electrical double layer, using O'Briens model, still a slight maximum exists. The results provided an estimate of the Stern layer thickness. An analysis using the hairy layer model, when surface conductance was neglected, provided an estimate of the hairy layer thickness.