Time‐dependent behavior and wall slip in concentrated shear thickening dispersions

Abstract

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.

Viscoelastic properties of concentrated shear-thickening dispersions

“…These clusters can span the whole gap and give rise to flow blockage and higher values of G'. In another study (22) we showed that in steady shear flow the presence of large clusters and plug-flow in the gap could be visualized using a photographic technique.…”

“…Measurements were made as quickly as possible. This means that a value for the torque at a (22). After the measuring system was filled, with care being taken not to include air in the system, the system was fitted into the rheometer.…”

“…At higher shear rates dependence of the torque on the size of the measuring geometry was detected. The resuits with the largest geometry were taken to be the most reliable (22).…”

Heterogeneous Multiscale Simulations of Suspension Flow

“…Some experiments on real suspensions suggest that the shear-thickening regime is again followed by a shear-thinning region [1], [14]. On the contrary, it can be argued that in experiments on real suspensions, the influence of the experimental setup and additional forces between the particles cannot be absolutely excluded in most cases [15], [17], [18], [19]. Also in our simulations we see a continued increase of ν app to the highest-shear rates allowed by the methods we used [20] in contrast to comparable simulations that resemble more the experimental setup of real viscometers [21], [22], [23].…”

“…For ϕ > ϕ c , a sudden increase in ν app ðγ : Þ can be observed. In the literature, values for ϕ c vary with suspension type and particle size distribution between 0.52 ≤ ϕ c ≤ 0.575 [13], [14], [15]. In the present work, we concentrate on 2D suspensions at global volume fractions ϕ ¼ 0.3 and ϕ ¼ 0.5, in which the local ϕðxÞ was apparently well below a critical ϕ ð2DÞ c not known to us.…”