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Dispersions showing a high concentration of solid matter or gel-like particles; ceramic suspensions, starch dispersions, paper coatings, plastisol pastes (containing not enough plasticizer), natural rubber (NR), highly filled elastomers (such as polybutadiene or butyl rubber BR), dental composites, shock-resistant “smart fluids”

The terms shear-thickening and dilatant are identical in their meaning; sometimes the terms shear-hardening, shear-stiffening or solidifying can be heard. The Note on the term “apparent viscosity” of Chapter 3.3.2 also applies here.

Problems with flow processes should always be taken into account when working with shear-thickening materials. Flow should be observed carefully for the occurrence of wall-slip effects and separation of the material, e. g. on surfaces of measuring geometries, along pipeline walls or between individual layers of the sample. This can be investigated by repeating the test several times under identical measuring conditions, comparing the results with regards to reproducibility.

For dispersions, shear-thickening flow behavior should be taken into account

 at a high particle concentration

 at a high shear rate

Figure 3.17 presents the dependence of viscosity on the particle concentration (here with the volume fraction solid Φ). For Φ = 0 (i. e. pure fluid without any particle), the liquid shows ideal- viscous flow behavior. For 0.2 ≤ Φ ≤ 0.4, the suspension displays shear-thinning, particularly in the low-shear range. For Φ > 0.4, on the one hand there is shear-thinning in the low-shear range, but on the other hand occurs shear-thickening in the medium and high-shear range. At higher concentrations, the range of shear-thickening behavior is beginning at lower shear rate values already.

Shear-thickening materials are much less common in industrial practice compared to shear-

thinning materials. Nevertheless, shear-thickening behavior is desirable for special applications and is therefore encouraged in these cases (example: dental composites). Usually, however, this behavior is undesirable and should never be ignored since it may lead to enormous technical problems and in some cases even to destruction of equipment, e. g. pumps or stirrers.

Figure 3.17: Viscosity functions of dispersions:

dependence on the particle concentration (with the volume fraction solid Φ)

Figure 3.18: Viscosity curve of a shear-thickening material showing a “dilatancy peak” at a high shear rate

Note 1: Shear-thickening, time-dependent and independent of time

Sometimes, the term “shear-thickening” is used to describe time-dependent flow behavior at a constant shear load (see Figure 3.37: no. 3; and Figure 3.39: left-hand interval). There is a difference between time-dependent shear-thickening behavior (see Chapter 3.4) and shear-thickening behavior which is independent of time (as explained in this section; see also Note 1 in Chapter 3.3.2). If no other information is given, the term should be understood in common usage as the latter one.

Note 2: Dilatancy peak

Sometimes with highly concentrated dispersions, shear-thickening does not occur until higher shear rates are reached. If this behavior is presented in a diagram on a logarithmic scale, the viscosity curve often shows initially shear-thinning behavior up to medium shear rates before a “dilatancy peak is occurring at higher shear rates finally (see Figure 3.18, in logarithmic scales).