The Rheology Handbook. Thomas Mezger
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Название: The Rheology Handbook
Автор: Thomas Mezger
Издательство: Readbox publishing GmbH
Жанр: Химия
isbn: 9783866305366
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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).
Example 1: Plastisols in automotive industry
A PVC plastisol – as a paste-like micro-suspension – showed shear-thinning behavior in the range of γ ̇ = 1 to 100 s-1, and then a dilatancy peak at γ ̇ = 500 s-1. This may cause problems when it is sprayed as an automotive underbody coating or seam sealing (e. g. by blocking the flatstream spray nozzle).
Example 2: Paper coatings
After a certain shear-thinning range, highly concentrated preparations of paper coatings (suspensions showing a pigment content of about 70 weight-%) may display shear-thickening behavior beginning at around γ ̇ = 1000 s-1, often followed by a dilatancy peak. The peak was found to be shifted towards higher shear rate values for coatings showing a lower pigment concentration. Shear-thickening may cause problems during a coating process leading to coating streaks including the danger of tearing the paper [3.23].
Example 3: Hair shampoos containing surfactant superstructures
In the range of γ ̇ = 1 to 15 s-1 a shampoo displayed shear-thinning behavior and at γ ̇ = 30 s-1 occurred a dilatancy peak. The shampoo manufacturer wanted the shampoo to show a certain superstructure in this shear rate range, which corresponds to the process when flowing out of the shampoo bottle. Consumers then subconsciously believe that they have bought a liquid showing “body”. In this case, the goal was reached via superstructures, built up by visco-
elastic surfactants. (VES; see also Chapter 9.1.1d: worm-like micelles).
Note 3: Temperature-dependent shear thickening of elastomers .
Different temperatures can lead to fundamentally different kinds of flow behavior of filled and unfilled elastomers and rubber). mixtures.
Figure 3.19: Viscosity curve of a highly filled suspension showing immediate shear thickening when reaching the critical shear rate γ ̇ crit (“dilatant switch”)
Example 4: Shear-induced or flow-induced dilatant behavior of elastomers
A filled elastomer showed shear-thinning behavior at T = +80 °C across the whole shear rate range of γ ̇ = 0 to 100 s-1. At T = +40 °C it first indicated shear-thinning, followed by shear-
thickening when exceeding γ ̇ = 50 s-1. At T = +23 °C finally, already at around γ ̇ = 10 s-1 pronounced shear-thickening was occurring. Possible reason: At low temperatures, shear-induced or flow- induced crystallization can be expected for this material. A result of this is shear thickening and hardening, [3.16]. See also Chapter 9.2.2: Shear-induced effects.
Note 4: Composite materials as a “dilatant switch”
Shear-thickening fluids (STF) were developed with the aim of immediate thickening as soon as a defined limiting value of loading is exceeded.
Example 5: “Nano-fluids” or “smart fluids” for shock-resistant or bullet-proof materials
These water-based dispersions are mixtures of PEG (polyethylene glycol) and colloidal silica particles (60 weight-%, with d = 400 nm, mono-dispersely distributed). After initial shear-thinning behavior showing η = 100 Pas at γ ̇ = 0.001 s-1 and η = 2 Pas at γ ̇ = 40 s-1, the viscosity value at the “critical shear rate value” of γ ̇ crit = 50 s-1 immediately steps upwards to η > 500 Pas; see Figure 3.19. At this limiting value of the loading velocity, the silica particles are agglomerating, abruptly forming СКАЧАТЬ