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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Example: Creaming effect of pharmaceuticals and cosmetic products
The “creaming effect” is a result of this continued dispersion process. When spreading and rubbing corresponding emulsions such as creams, lotions and ointments, on the skin, a “whitening effect” may occur which is often leading to tacky, and even stringy, behavior, therefore causing of course an unpleasant skin sensation.
Note 2: Observation and visualization of flowing emulsions using a rheo-microscope
Using special measuring devices, flow behavior of emulsions at defined shear conditions can be observed simultaneously as well in the form of the measured viscosity function as well as visually, for example, in order to observe the onset of breaking up the droplets. This process can be recorded via digital photography or video, measuring point by measuring point. (See also Chapters 10.8.2.2 and 10.8.2.5: Rheo-optics, microscopy and SALS).
Note 3: Difference between dilatancy and dilatation
Sometimes, also due to historic reasons, these two terms are used falsely as synonyms (see Chapter 14.2, 1883 Reynolds). In proper science, however, there should be made a difference: On the one hand, dilatant or shear-thickening behavior is a rheological effect. On the other hand, dilatation may occur with a shear deformation on bulk materials including relatively coarse particles, e. g. such as dry and humid sand (see Chapter 13.2.2: Pre-shear of powder or bulk solids). Example: The following can be seen when setting one foot after another in humid sand during a stroll along the seaside. The area around the feet seems to dry immediately. This happens due to the compression stress onto the sand grains and the resulting deformation. Initially there was a highly ordered cubic closest ball packing of the sand grains, showing therefore a higher density. When this state is disarranged, as a consequence, the surrounding water is sucked rapidly into the now enlarged amount of hollow space between the particles. Imagine a volume element within this bulk material, this process leads to a certain volume increase finally. This effect may be even stronger, if besides the compression there is also a shear load acting on the sand grains, and if some particles have to move across other ones. Dilatation describes a change of the
geometrical shape and volume.
Summary: There should be made a clear difference between the two terms (rheological) dilatance and (volumetric) dilatation, since both of them are specifying clearly different physical phenomena. Both effects may occur simultaneously, however, mostly this is not the case. In order to prevent any confusion, some authors even recommend to use for the rheological thickening behavior the term shear-thickening only, and to avoid in this context the terms dilatant and dilatancy [3.81] [3.82].
Figure 3.20: Toothpaste – our daily struggle with the yield point
3.3.4Yield point
Experiment 3.2: Squeezing toothpaste out of the tube (see Figure 3.20)
A certain amount of force must be applied before the toothpaste starts to flow. A sample with a yield point begins to flow not before the external forces Fext acting on the material are larger than the internal structural forces Fint. Below the yield point, the material shows elastic behavior, i. e. it behaves like a rigid solid, exhibiting under load only a very small degree of deformation, which however recovers completely after removing the load. The following applies: If Fext < Fint the material is deformed to such a small degree only that it is hardly perceptible to the human eye. The sample does not begin to flow before Fext > Fint. The yield point is also referred to as yield stress or yield value . Also, other terms were proposed formerly, e. g. such as stiffness-at-rest (see Chapter 14.2: T. Schwedoff in 1880).
Experiment 3.3: Sticking rods into hand cream and silicone
Small rods are put into the following two materials, in order to observe their motion.
1 Hand cream: The rod remains standing straight in the cream, thus here, a yield point exists.
2 Silicone polymer (uncrosslinked PDMS): The rod moves very slowly to the side, i. e., although the highly viscous silicone displays a clear flow resistance, there is no yield point. It exhibits behavior of a viscoelastic liquid indicating a high value of the zero-shear viscosity in the low-shear range (see also Chapter 3.3.2.1a).
3.1.2.1.3Examples of materials which may show a yield point
Gels, dispersions with a high concentration of solid particles such as plastisol pastes, conductor pastes (electrotechnics), toothpaste, sealants, putties, emulsion paints, printing pastes, ceramic masses, lipstick, creams, ketchup, mayonnaise, chocolate melts, margarine, yogurts; semi-solid materials, concentrated surfactant systems
Yield points have great importance for practical users, and therefore, various methods for the acquisition of appropriate measurement values have been developed over the years – with quite a lot of creativity, obtaining more or less useful results. See also Chapters 3.3.6.4 (model functions, e. g. according to Bingham), 11.2.3d (slump test), 11.2.4a/c (inclined plate), 11.2.6d/e (inclined channel, Casagrande Apparatus), 11.2.7c (Kasumeter), 11.2.8b2 (falling rod), 11.2.9 (penetrometers), 11.2.11a/e/i (gelation test, Mini-Rotary), as well as Chapter 12.4.1a (guideline) and the index. Sometimes, there is distinguished between tests to determine the “apparent” or the “really existing” yield point (for more information on this discussion, see also [3.9] [3.24] [3.82]). Materials having a yield point often show “plastic behavior”, as they tend to flow inhomogeneously, and then, wall-slip effects should be taken into account (see also Chapter 3.3.4.3c).
3.3.4.1Yield point determination using the flow curve diagram
a) With controlled shear rate (CSR): Yield point calculation via curve fitting models
Here, rotational speeds (or shear rates, resp.) are preset in the form of steps or as a ramp (see Figures 3.1 and 3.2). However, using this kind of testing, a yield point cannot be determined directly. Therefore, it is calculated by use of a fitting function which is adapted to the available measuring points of the flow curve. Curve fitting is carried out using one of the various model functions, e. g. according to the models of Bingham, Casson or Herschel/Bulkley (see Chapter 3.3.6.4). For all these approximation models, the yield point is determined by extrapolation of the flow curve towards the shear rate value γ ̇ = 0, or at the intersection point of the fitting function and the τ-axis, respectively (as described in the meanwhile withdrawn DIN 53214). The different model functions usually produce different yield point values because each model uses a different basis of СКАЧАТЬ