Читать книгу The Rheology Handbook - Thomas Mezger - Страница 88

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