Читать книгу The Rheology Handbook - Thomas Mezger - Страница 210

Water as an ideal-viscous liquid exhibits the usual flow behavior during stirring, forming a vortex around the stirrer axis showing the lowest water level in the center of the vessel. The highest level occurs at the wall of the vessel. In contrast to that, the viscoelastic polymer solution creeps up the stirrer shaft when stirring; this is called the Weissenberg effect.

Karl Weissenberg (1893 to 1976) studied VE effects in detail [5.7]. In 1951, he was the first one who presented an instrument which really deserves to be called a “rheometer” (see also Chapter 14.4). When shearing VE liquids at certain conditions, such as sufficiently high rotational speeds and at low temperatures, they are displaying the Weissenberg effect, which is also called the “rod climbing effect ”. For these kinds of fluids, the pressure and flow conditions in the vessel are changing compared to the behavior of an ideal-viscous fluid, and as a consequence, reversed flow direction of the VE liquid can be observed in a bypass pipeline as illustrated in Figure 5.4. Of course, these effects might have an enormous impact on the effectiveness of a stirring process, and the success in mixing corresponding liquids might be greatly limited. By the way, this effect also occurs if a magnetic stirrer is used, i. e. without using a stirrer shaft. In this case, a rising “mountain of liquid” can be seen in the center of the container above the rotating stirring rod. Further information on the Weissenberg effect, including images, can be found in [5.3] [5.4] [5.5] [5.6].

Figure 5.4: Liquids in two stirrer vessels, left: displaying ideal-viscous flow behavior, and right: visco­elastic behavior showing the Weissenberg effect (with the pressures pA at the inner wall and pB in the center of the vessel)