Weissenberg effect
The Weissenberg effect (after the physicist Karl Weissenberg ) is used in rheology to describe the behavior of some non-Newtonian liquids that rise on a rotating rod. This effect is particularly pronounced with spaetzle dough .
In everyday life the phenomenon occurs when making tarts or cakes or when stirring a dough . The dough then climbs up the stir stick instead of staying in the bowl. Like the dough, ketchup also shows this Weissenberg effect. If you put some ketchup in a water glass and stir with a stick, then the ketchup rises up the stick instead of keeping a smooth surface. The reason for this is the very complex diffusion behavior in liquids such as ketchup, dough, but also honey . This property is also called a “rheological” feature. Such phenomena are described across disciplines: soft matter physics , chemistry, biology and materials research are all participating disciplines.
The fact that the fluid rises in the middle of the stirred vessel does not depend on whether there is a stirrer shaft there as a climbing aid. When stirring with a magnetic stirrer, a ridge also forms in the middle. However, it is less pronounced.
The Weissenberg effect is one of the normal stress effects .
Technical importance
The Weissenberg effect is primarily known as a disruptive effect in stirring processes. In stirred tank reactors, for example, if the design is poor, stirrers can fail because the stirred product clogs the bearing or the motor. This effect is also undesirable for large-scale dough stirring.
Web links
- The Weissenberg Effect (3 sample liquids) Youtube video
Individual evidence
- ^ A b Günter Jakob Lauth, Jürgen Kowalczyk: Introduction to the physics and chemistry of interfaces and colloids . Springer, Berlin, Heidelberg 2016, ISBN 978-3-662-47017-6 , pp. 354 f ., doi : 10.1007 / 978-3-662-47018-3_10 .