Weissenberg effect

In fluid dynamics, the Weissenberg effect is a phenomenon that occurs when a spinning rod is inserted into a solution of elastic liquid. Instead of being thrown outward, the solution is drawn towards the rod and rises up around it.^[1] This is a direct consequence of the normal stress that acts like a hoop stress around the rod. The effect is a common example of non-Newtonian fluid dynamics, which has been shown to occur for polystyrene.^[2]

Illustration of the Weissenberg effect on a 2% solution of high molecular weight polyacrylamide

However, it is not always necessary to insert a rod to observe this effect. A rotating disk at the bottom of a viscoelastic non-Newtonian fluid can produce a similar phenomenon: the fluid rises at the center, whereas a Newtonian fluid forms a parabolic depression, ^[3] as seen when stirred with a magnetic stirrer.

A Newtonian fluid forming a parabolic depression when stirred with a magnetic stirrer.

The height of the fluid at the center increases with both the rotational speed of the disk and the elasticity of the fluid. ^[4]

The effect is named after Karl Weissenberg who published about it in 1947.^[5]