Weissenberg effect

The Weissenberg effect is a common phenomenon that occurs when a spinning rod is placed into a solution of liquid polymer. Instead of being thrown outward, entanglements cause the polymer chains to be drawn towards the rod. It is named after Karl Weissenberg.

Technically, it is due to a non-zero first normal stress difference. The strain tensor of the motion of turning the rod produces a non-zero difference between the normal components of the resulting stress tensor, so there is a force up and down.

The polymer chains get wrapped around the rod and then as it keeps turning, the free ends in the bulk solution are trapped in the tangled bulk. As the rod is rotated, the end of the chain wrapped on the rod is under tension (a force on each end). To try to reduce the distance between the two ends it tries to move up or down the rod to a region where fewer chains are wrapped round and hence the effective diameter (the diameter of the rod plus wrapped-around chains) is less and hence the distance is shorter.

References

• [1]
• The Isolation of, and the Initial Measurements of the Weissenberg Effect

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