- Weissenberg effect (rod-climbing effect)
When a spinning rod stirs a cup of Newtonian fluid, for example, water, a sunk-
curvature is normally observed at the surface region near the rotating rod because of
the centrifugal forces. In contrast, for concentrated polymer solutions or melts, a
convex curvature occurs at the surface region near the rotating rod, as illustrated in
Fig.7.14. This effect of non-Newtonian-fluid behavior is namedWeissenberg effect
(Weissenberg 1947 ). This effect is because closer to the rotating rod, the shear rate
appears larger and correspondingly the normal stresss 11 along the flow circle
becomes stronger. When polymer chains spin away from the rod under the work
of centrifugal forces, they cannot immediately release the normal stress along the
flow circle due to their entanglements. Therefore, similar to a rubber band
surrounding the wrist, the inter-hooked chains surrounding the rod make an inward
force to compensate the centrifugal forces and even more, to compensate the
gravitation force to raise the liquid surface. The flow that raises the liquid surface
is called the secondary flow, which is responsible for the vortex near the entrance
corner of the extrusion die.
- Barus effect (die-swell or extrudate-swell effect)
The flow velocity of polymer melt is normally large within an extrusion die. Once it
exits the die, a significant transverse swell occurs, as illustrated in Fig.7.15. If the
course of extrusion die is short, the extensional deformation has not yet completely
relaxed at the exit, and accordingly the die swell can recover the loss of the coil size
from the extensional deformation. This effect is also called the memory effect of
elasticity, appearing as a recovery of coil sizes upon the removal of the transverse
restriction at the exit. In fact, the slow-down at the exit of extrusion die is mainly
responsible for the die-swell effect, exactly in reverse to the acceleration at the
entrance causing extensional deformation. Note that the extensional deformation
can gradually relax back to the coil state upon further flow along the extrusion die.
Therefore, the die swell is not necessary due to a recovery of the coil state, but rather
due to slowing-down for a transverse deformation beyond the original coil size.
Fig. 7.14Illustration of (a) the sunk-curved liquid surface when stirring the Newtonian fluids and
(b) the convex-curved liquid surface when stirring polymer melt or concentrated solutions
7.3 Viscoelastic Phenomena of Polymer Flow 141