92168.pdf

Rheology 261

another without weakening the material, the forces (specific and non-

specific) between them must be overcome and then reinstated at the

same rate in new positions. If the deforming stress is such that these

forces are not reinstated as rapidly as they are overcome, the material

becomes structurally weaker. The remaining forces in certain cross-

sections between the structural units are then overcome even more

readily by the applied stress, and cracks might appear in the sample.

Materials with low linear viscoelastic limits are, therefore, those

which are readily work-softened.

The Weissenberg effect

A characteristic of viscoelastic behaviour is the tendency for flow to

occur at right angles to the applied force. An extreme example of this

behaviour is illustrated in Figure 9.9. When a rotating rod is lowered

into a Newtonian liquid, the liquid is set into rotation and tends to

move outwards, leaving a depression around the rod. When the

rotating rod is lowered into a viscoelastic liquid, the liquid may

actually climb up the rod. The rotation of the rod causes the liquid to

be sheared circularly and, because of its elastic nature, it acts like a

stretched rubber band, tending to squeeze liquid in towards the

centre of the vessel and, therefore, up the rod.

(b)

Figure 9.9 The Weissenberg effect: (a) Newtonian liquid; and (b) viscoelastic liquid