Viscoelastic Phenomena of Polymer Flow

In shear flows, polymer melt experiences significant elastic deformations, and the normal stress difference reflects its resistance to the shear stress. As illustrated in Fig. 7.13, the direction of flow is labeled as 1 , the direction of velocity gradient is labeled as 2 , and the direction of equal velocity (normal to the book) is labeled as 3 . Since the deformation along direction 1 is large, part of the normal stress initially along direction 2 will be added onto direction 1, resulting in the order of normal stresses as 

Therefore, the first normal stress difference is defined as 

Owning to large normal stress differences, the following-listed viscoelastic phenomena are unique in polymer flows. 

Barus effect (die-swell or extrudate-swell effect) 

The siphon phenomenon is normally conducted by a tube. Polymer melt can suck the liquid into the tube even if the tube end locates high above the liquid surface, because of a significant viscoelastic effect. Such a flow is also called Fano flow (Fano 1908). Another situation is the pouring of the viscous polymer melt from a cup. Once the flow starts, the polymer melt can be continuously poured out, even if the liquid surface inside the cup becomes lower than the cup edge, as illustrated in Fig. 7.16. So far, a clear theoretical descriptirMi of this phenomenon is still lacking. 

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