Interpretation of the Normal Stresses

The presence of deviatoric normal stress components in steady viscometric flow implies fluid elasticity, and they are physically real quantities. The well-known fVeissenberg effect is a good example. When a vertical rotating rod is immersed 

Similar reasoning leads to the conclusion that in cone-and-plate or disk-plate viscometers (Sections 16.7 and 16.8) there will be forces tending to push the cone and plate or the disk and plate apart. Indeed, there are, and the measurement of these forces provides a means of determining the functions Ni j) and Also, for the case of torsional flow, Maxwell and Scalora showed that if a hole is drilled through the plate along the axis of rotation, a screwless extruder is obtained, as the axial normal stress pushes a polymer melt through the hole. Unlike a screw extruder, however, this device works only with viscoelastic fluids. 

Coleman, B. D., H. Markovitz, and W. Noll, Viscometric Flows of Non NewtonUm Fltdds Springer, New York, 1966. 

Middleman, S., The Flow of High Polymers Wiley, New York, 1968. 

Christensen, R, M., Theory of Viscoelasticity An Introduction, Academic, New York, 1971. 

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