Rod-climbing phenomenon

When a viscoelastic material is sheared between two parallel surfaces at an appreciable rate of shear, in addition to the viscous shear stress T 2, there are normal stress differences Wi s Tn - 722 and N2 s 722 - T23. Here 1 is the flow direction, 2 is perpendicular to the surfaces between which the fluid is sheared, as defined by eq 1.4.8, and 3 is the neutral direction. The largest of the two normal stress differences is N, and it is responsible for the rod climbing phenomenon mentioned at the beginning of this book. For isotropic materials, Ni has always been found to be positive in sign (unless it is zero). In a cone and plate rheometer this means that the cone and plate surfaces tend to be pushed q>art. N2 is usually found to be negative and smaller in magnitude than Ni typically the ratio —N2/N1 lies between 0.05 and 0.3 (Keentok et al., 1980 Ramachandran et al., 1985). Figure 4.2.1 shows the... 

In some ways a simpler approach to obtaining normal stresses from the concentric cylinder system is to use the rod climbing phenomenon quantitatively. Because the flow is complex, analysis requires the assumption of some constitutive relation. Joseph and Fosdick (1973) have done this using the second-order fluid, which should be an exact representation of elastic liquids in the limit of slow flows (see Section 4.3). They derive a power series for the... 

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