Weissenberg effect, viscoelasticity measurements

Viscoelastic Measurement. A number of methods measure the various quantities that describe viscoelastic behavior. Some requite expensive commercial rheometers, others depend on custom-made research instmments, and a few requite only simple devices. Even quaHtative observations can be useful in the case of polymer melts, paints, and resins, where elasticity may indicate an inferior batch or unusable formulation. Eor example, the extmsion sweU of a material from a syringe can be observed with a microscope. The Weissenberg effect is seen in the separation of a cone and plate during viscosity measurements or the climbing of a resin up the stirrer shaft during polymerization or mixing. 

For a one-dimensional steady shear flow of a fluid between two planes, the velocities of an inflnitesimal element of fluid in the y- and z-directions are zero. The velocity in the x-direction is a function of y only. Note that in addition to the shear stress Tyx (refer to r subsequently) there are three normal stresses denoted by Txx, tyy, Tzz within the sheared fluid. Weissenberg in 1947 [6] was the first to observe that the shearing motion of a viscoelastic fluid gives rise to unequal normal stresses, known as Weissenberg effects. Since the pressure in a non-Newtonian fluid cannot be defined, and as the normal stress differences [2, 3], Txx — Tyy = Vi and Tyy — Tzz = V2, are more readily measured than the individual stresses, it is therefore customary to express N and N2 together with the shear stress t as functions of the shear rate /yx to describe the viscoelastic behavior of a material in a simple shear flow. 

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