Viscosity measurements overshoot

As mentioned above, it is far more difficult to measure extensional viscosity than shear viscosity, in particular of mobile liquids. The problem is not only to achieve a constant stretch rate, but also to maintain it for a sufficient time. As shown before, in many cases Hencky strains, e = qet, of at least 7 are needed to reach the equilibrium values of the extensional viscosity and even that is questionable, because it seems that a stress overshoot is reached at those high Hencky strains. Moreover, if one realises that that for a Hencky strain of 7 the length of the original sample has increased 1100 times, whereas the diameter of the sample of 1 mm has decreased at the same time to 33 pm, then it will be clear that the forces involved with those high Hencky strains become extremely small during the experiment. 

To is a terminal relaxation time describing chain motions. Other global relaxation times can be defined experimentally, for example as the inverse of the frequency of the maximum in the terminal dispersion in the loss modulus, Tmax from the time for equilibration following cessation of nonlinear shear flow, XR rj, measured by recovery of the overshoot in the transient viscosity the corresponding time for recovery of the overshoot in the second normal... 

Photographs of the channel were taken every 5 seconds. A comparison between the experimental results and the present model is shown in Figure 1.17. We observe that the yj t law fits well the experiments, with the constant value of the static contact angle. A small overshoot is observed but it is not significant. It is within the error margin of the uncertainties of measurement of the surface tension, viscosity and contact angle. 

The measurement of the entrance viscosity at the low apparent shear rates requires calibration of the pressure transducer to its down resolution limit, which is demonstrated in Figures 3-4. Here the effect of such calibration on the entrance viscosity shape is clearly visible for highly branched LDPE melt, especially for low shear rate range. As it can be seen, the overshoot in the entrance viscosity (typical for branched polymers, see [8]) can be identified in this case only if the pressure transducer is calibrated to its down resolution limit. 

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