Viscometers to measure extensional viscosity

How do we measure extensional viscosity Apart from a limited number of applications where it is possible to form filaments and sheets and puU them in one or two directions, the answer is that it is much more difficult to perform extensional viscosity measurements than their shear viscosity equivalents. For polymer melts, it is possible to chiU the clamps used to hold the melt, and so secure the sample, see figure 26. Sometimes, it is even possible to use a super-glue to attach the sample to the moving clamps  

However, when we have mobile solutions, the methods described above are impossible, and we have to try to create an extensional flow within a flowing system. This has been done in a number of ways, as shown in figure 26. The method used depends on the liquid of interest being spinnable or not spinnable. If the solution is spinnable, then it is possible to wind up the liquid thread on a rotating drum. In all these cases the average extension rate is measured using a camera system to record the profile of the stretched liquid. 

Reaction measured Test liquid pumped on one arm equally up both arms 

In all these measurements there is one great recurring problem—one is never sure if steady state has been achieved. In simple rotational shear instruments, the attainment of steady state is just a matter of waiting. However in extensional instruments the amount of deformation is usually small compared with that necessary to reach equilibrium, which for some polymeric systems—as we have seen—can sometimes only be reached at over 1000 times extension of the undeformed sample at its rest state. 

There is one simple experiment that can be performed to obtain a fair estimate of the extensional viscosity in practical situations, this is by measuring the entrance pressure into a capillary of radius a, see figure 27. Then a reasonable expression for the extensional stress is the entrance pressure drop Pe and an equally simple expression for the extension rate is Q/m. The next level of sophistication is to carry out a power-law analysis, this was done by Cogswell [7] who the suggested that 

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