2102Filamentstretching techniques

In this method, the sample is held between two discs, the lower of which is attached to a shaft whose movement is controlled by a computer capable of generating an exponentially varying voltage, the shaft velocity being proportional to the applied voltage. The upper disc is attached to a load measuring device and an optical system is used to measure filament diameter [Sridhar etal., 1991].

A reverse-flow near the plates causes a delay in the development of the uniform cylindrical column, and a difference between the local and imposed extension rates at early times [Shipman etal., 1991]. Problems may also be encountered due to adhesion of the fluid to the plates [Spiegelberg and McKinley, 1996] but this may be compensated for [Tirtaatmadja and Sridhar, 1993]. Notwithstanding these difficulties, the technique can provide meaningful extensional viscosity data for polymer solutions, as the deformation is uniform and independently imposed, and the total strain is measurable.

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