Extensional flow viscometry

In uniaxial extension the material is stretched in one direction and compressed equally in the other two. For uniaxial extension or simple extension of a liquid sample of length, L, one meaningful measure of strain is (Dealy, 1982)  

When the strain rate, s, is maintained constant, the deformation obtained is called steady simple extension or steady uniaxial extension (Dealy, 1982) and the extensional viscosity, is related to the normal stress difference  

One application of the above equation is to be able to estimate magnitudes of the normal stress difference (an - 022) as the product of e and rj. However, to obtain magnitudes of i e from the above equation, one must have the means to measure both the normal stress difference and the strain rate, e. A fluid is said to be tensionthickening when i e increases with increasing e, and tension-thinning when i e decreases with increasing . For the special case ofaNewtonian fluid with viscosity, tj  

For a shear thinning fluid with a zero shear viscosity, t]o  

The Trouton ratio, 7r, is defined as the ratio of the extensional viscosity to the shear viscosity  

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Finally, because the results obtained in capillary viscometry, especially for capillaries of small UR, are influenced by both extensional and shear flow phenomena associated with the fluid spatial accelerations at the capillary entrance, it is necessary to correct the values of tw given in Eq. E3.1-3. Chapter 13 covers the nature, magnitude, and significance of these, commonly known as Bagley corrections. 

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