Extensional Viscosity

Extensional flow, also known as elongational flow or stretching flow, was defined in the first chapter, and, in this flow field, the velocity increases in the flow direction. Industrially, this kind of flow is encountered during fiber spinning and film blowing of polymer melts, during solution spinning of textile fibers, inkjet printing, baking bread, and extrusion puffing, for example. It is a “strong” flow in that polymer molecules stretch, uncoil, and even rupture. Not surprisingly, fluid stresses can reach large values, and their measurement as a function of deformation rate, temperature, and molecular variables has occupied rheologists for a long time. As the contents of this chapter reveal, this has been a fascinating journey, and many instruments of very different design have been proposed for the determination of the uniaxial extensional viscosity. In the case of polymer melts, progress has been made in making the measurements more easily accessible. Similarly, much progress has been made in measuring the extensional viscosity of polymer solutions. In the case of wheat doughs, an original analysis of the Alveograph, a commonly used bubble-inflation technique, has been included in the Appendix to obtain rheologically meaningful and practically useful measures. Data are useful in characterizing materials, guiding processing operations, and also for developing and testing rheological constitutive equations for polymers and foods. Among foods, such measurements have been used to estimate extensional viscosities of potato powder and corn meal during extrusion cooking, wheat dough, various solutions, and spreads.