A Critical Appraisal of Available Methods for the Measurement of Extensional Properties of Mobile Systems

Abstract

There is now a general consensus in the field of non-Newtonian fluid mechanics that the extensional viscosity of mobile highly elastic liquids can be a very important influence in determining flow characteristics. Depending on the complexity of the flow geometry, it can be at least as important as the more widely measured shear viscosity, and in some cases it may be the more dominant influence. Not surprisingly, this realization has motivated the development of extensional rheometers. The process has been reasonably successful for (stiff) highly viscous liquids like polymer melts, although it must be conceded that even in this case there can be severe limitations on the attainable range of equilibrium strain rates. The problems for mobile systems, like polymer solutions, are more acute and they are of a different nature to those encountered with stiff systems. Basically, the difficulties are associated with the need to generate flows with a well-defined extensional flow field, free of a substantial shear component.