Extensional Flow
“First off, you need to understand what rheology means. In short, rheology is the study of the fluid response of a material to an imposed stress. Extensional rheology focuses on pulling on a piece of material in a purely extensional manner, i.e. no shear. This is the type of flow you would see if you stretched a rubber band. A shearing flow would be the response of the rubber band if you laid it on a flat surface and slid your hand over it. The rubber band extends slightly, then tumbles over and contracts, repeating this action over and over again. In an extensional flow, the rubber band will continue to elongate until you can not stretch it any further.”
“It is easy to see that the amount of force required to stretch the rubber band is considerably greater than than required to shear the rubber band. The stress, or the force/area, is higher in the extensional deformation due to extent of the deformation. Entropic forces, which want to randomize the configuration of the rubber band, resist the deformation, which aligns the macromolecules that make up the rubber band. Whence, the stress increases as the deformation increases.”
Recent Journal Articles
Estimation of Elongation Viscosity of Polyethylene Melts
(367-376) Journal of Thermoplastic Composite Materials 24 #3 (2011)
Abstract
Liang and Zhong of South China University of Technology, China, introduced temperature T into H function which represents the destruction rate of network junctions and a new viscosity equation was derived based on Phan-Thien—Tanner model for extensional flow. The elongation viscosities of a high-density polyethylene (HDPE) and a low-density polyethylene (LDPE) melts were estimated using this equation at 150°C, and the calculations were compared with the measured data from the extension experiments of the HDPE and LDPE melts reported in reference. Good agreement was showed between the predictions and the measured data. Furthermore, the parameters in this equation were easy to be determined. (RDC 5/25/2011) (See also Polyethylene )