Paper Number
TM16
Session
Rheometry: Advanced Techniques and Methods
Title
Stability of liquid filament stretching and implications for rheometry
Presentation Date and Time
October 22, 2019 (Tuesday) 10:15
Track / Room
Track 1 / Room 305A
Authors
- Hassager, Ole (Technical University of Denmark, Department of Chemical and Biochemical Engineering)
Author and Affiliation Lines
Ole Hassager
Department of Chemical and Biochemical Engineering, Technical University of Denmark, Kgs. Lyngby, Denmark
Speaker / Presenter
Hassager, Ole
Text of Abstract
In a recent series of landmark publications [1, 2] Hoyle and Fielding have derived criteria for extensional necking instability in extensional filament stretching. For most complex fluids undergoing start-up of constant strain-rate, a necking instability is predicted to occur at relatively low Hencky strains. In apparent contrast to this, the technique of Filament Stretching Rheometry (FSR) is routinely used to characterize the non-linear extensional properties of complex liquids including their steady extensional viscosity. As shown, here there is in fact no conflict between the application of the FSR technique to access extensional viscosity and the Hoyle-Fielding necking criteria. For start-up of constant strain rate extensional flow, Hoyle and Fielding have derived two criteria and associated necking modes: The so-called “Stress curvature mode” and the “Elastic Considere mode”. Both of these modes will be considered for a spectrum of fluids ranging from the highly stain hardening Upper Convected Maxwell model to the extremely stain softening non-stretch Rolie-Poly model. The two modes act rather differently with respect to the wavelength of the resulting deformation, with the elastic Considere mode having the shorter wavelength. The main conclusion, however, is that even in the presence of both modes the FSR technique is perfectly able to access steady extensional viscosity. The stability of a liquid filament after an interrupted extensional strain ramp will be examined theoretically in the light of the Hoyle-Fielding criteria and experimentally for specific complex fluids. It will be demonstrated how meaningful stress relaxation measurements may be obtained even for fluids prone to the so-called delayed necking phenomenon. [1] D.M. Hoyle and S.M. Fielding, J. Rheol. (2016) [2] D.M. Hoyle and S.M. Fielding, J. Non-Newtonian Fluid Mech. (2017)