"Planar Elongational Flow and Elastic Instabilities in an Optimized-Shape Cross-Slot Extensional Rheometer" Simon J. Haward

Seminar by Dr. Simon J. Haward, Research Scientist of Faculdade de Engenharia da Universidade do Porto

Date:     Thursday, June 19, 2014
Time      10:00 - 11:00 (incl. Q&A)
Venue:  C209, Center Building

Simon James Haward, Ph.D.

Research Scientist
Faculdade de Engenharia da Universidade do Porto

Biography:
Dr. Haward obtained his Ph.D. in Physics from the University of Bristol, UK in 2002, specializing in the experimental study of cyclic extensional flows of polymer solutions under the supervision of Dr. Jeff Odell. After completing postdoctoral research at Bristol in the field of scanning-probe microscopy, he returned to the subject of extensional rheology as a postdoc in the group of Prof. Gareth McKinley at MIT. In 2011 he obtained a Marie Curie International Research Fellowship and is currently based in the Chemical Engineering Department at the University of Porto in Portugal. His current research interests include macromolecular dynamics in extensional flows, rheology of complex fluids, microfluidic rheometry and viscoelastic flow instabilities.

Seminar Title: 
"Planar Elongational Flow and Elastic Instabilities in an Optimized-Shape Cross-Slot Extensional Rheometer"

Seminar Abstract:

A cross-slot flow geometry, with a shape that has been optimized by numerical simulation of the fluid kinematics, is fabricated using wire electro-discharge machining and employed to measure the extensional viscosity of a range of dilute and semi-dilute polymer solutions.  MicroPIV is used to verify the homogeneity of the kinematics and full-field birefringence microscopy is used to monitor the evolution and growth of macromolecular anisotropy along the stagnation point streamline.  We observe the formation of a strong and uniform birefringent strand when the dimensionless flow strength exceeds a critical Weissenberg number, Wi  0.5.  Birefringence and bulk pressure drop measurements in this Optimized Shape Cross-slot Extensional Rheometer (OSCER) provide self-consistent estimates of the planar extensional viscosity of viscoelastic test fluids over a wide range of deformation rates (up to ~500 s^-1).  Measurements with a model dilute polymer solution are also in close agreement with numerical simulations performed using a finitely extensible non-linear elastic (FENE) dumbbell model.  As the imposed extension rate in the OSCER device is increased the homogeneous planar elongational flow ultimately becomes unstable.  High-frame rate video-imaging of the birefringence field is used to construct space-time diagrams of the evolution in the flow for 7 different viscoelastic test fluids and to construct the first stability diagram for planar extensional flows in cross-slot devices.  The mode of instability is found to depend on the elasticity number of the fluid (El = Wi/Re, where Re is the Reynolds number), with a steady symmetry-breaking purely-elastic bifurcation observed at high El > 1, and time-dependent three-dimensional inertio-elastic instabilities dominant for El < 1.