Paper Number
PO66 

Session
Poster Session

Title
Rheological effects on the cross-slot flow instability

Presentation Date and Time
October 12, 2022 (Wednesday) 6:30

Track / Room
Poster Session / Riverwalk A

Authors (Click on name to view author profile)

1. Yokokoji, Arisa (Okinawa Institute of Science and Technology)
2. Varchanis, Stylianos (Okinawa Institute of Science and Technology)
3. Haward, Simon J. (Okinawa Institute of Science and Technology)
4. Shen, Amy Q. (Okinawa Institute of Science and Technology)

Author and Affiliation Lines (in printed abstract book)
Arisa Yokokoji, Stylianos Varchanis, Simon J. Haward and Amy Q. Shen
Okinawa Institute of Science and Technology, Onna, Kunigami-gun, Okinawa 904-0495, Japan

Speaker / Presenter 
Yokokoji, Arisa

Keywords
flow-induced instabilities; polymer solutions

Text of Abstract
The cross-slot geometry generates a planar extensional flow field with a stagnation point at the center of the cross. Flows of polymer solutions in the cross-slot geometry can become unstable and asymmetric above a critical value of the Weissenberg number Wi=?ε, where ? is the relaxation time of the fluid and ε is the applied extension rate. However, the origin of the asymmetric flow pattern is still not clearly understood. Inside the cross-slot channel, there is a complex system of extensional flow close to the stagnation point and shear flow near the channel walls. In this work, we investigate how the rheological properties of polymeric fluids under extensional flow (strain hardening, or elasticity), and shear flow (shear thinning) affect the onset and development of the flow asymmetry. For this we employ aqueous solutions of hydrolyzed polyacrylamide at concentrations 0.002 < c < 0.3 wt% with sodium chloride at concentrations 0, 0.1, 0.5 M, which display a wide variety of rheological properties. We quantify ? by capillary breakup extensional rheometry and use Wi to assess the elasticity of the flow in the cross-slot. From the steady flow curve measured by rotational rheometry, we compute a 'shear thinning parameter'S [1] in order to assess the influence of shear thinning in the cross-slot. Micro-particle image velocimetry is used to measure the flow patterns of the different solutions and evaluate the degree of flow asymmetry I as a function of both Wi and S. We also compare the experiments against numerical simulations based on the linear Phan-Thien and Tanner model. Finally, we summarize the trends from the experiments and simulations as a flow diagram in Wi-S-I state space, showing the relationship between rheological properties, applied flow conditions, and the degree of flow asymmetry. We demonstrate that the degree of strain hardening, and shear thinning are both important factors that influence elastic instabilities in the cross-slot channel.[1] Haward & McKinley, Physical ReviewE85,031502(2012)