Tracking polymer orientation and flow leading to unsteady cross-slot flow: High-speed imaging and modeling

Viscoelastic flow instabilities limit polymer processing rates. High-speed optical measurements of stress and flow are used to provide insight into the relationships between polymer orientation and flow field that lead to viscoelastic fluctuations and instability. The flow of high-molar-mass polyethylene oxide solutions through a cross-slot geometry transitions from a symmetric flow into an asymmetric flow that continually switches its asymmetric configuration at sufficiently high flow rates. Data was acquired by synchronized particle velocimetry and polarization imaging at sub-ms resolution. Three-dimensional numerical simulations using the Giesekus constitutive model demonstrate similar flow switching behavior. Both experiments and simulations show a growth of flow–polymer misalignment near stagnation points prior to switching of the flow asymmetry direction. The role of polymer misalignment demonstrates the important role of stagnation points in flow fields, and this understanding may suggest ways to improve control of instabilities for more efficient processing.