An Optimized Cross-Slot Flow Geometry for Microfluidic Extensional Rheometry

A precision-machined cross-slot flow geometry with a shape that has been optimized by numerical simulation of the fluid kinematics is fabricated and used to measure the extensional viscosity of a dilute polymer solution. Full-field birefringence microscopy is used to monitor the evolution and growth of macromolecular anisotropy along the stagnation point streamline, and 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 provide self-consistent estimates of the planar extensional viscosity of the fluid over a wide range of deformation rates -1 -1 (26 s 435 s ) ε ≤ ≤ and are also in close agreement with numerical simulations performed using a finitely extensible non-linear elastic (FENE) dumbbell model. PACS numbers: 47.57.Qk, 83.85.Rx, 47.80.Jk, 78.20.hb