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Optimized cross-slot flow geometry for microfluidic extension rheometry

Haward, S.J. and Oliveira, Monica and Alves, M.A. and McKinley, G.H. (2012) Optimized cross-slot flow geometry for microfluidic extension rheometry. Physical Review Letters, 109 (12). ISSN 0031-9007

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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 Wicrit 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 (26 s1 "_ 435 s1) and are also in close agreement with numerical simulations performed by using a finitely extensible nonlinear elastic dumbbell model.