title: Optimized cross-slot flow geometry for microfluidic extensional rheometry.
abstract: 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(crit) ≈ 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 s(-1) ≤ ε ≤ 435 s(-1)) and are also in close agreement with numerical simulations performed by using a finitely extensible nonlinear elastic dumbbell model.
authors.affiliation: Hatsopoulos Microfluids Laboratory, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, 02139, USA.
authors.forename: Simon J
authors.initials: SJ
authors.lastname: Haward
authors.forename: Mónica S N
authors.initials: MS
authors.lastname: Oliveira
authors.forename: Manuel A
authors.initials: MA
authors.lastname: Alves
authors.forename: Gareth H
authors.initials: GH
authors.lastname: McKinley