Extensional rheometry of mobile fluids. Part I: OUBER, an optimized uniaxial and biaxial extensional rheometer

Numerical optimization of a “six-arm cross-slot” device yields several three-dimensional shapes fluidic channels that impose close approximations to an ideal uniaxial (biaxial) stagnation point extensional flow under the constraints having four inlets and two outlets (two outlets) for Newtonian creeping flow. One numerically designed geometries is considered suitable fabrication at microscale, numerical simulations with Oldroyd-B Phan-Thien Tanner models confirm optimal fields are observed in geometry both constant viscosity shear thinning viscoelastic fluids. The geometry, named optimized biaxial rheometer (OUBER), microfabricated high precision by selective laser-induced etching fused-silica substrate. Employing refractive index-matched viscous fluid, microtomographic-particle image velocimetry enables measurement field substantial volume around point. velocimetry, performed low Reynolds number (<0.1), confirms accurate imposition desired predicted flows, pure essentially uniform deformation rate being applied over wide region In Part II this paper [Haward et al., J. Rheol. 67, 1011–1030 (2023)], pressure drop measurements OUBER used assess rheometry dilute polymeric solutions, comparison made planar extension using optimized-shape cross-slot [OSCER, Haward Phys. Rev. Lett. 109, 128301 (2012)].