Extensional flow of Newtonian and Boger fluids through a flow focusing microdevice

Abstract

In this work we perform a numerical and experimental study on the flow of Newtonian and viscoelastic fluids through a microfluidic device in which hydrodynamic flow focusing is produced using two balanced lateral sheath streams that shape a third inlet stream. The flow focusing device used was conceived to achieve a nearly constant extensional rate along the centerline and is shaped much like a conventional cross-slot except for comprising three inlets and one exit channel. The work undertaken comprises experimental flow visualization as well as 2D and 3D numerical calculations using a finite volume method. The Newtonian fluid used in the experiments was distilled water and the viscoelastic fluid was an aqueous solution containing 125 ppm (w/w) of polyacrylamide (PAA, Mw = 18×10(6) g/mol), to which 1% of salt (NaCl) was added. The addition of salt to a shear-thinning PAA solution resulted in a low viscosity Boger fluid, i.e., a viscoelastic fluid with a nearly constant viscosity. The combination of small length scales characteristic of microfluidics with this type of fluid allows us to observe strong elastic effects in the absence of shear-thinning and inertial effects (or at least when these effects are weak). We report a rich variety of flow features, which depend on the fluid used as well as on the operational conditions. Additionally, we show good qualitative agreement between the experimental observations and the numerical predictions.