Microfluidic contraction devices have been proposed for extensional rheometry measurements, in particular as a useful method for determining the extensional viscosity of low elasticity solutions. The first commercially avail- able “Extensional Viscometer-Rheometer-On-a-Chip ”(e-VROC TM ), developed by Rheosense, is a hyperbolically- shaped contraction/expansion geometry which incorporates pressure-drop measurement capabilities. To better understand the underlying flow kinematics within this geometry we have conducted a numerical study perform- ing three-dimensional numerical simulations for both Newtonian and viscoelastic fluids. For the viscoelastic fluids the simplified Phan-Thien and Tanner (sPTT) and the Finitely Extensible Nonlinear Elastic models (FENE-P) are employed, in order to investigate the efficiency of this configuration in terms of increasing Weissenberg numbers and to understand the effects of various model parameters on the flow field. Our Newtonian fluid results suggest that the e-VROC TM geometry produces only a small region of extensional flow and is mainly shear-dominated, po- tentially suggesting any pressure-drop measurements from this device may be related to viscoelastic first normal- stress differences developed via a combination of shear and extension, rather than solely pure extension. By a careful selection of the sPTT and FENE-P model parameters, such that steady-state viscometric properties in ho- mogeneous flows are matched, we are able to show that a small enhanced pressure-drop is seen for both models, which is larger for the FENE-P model.
- extensional flow
- converging/diverging channels
- viscoelastic fluids