Microfluidic extensional rheometry using a hyperbolic contraction geometry

Microfluidic devices are ideally suited for the study of complex fluids undergoing large deformation rates in the absence of inertial complications. In particular, a microfluidic contraction geometry can be utilized to characterize the material response of complex fluids in an extensionally-dominated flow, but the mixed nature of the flow kinematics makes quantitative measurements of material functions such as the true extensional viscosity challenging. In this paper, we introduce the ‘extensional viscometer-rheometer-on-a-chip’ (EVROC), which is a hyperbolically-shaped contraction-expansion geometry fabricated using microfluidic technology for characterizing the importance of viscoelastic effects in an extensionally-dominated flow at large extension rates ( $lambda dot varepsilon _a gg 1$ , where $lambda $ is the characteristic relaxation time, or for many industrial processes $dot varepsilon _a gg 1$ s $^{-1}$ ). We combine measurements of the flow kinematics, the mechanical pressure drop across the contraction and spatially-resolved flow-induced birefringence to study a number of model rheological fluids, as well as several representative liquid consumer products, in order to assess the utility of EVROC as an extensional viscosity indexer.