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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 (λ[. over ε][subscript a] ≫ 1, where λ is the characteristic relaxation time, or for many industrial processes . over ε][subscript a] ≫ 1 s[superscript −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., National Science Foundation (U.S.). Graduate Research Fellowship, United States. National Aeronautics and Space Administration (Microgravity Fluid Sciences Grant NNX09AV99G), European Commission. Marie Curie Actions (FP7-PEOPLE-2011-IIF Grant 298220)

Although many constitutive models for wormlike micellar solutions have been proposed, few quantitative comparisons have been made with detailed rheological measurements. The majority of comparative studies focus on the linear viscoelastic properties of micellar solutions, which are well described by monoexponential Maxwell-like behavior. In the present work we compare the predictions of a prototypical two-species reptation-reaction model [developed in Part 1, Vasquez et al., “A network scission model for wormlike micellar solutions: I. Model formulation and viscometric flow predictions,” J. Non-Newtonian Fluid Mech. 144(2–3), 122–139 (2007)] with rheological measurements performed using a concentrated cetyl pyridinium chloride/sodium salicylate (CPyCl/NaSal) solution in a range of steady and transient shear flows. The model captures the continuous rupture and reformation of the long entangled chains that form a physically entangled viscoelastic network and the enhanced breakage rates that occur during imp...

The development and growth of microfluidics has stimulated interest in the behaviour of complex liquids in micro-scale geometries and provided a rich platform for rheometric investigations of non-Newtonian phenomena at small scales. Microfluidic techniques present the rheologist with new opportunities for material property measurement and this review discusses the use of microfluidic devices to measure bulk rheology in both shear and extensional flows. Capillary, stagnation and contraction flows are presented in this context and developments, limitations and future perspectives are examined.

We investigate the use of two distinct and complementary approaches in measuring the viscometric properties of low viscosity complex fluids at high shear rates up to 80,000 s−1. Firstly, we adapt commercial controlled-stress and controlled-rate rheometers to access elevated shear rates by using parallel-plate fixtures with very small gap settings (down to 30 μm). The resulting apparent viscosities are gap dependent and systematically in error, but the data can be corrected—at least for Newtonian fluids—via a simple linear gap correction originally presented by Connelly and Greener, J. Rheol, 29(2):209–226, 1985). Secondly, we use a microfabricated rheometer-on-a-chip to measure the steady flow curve in rectangular microchannels. The Weissenberg–Rabinowitsch–Mooney analysis is used to convert measurements of the pressure-drop/flow-rate relationship into the true wall-shear rate and the corresponding rate-dependent viscosity. Microchannel measurements are presented for a range of Newtonian calibration oils, a weakly shear-thinning dilute solution of poly(ethylene oxide), a strongly shear-thinning concentrated solution of xanthan gum, and a wormlike micelle solution that exhibits shear banding at a critical stress. Excellent agreement between the two approaches is obtained for the Newtonian calibration oils, and the relative benefits of each technique are compared and contrasted by considering the physical processes and instrumental limitations that bound the operating spaces for each device.