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We present a microfluidic rheometer that uses in situ pressure sensors to measure the viscosity of liquids at low Reynolds number. Viscosity is measured in a long, straight channel using a PDMS-based microfluidic device that consists of a channel layer and a sensing membrane integrated with an array of piezoresistive pressure sensors via plasma surface treatment. The micro-pressure sensor is fabricated using conductive particles/PDMS composites. The sensing membrane maps pressure differences at various locations within the channel in order to measure the fluid shear stress in situ at a prescribed shear rate to estimate the fluid viscosity. We find that the device is capable to measure the viscosity of both Newtonian and non-Newtonian fluids for shear rates up to 104 s−1 while keeping the Reynolds number well below 1.

We present a novel method of testing creep recovery in a microfluidic device. This method allows for the measurement of relaxation time of fluids at low strain. After applying a steady pressure-driven flow along a microchannel, the pressure is released and the fluid is allowed to relax and come to rest. Local strains are observed via the time-dependent velocity profiles and are fit to a general viscoelastic model to obtain the fluids' relaxation times. The use of polymeric solutions of various molecular weights allows for the observation of time scales for strains ranging from 0.01 to 10. Results are consistent with data obtained in a macroscopic rheometer and with a viscoelastic constitutive model.

We investigate the rheological characteristics of human blood plasma in shear and elongational flows. While we can confirm a Newtonian behavior in shear flow within experimental resolution, we find a viscoelastic behavior of blood plasma in the pure extensional flow of a capillary break-up rheometer. The influence of the viscoelasticity of blood plasma on capillary blood flow is tested in a microfluidic device with a contraction-expansion geometry. Differential pressure measurements revealed that the plasma has a pronounced flow resistance compared to that of pure water. Supplementary measurements indicate that the viscoelasticity of the plasma might even lead to viscoelastic instabilities under certain conditions. Our findings show that the viscoelastic properties of plasma should not be ignored in future studies on blood flow., Comment: 4 figures, 1 supplementary material Highlighted in http://physics.aps.org/articles/v6/18

In this paper, we present a microfluidic-based rheometer that uses in-situ pressure signals to measure the shear viscosity of liquids. The shear-viscosity is measured using a PDMS-based microfluidic device that consists of a channel layer and a sensing membrane integrated with an array of piezoresistive pressure sensors via plasma surface-treatment. The micro-pressure sensors are fabricated using conducting particles-PDMS composites, synthesized by mixing 82 wt% micrometer-sized silver particles (1–3 μm in diameter) with PDMS gels. The simple design and fabrication enable nearly perfect bonding of a 120 μm-thick sensing membrane to different channel geometries that map the pressure differences at various locations within the channel in order to measure the fluid shear stress in-situ and to estimate the fluid viscosity. Experimental validation of this microfluidic-based rheometer is performed using calibration oil of known viscosity as well as macroscopic rheometric data, both of which achieve good consistency. The present work allows in-situ measurements of rheological properties of fluids at low Re.