High Shear Rate Viscometry

C. J. Pipe, T. S. Majmudar, G. H. McKinley

We investigate the use of two distinct and complementary approaches to 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  m). 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 & Greener (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/flowrate
relationship into the true wall-shear rate and the corresponding shear-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.