## 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.