High-shear rate rheometry of micro-nanofibrillated cellulose (CMF/CNF) suspensions using rotational rheometer

Suspensions of cellulose micro- and nanofibrils are widely used in coatings, fibre spinning, 3D printing and as rheology modifiers where they are frequently exposed to shear rates > 104 s−1, often within small confinements. High-shear rate rheological characterisation for these systems is therefore vital. Rheological data at high-shear rates are normally obtained using capillary and microfluidic rheometers, which are found in relative scarcity within research facilities compared to rotational rheometers. Also, secondary flows and wall depletion prevalent at such high-shear rates often go unnoticed or unquantified, rendering the measurement data unreliable. Reliable high shear rate rheometry using rotational rheometers is therefore desirable. Suspension of TEMPO-oxidised CMF/CNF was tested for its high-shear rate rheological properties using parallel plate geometry at measurement gaps 150–40 µm and concentric cylinder at 1 mm gap. The errors from gap setting, radial dependence of shear stress and wall depletion were quantified and accounted for. Viscosity data from 0.1 to 30,000 s−1 shear rates was constructed using both geometries in agreement. Possibilities of secondary flows, radial migration of fluid and viscous heating were ruled out.