Rheological characterization of micro-fibrillated cellulose fibre suspension using multi scale velocity profile measurements

A rheometric method based on velocity profiling simultaneously by optical coherence tomography and the ultrasound velocity profilometry was introduced and used in a preliminary study of the rheological and boundary layer flow properties of microfibrillated cellulose. The two velocity profiling methods appear adequate and complementary for rheoligical characterization of opaque complex fluids. The ultrasound method is useful in measuring the velocity profile in the interior parts of the tube, while the optical technique is capable of high-resolution measurement of the boundary layer flow close to the tube wall. The preliminary results obtained for a 0.4% micro-fibrillated cellulose suspension show typical shear thinning behaviour in the interior part of the tube while the near wall behaviour shows existence of a slip layer of thickness ~200 um. Both the velocity profile measurement and the imaging mode data obtained by the optical coherence tomopraphic method indicate that the slip layer is releated to a concentration gradient appearing near the tube wall. In a sublayer of thickness ~100 um, the fluid appears nearly Newtonian, and the viscosity value approaches that of pure water with decreasing distance from the wall.