Characterization of micro-fibrillated cellulose fiber suspension flow using multi scale velocity profile measurements

Rheological properties and boundary layer flow behavior of Micro Fibrillated Cellulose (MFC) suspended in water was studied using a novel velocity profiling rheometric technique. The method is based on measuring stationary velocity profiles of fluid flow in a straight tube simultaneously by Doppler Optical Coherence Tomography (DOCT) and by Ultrasound Velocity Profiling (UVP). The high resolution DOCT provides velocity profiles near the transparent tube wall, while UVP yields corresponding information in the interior parts of the flow. The data from the two instruments is combined into a comprehensive velocity profile including both the thin boundary layer near the wall and the interior parts of the flow. Within the boundary layer, concentration and thereby the viscosity of MFC is found to decrease towards the wall. At high flow rate, sublayer of virtually pure water is observed next to the wall, giving rise to apparent wall slip. The results from interior part of the flow show shear thinning behavior in qualitative agreement with results from conventional rheological methods. The results indicate that the new method can provide detailed experimental information on the rheology of MFC suspensions and their intricate boundary layer flow behavior, avoiding uncertainties inherent in many conventional rheological techniques.