Slip flow and wall depletion layer of microfibrillated cellulose suspensions in a pipe flow

Rheological properties and boundary layer flow behaviour of Micro Fibrillated Cellulose (MFC) suspended in water was studied using Ultrasound Velocity Profiling (UVP) and Optical Coherence Tomography (OCT). The high-resolution OCT provided velocity profiles near the transparent tube wall, while UVP yielded corresponding information in the interior parts of the flow. The results from interior part of the flow showed typical power law shear thinning behaviour of MFC suspension. Close to the wall the suspension was found to have strong (apparent) slip flow associated with depletion of MFC particles from the wall. In this thin depletion layer the consistency and the viscosity of the suspension decrease rapidly with decreasing distance from the wall. In the immediate vicinity of the wall the viscosity of the suspension was found to be close to that of pure water.