ORIGINAL PAPER Analysis of rheology and wall depletion of microfibrillated cellulose suspension using optical coherence tomography Janne Lauri . Antti Koponen . Sanna Haavisto . Jakub Czajkowski . Tapio Fabritius Received: 22 May 2017 / Accepted: 8 September 2017 / Published online: 13 September 2017 Ó Springer Science+Business Media B.V. 2017 Abstract A rheometric method based on velocity profiling by optical coherence tomography (OCT) was used in the analysis of rheological and boundary layer flow properties of a 0.5% microfibrillated cellulose (MFC) suspension. The suspension showed typical shear thinning behaviour of MFC in the interior part of the tube, but the measured shear viscosities followed interestingly two successive power laws with an identical flow index (exponent) and a different con- sistency index. This kind of viscous behaviour, which has not been reported earlier for MFC, is likely related to a sudden structural change of the suspension. The near-wall flow showed existence of a slip layer of 2–12 lm thickness depending on the flow rate. Both the velocity profile measurement and the amplitude data obtained with OCT indicated that the slip layer was related to a concentration gradient appearing near the tube wall. Close to the wall the fluid appeared nearly Newtonian with high shear rates, and the viscosity approached almost that of pure water with decreasing distance from the wall. The flow rates given by a simple model that included the measured yield stress, viscous behavior, and slip behavior, was found to give the measured flow rates with a good accuracy. Keywords Shear viscosity  Yield stress  Lubrication layer  Slip velocity  Velocity profile  Optical coherence tomography Introduction Recently, there has been a growing interest towards disintegrating pulp fibres to their elementary fibrils, thereby producing a renewable, recyclable and biodegradable raw stock for novel cellulosic materials. The disintegration can be performed using a combi- nation of mechanical grinders, together with chemical or enzymatic treatments that typically yield an aque- ous suspension of fibrils. The dimensions of the produced fibrils, called micro (nano) fibrillated cellulose fibres (MFC), can be diverse: the length can vary from hundreds of nanometers to tens of micrometers (Jonoobi et al. 2015). Thus, the mechanical properties of MFC fibres J. Lauri  J. Czajkowski  T. Fabritius Optoelectronics and Measurement Techniques Unit, Faculty of Information Technology and Electrical Engineering, University of Oulu, P.O. Box 4500, FI-90014 Oulu, Finland A. Koponen (&) VTT Technical Research Centre of Finland Ltd, P.O. Box 1603, FI-40101 Jyva ¨skyla ¨, Finland e-mail: Antti.Koponen@vtt.fi S. Haavisto Department of Physics, University of Jyva ¨skyla ¨, P.O. Box 35, FI-40014 Jyva ¨skyla ¨, Finland S. Haavisto Spinnova Ltd., Asematie 11, 40800 Vaajakoski, Finland 123 Cellulose (2017) 24:4715–4728 DOI 10.1007/s10570-017-1493-5