Application of Doppler Optical Coherence Tomography in Velocity Profiling Rheometry of Complex Fluids

Abstract

In this thesis, Doppler Optical Coherence Tomography (DOCT) was applied to velocity profiling rheometry in combination with rotational rheometer and pipe rheometer. In addition to the velocity profiling, the imaging modality of the OCT was utilized in characterizing the structural behavior of the materials under study. Furthermore, a novel hybrid multi-scale velocity profiling method, based on measuring stationary velocity profiles of fluid flow in a straight tube simultaneously by DOCT and ultrasound velocity profiling (UVP), was developed. The data from the two instruments can be combined into a comprehensive velocity profile including information on the thin boundary layer near the tube wall as measured by DOCT, and in the interior parts of the of flow as measured by UVP. The experimental methods were used to investigate the rheology of aqueous microfibrillated cellulose (MFC) suspensions, which show complex rheological properties. It was found that flow curve measurements with rotational rheometer and smooth concentric cylinders are greatly affected by shear localization effects. In particular, conventional analysis of the bulk properties of MFC suspensions at shear stress values below their yield stress did not appear appropriate – despite the seemingly reasonable power-law flow curves thereby obtained. Experiments with the pipe rheometer and hybrid multi-scale method showed that within the boundary layer concentration and thereby the viscosity of MFC decreases towards the wall. At high values of flow rate, sublayer of virtually pure water was observed next to the wall, giving rise to apparent wall slip. The results from the interior part of the flow showed shear-thinning behavior in qualitative agreement with results obtained by using conventional rheological methods. Velocity profiling rheometry with DOCT and hybrid multi-scale methods appear useful in analyzing the rheological behavior of complex fluids. They 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.