Rheological characterization of microfibrillated cellulose suspension using optical coherence tomography

Fiber suspensions, such as microfibrillated cellulose, are a challenge for conventional rheometers to measure. This is because rheometers have small flow channel dimensions that can restrict flocculation. Often, questionable assumptions are also made about the fluid behavior in the gap. A pipe rheometer and ultrasound velocity profiling-pressure difference (UVP-PD) concept can be used, by which the real flow behavior is used for the rheological analysis of the bulk properties of the suspension. Unfortunately, the resolution of UVP is too low for studying near-wall phenomena, such as the lubrication layer, that are often very important for understanding the rheology and to upscale the results to industrial flows. To address this problem, we have widened the UVP-PD concept with optical coherence tomography measurements. This enables us to measure the bulk and wall-layer behavior simultaneously. Our results demonstrate the benefits of having direct, detailed measurement of the velocity profile inside the rheometer.