Online measurement of floc size, viscosity, and consistency of cellulose microfibril suspensions with optical coherence tomography

Janne Lauri,Juha Salmela,Arttu Miettinen,Tapio Fabritius,Sanna Haavisto,Antti I. Koponen

doi:10.1007/s10570-021-03745-6

Abstract

In this study, cellulose microfibril (CMF) suspensions were imaged during pipe flow at consistencies of 0.4%, 1.0%, and 1.6% with optical coherence tomography (OCT) to obtain images of the structure and the local velocity of the suspension. The viscosities obtained by combining pressure loss measurement with the OCT velocity data showed typical shear thinning behavior and were in excellent agreement with viscosities obtained with ultrasound velocity profiling. The structural OCT images were used to calculate the radial and the axial floc sizes of the suspension. A fit of power law to the geometrical floc size–shear stress data gave the same power law index for all consistencies, suggesting that floc rupture dynamics is independent of consistency. The dependence of viscosity and floc size on shear stress was similar, indicating that the shear thinning behavior of CMF suspensions is closely related to the rupture dynamics of flocs. The results also showed that an apparent attenuation coefficient of the OCT signal can be used to determine the consistency of CMF suspensions.

Highlights

Cellulose microfibrils (CMFs) are a sustainable and a biodegradable material that make it possible to develop novel, all-cellulose products due to, for instance, its lightness, mechanical robustness, and barrier properties (Klemm et al 2011; Lavoine et al 2012; Moon et al 2016)
Local viscosity of the CMF suspension above the wall-depletion layer can be directly calculated from Eq (2) by utilizing the wall shear stress and the shear rate calculated from the optical coherence tomography (OCT) velocity profile, as explained in the Introduction
Floc size was systematically smaller in the radial direction than in the axial direction, most likely because the laminar pipe flow did not exhibit elongational stresses and the flocs were mainly broken by radial shear stress

Summary

Introduction

Cellulose microfibrils (CMFs) are a sustainable and a biodegradable material that make it possible to develop novel, all-cellulose products due to, for instance, its lightness, mechanical robustness, and barrier properties (Klemm et al 2011; Lavoine et al 2012; Moon et al 2016). The specific surface area (and, hydroxyl group surface density) is much higher for CMF fibrils than for regular cellulose fibers. For these reasons, CMFs form networks that are often encountered as flocs, gels, and films, and the gross structure of the CMF suspensions is much larger than the size of individual fibrils (Hubbe et al 2017; Karppinen et al 2012; Paakkonen et al 2016; Raj et al 2017)

Methods

Results

Conclusion