Dynamic characterization of cellulose nanofibrils in sheared and extended semi-dilute dispersions

TL;DR

This study investigates how cellulose nanofibrils (CNF) reorient and diffuse in different flow conditions, revealing multiple diffusion time scales and flow-dependent diffusion rates, which can help optimize material processing.

Contribution

It introduces a flow-stop experimental method combining POM and SAXS to analyze CNF rotary diffusion and orientation distribution in process-relevant flows.

Findings

Rotary diffusion of CNF occurs at multiple time scales.

Diffusion is faster in shear flows than in extensional flows.

The experimental setup enables quick characterization of CNF dynamic properties.

Abstract

New materials made through controlled assembly of dispersed cellulose nanofibrils (CNF) has the potential to develop into biobased competitors to some of the highest performing materials today. The performance of these new cellulose materials depends on how easily CNF alignment can be controlled with hydrodynamic forces, which are always in competition with a different process driving the system towards isotropy, called rotary diffusion. In this work, we present a flow-stop experiment using polarized optical microscopy (POM) to study the rotary diffusion of CNF dispersions in process relevant flows and concentrations. This is combined with small angle X-ray scattering (SAXS) experiments to analyze the true orientation distribution function (ODF) of the flowing fibrils. It is found that the rotary diffusion process of CNF occurs at multiple time scales, where the fastest scale seems to be dependent on the deformation history of the dispersion before the stop. At the same time, the hypothesis that rotary diffusion is dependent on the initial ODF does not hold as the same distribution can result in different diffusion time scales. The rotary diffusion is found to be faster in flows dominated by shear compared to pure extensional flows. Furthermore, the experimental setup can be used to quickly characterize the dynamic properties of flowing CNF and thus aid in determining the quality of the dispersion and its usability in material processes.