Cellulose nanomaterial (CNM) aspect ratio strongly influences sheet formation and resulting mechanical, optical, and barrier properties. However, there is a lack of fast and reliable methods for CNM aspect ratio determination, limiting the reliable production of nanocellulose at industrial-scale. Current laboratory approaches comprise microscopic (e.g. atomic force microscopy (AFM) and transmission electron microscopy (TEM)), and sedimentation methods, which are time-consuming and limited to specific CNM fibre sizes. Here, we describe a new rheological method to determine the aspect ratios for the whole size range of cellulose fibres using rheology. Cellulose nanocrystals (CNCs), cellulose nanofibres (CNFs), and wood fibres in the form of Bleached Eucalyptus Kraft (BEK) were investigated. The aspect ratios of these three scales of cellulose fibres were determined by measuring the specific viscosity profiles of their suspensions at different concentrations from high to low shear rates (2000–0.001 s−1), and evaluating whether the fibre suspensions exhibited entangled or disentangled behaviour. The rheological results agreed well with those produced by AFM and sedimentation methods. Furthermore, cellulose fibre aspect ratios determined with specific viscosity measurements were generated in 5 hours for each feedstock, while sedimentation and AFM required at least 2 days to produce the same results. Ultimately, we demonstrate that rheology is a rapid and accurate method to determine the aspect ratio for the whole range of cellulose fibre sizes, a critical step towards facilitating their full-scale application.

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