• Toivakka, Martti
• Nazari, Behzad
• Kumar, Vinay
• Bousfield, Douglas W.

Abstract

Cellulose nanofibers (CNFs) are an exciting new renewable material produced from wood fibers. Even at low solids content, CNF-water suspensions have a complex rheology that includes extreme shear-thinning as well as viscoelastic properties and a yield stress similar to other suspensions of nanoscale particles. When characterizing the rheology of CNF suspensions, the measurement method may influence the results due to a water layer expected at the boundary, but it is unclear how the behavior near walls influences the measurement method. Parallel-plate, Couette, and vane geometries were used to compare yielding and flow of CNF suspensions obtained by steady-state shear and oscillatory rheological measurements. Five different techniques were compared as methods to obtain a yield stress. Cone and plate geometries were found to lead to sample ejection at low shear rates: Floc-floc interactions can explain this ejection. The suspensions violated the Cox-Merz rule by a significant amount; this behavior has been explained in the past as weak gel structures that break down in shear, but for this material it seems that the acting mechanism involves the formation of a water-rich layer near the solid boundaries in steady shear, while for oscillatory tests, these layers do not form. For suspensions lower than 3% solids, the yield stress measured by different procedures was within 20% of each other, but for high solids suspensions, differences between the methods could be as large as 100%; the water-rich layer formation likely is the cause of these results. Oscillatory methods are suggested as a method to obtain yield stress values for this type of material. The Couette geometry data were below the power-law lines fitted to the parallel-plate geometry data from steady-shear measurements perhaps again attributable to different water-rich layers that form in these different geometries.

Topics

• impedance spectroscopy
• wood
• cellulose