Heterogeneous flow kinematics of cellulose nanofibril suspensions under shear

Abstract

The rheology of NFC suspensions that exhibited different microstructures and colloidal stability, namely TEMPO and enzymatic NFC suspensions, was investigated at the macro and mesoscales using a transparent Couette rheometer combined with optical observations and ultrasonic speckle velocimetry (USV). Both NFC suspensions showed a complex rheology, which was typical of yield stress, non-linear and thixotropic fluids. Hysteresis loops and erratic evolutions of the macroscale shear stress were also observed, thereby suggesting important mesostructural changes and/or inhomogeneous flow conditions. The in situ optical observations revealed drastic mesostructural changes for the enzymatic NFC suspensions, whereas the TEMPO NFC suspensions did not exhibit mesoscale heterogeneities. However, for both suspensions, USV measurements showed that the flow was heterogeneous and exhibited complex situations with the coexistence of multiple flow bands, wall slippage and possibly multidimensional effects. Using USV measurements, we also showed that the fluidization of these suspensions could presumably be attributed to a progressive and spatially heterogeneous transition from a solid-like to a liquid-like behavior. As the shear rate was increased, the multiple coexisting shear bands progressively enlarged and nearly completely spanned over the rheometer gap, whereas the plug-like flow bands were eroded.

Current Progress in Rheology of Cellulose Nanofibril Suspensions

“…Copyright 2004 EDP Sciences), which allows recording of (e) velocity maps (adapted from ref. 33 . Copyright 2015, with permission of 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 23…”

“…In addition to the visual observation of the CNF structure discussed above, Martoïa et al 33 compared the rheological behavior of enzymatically hydrolyzed and TEMPO-oxidized CNF suspensions using Couette geometry combined with USV. In their work, it was shown that for both types of CNF suspensions the flow was affected by the presence of wall-slip and multiple flow bands, which evolved with applied shear rate.…”

Cellulose nanomaterials: size and surface influence on the thermal and rheological behavior

“…Floculation, fiber entanglement and wall slippage are phenomena present in this system due to the particles characteristics, i.e. size polidispersity, flexibility and surface charge [26,29] . …”

Ultrasound Imaging Velocimetry: a review

“…In this case the torque, rather than the pressure drop, is used to quantify viscous losses. Ultrasound speckle velocimetry was used to characterise fatigue due to shear in gels (Gibaud et al 2016), flow banding in cellulose nanofibril suspensions (Martoïa et al 2015), flow instabilities and shear banding in a non-Newtonian micellar solution (Gallot et al 2013) and avalanche-like fluidisation events in suspensions (Kurokawa et al 2015). Again, most of these systems are inherently echogenic and do thus not require seeding.…”