Pipe rheology of microfibrillated cellulose suspensions

Abstract

The shear rheology of two mechanically manufactured microfibrillated cellulose (MFC) suspensions was studied in a consistency range of 0.2-2.0% with a pipe rheometer combined with ultrasound velocity profiling. The MFC suspensions behaved at all consistencies as shear thinning power law fluids. Despite their significantly different particle size, the viscous behavior of the suspensions was quantitatively similar. For both suspensions, the dependence of yield stress and the consistency index on consistency was a power law with an exponent of 2.4, similar to some pulp suspensions. The dependence of flow index on consistency was also a power law, with an exponent of -0.36. The slip flow was very strong for both MFCs and contributed up to 95% to the flow rate. When wall shear stress exceeded two times the yield stress, slip flow caused drag reduction with consistencies higher than 0.8%. When inspecting the slip velocities of both suspensions as a function of wall shear stress scaled with the yield stress, a good data collapse was obtained. The observed similarities in the shear rheology of both the MFC suspensions and the similar behavior of some pulp fiber suspensions suggests that the shear rheology of MFC suspensions might be more universal than has previously been realized.

The effect of consistency on the shear rheology of aqueous suspensions of cellulose micro- and nanofibrils: a review

“…Finally notice that the effect of slip flow on the rheological analysis can be totally eliminated by combining the rheometer with a velocity profiling technique, such as Ultrasound Velocity Profiling (UVP), Nuclear Magnetic Resonance Imaging (MRI), or Optical Coherence Tomography (OCT) . Of the studies reviewed here, this approach was used in Kataja et al (2017) and Turpeinen et al (2019). Notice that the scaling parameters obtained in these two studies are very similar to those obtained in the other studies (see Table 3).…”

“…Tatsumi et al (2002), the shear stress-shear rate rheograms have regions where shear stress is constant. In both studies the shear stress was probably below the yield stress, and slip flow had probably taken place at the lowest shear rates (Vadodaria et al 2018;Turpeinen et al 2019). With higher (Hubbe et al 2017).…”

“…While the values of parameters K and n have been reported for CMNF suspensions in numerous studies, there are only a few studies where the dependence of the consistency index K and the flow index n on consistency have been studied systematically. In Schenker et al (2018) and Turpeinen et al (2019) these relations were found to be power laws, i.e. K $ c m K and n $ c m n , where c is the mass consistency of the suspension.…”

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

“…Similarly to pulp fibres or particle suspensions in general (Barnes 1995(Barnes , 2000, CMF suspensions have a strong tendency to apparent slip flow at solid walls (Hubbe et al 2017;Kumar et al 2016;Martoïa et al 2015;Nechyporchuk et al 2014;Saarikoski et al 2012;Turpeinen et al 2020). The apparent slip is caused by a wall depletion layer, where the consistency changes from a nearly fibril-free zone to bulk consistency (Kataja et al 2017;Lauri et al 2017;Saarinen et al 2014).…”

“…For simplicity, we will omit the term 'apparent' from slip below. The slip behavior between CMF grades can be quite different and often causes problems in rheometers by making it challenging to obtain reliable information about shear rheology (Haavisto et al 2015;Schenker et al 2018;Turpeinen et al 2020;Vadodaria et al 2018). In addition to slip behavior, sample preparation and shear history may also change the properties of the CMF and can make reliable and repeatable characterizations of them difficult (Liao et al 2020;Naderi and Lindström 2015).…”