The yielding of attractive gels of nanocrystal cellulose (CNC)

Abstract

The flow behavior of nanocrystalline cellulose (CNC) hydrogels in the presence of a monovalent electrolyte (NaCl) as a function of CNC and salt concentration is explored using a variety of linear and nonlinear rheological tests. We have first observed an apparent “slip yield stress” at small wall shear stress values that is mainly due to the onset of solidlike slippage of the hydrogels before their true yielding and deformation. This flow regime and the “slip yield stress” can be eliminated using a sandpaper of an optimum grit size. At higher wall shear stress values, two yielding points for the CNC/salt network are seen in strain sweep tests due to network disruption and cluster deformation that depend on the concentration of both CNC (1–5 wt. %) and electrolyte (0–100 mM). The first yield stress is due to yielding and flow of clusters, while the second one is due to breakage of clusters to small flocs and individual fibers. These yielding stresses were obtained by a variety of tests including strain amplitude sweep, creep, and steady shear, and their values are compared confirming their existence. Rheo-SALS (small angle light scattering) measurements confirmed structural changes as the scattering patterns change from isotropic to highly anisotropic with an increase of deformation and rate of deformation. Moreover, confocal laser scanning microscopy and polarized microscopy images confirm the gradual breakup of clusters to smaller ones and eventually to nearly individual fibers with an increase in the applied shear strain and rate.

Phase transitions of cellulose nanocrystal suspensions from nonlinear oscillatory shear

“…We note that the viscous LB diagrams show self-intersecting loops for the biphasic and liquid crystalline phases. Self-intersecting loops have also been observed by Moud et al (2021) for highly concentrated hydrogels with cellulose nanocrystals and was attributed to an overshoot in shear stress response. The strain-stiffening (S) and shear-thickening (T) dimensionless ratios are compiled in Fig.…”

“…10 wt% CNC, were found not to exhibit wall slip (Derakhshandeh et al 2013). Danesh et al (2021) showed that concentrations 1-5 wt% CNC obeyed the noslip boundary condition, while the addition of NaCl caused wall slip.…”

“…Another study on CNC in aqueous media with added NaCl revealed that the nonlinear viscoelastic measurements are significantly sensitive to detect changes in the microstructure and yield valuable information about the network structure (Abbasi Moud et al 2020). Furthermore, performing the nonlinear oscillatory shear analysis enabled detecting the formation of a double network in CNC-PVA/salt hydrogels at high CNC loadings (Moud et al 2021). Several studies indicate that some materials can exhibit unique nonlinear signatures with a non-quadratic scaling of I 3∕1 with the strain amplitude, I 3∕1 ∝ n 0 , n ≠ 2 , that could be related to microstructural or molecular material properties.…”

Chiral Liquid Crystalline Properties of Cellulose Nanocrystals: Fundamentals and Applications

“…Because of the multiple uses those iridescent inks created by CNCs might give, it is possible that they will attract attention soon. There are other challenges as well that need proper investigation such as slip effects that affect the local organization of CNCs like other colloids 243 during a process adjacent to the walls. Investigation with this sort is largely unexplored in the literature.…”

Mechanically Tuned Cellulose Nanocrystal (CNC)-based Bio-aimed Hydrogels: Are Hydrogel Meeting Bio-based Application Demands?

“…Mathematically to incorporate yielding into flow equation of the hydrogel one can add an intercept into power law equation introduced earlier to produce Herschel-Bulkley model. Several hydrogel publications in the literature have used this paradigm 88,[220][221] .…”