## 3 Results and Discussion
### Characterization of Nanocellulose Gels and Films
#### 3.2.2 Rheology of CNF Gel  
The results of the rheological analyses of CNF gels are shown in Figure 4. Both CNF gels present similar rheological flows. CNF\({}_{1}\) and CNF\({}_{2}\) gels exhibit shear-thinning behavior, as the shear stress increases with the increasing strain rate. As mentioned previously, CNF\({}_{1}\) and CNF\({}_{2}\) gels come from different chemical pulps, respectively, sulfite and sulfate. Differences in the properties of these two pulps have been reported in the literature [19]. Shear-thinning is common in nanoellulose suspensions due to the orientation and releasing of nanofibers under shear. CNF\({}_{2}\) shows a higher shear stress at all strain rates compared with CNF\({}_{1}\), indicating that CNF\({}_{2}\) has a stronger internal network structure, revealing  
Figure 3: Chromatogram of fatty acid X-8, P-acidic (**left**) and P-neutral (**right**).  
higher viscosity [20]. CNF\({}_{1}\) exhibits lower shear stress, probably due to its slightly lower solids content. At lower concentrations, the resistance to flow decreases because there are fewer interactions between the particles and the medium. Consequently, more diluted suspensions, with pseudoplastic behavior, generally have lower viscosity and shear stress.  
CNF\({}_{1}\) gel exhibits a lower shear stress, probably due to the slightly lower solids content and weaker fibril interactions. This result can also suggest that films produced from CNF\({}_{1}\) gel may exhibit weaker mechanical properties compared with films produced from CNF\({}_{2}\) gel [21].  
Two regions are observed. For the low strain rate, less than 1000 s\({}^{-1}\), both gels show a rapid increase in shear stress, characteristic of yield stress behavior [22]. Concerning the high strain rate region, CNF\({}_{1}\) tends to plateau, suggesting that the increase in shear stress becomes less significant. CNF\({}_{2}\) continues to increase more significantly, indicating a greater resistance to flow and therefore higher viscosity. These differences can be explained by the higher nanocellulose concentration of CNF\({}_{2}\) gel, which may provide stronger hydrogen bonding. This result may be confirmed by the FTIR results of the films, where the hydrogen bond band at 3300 cm\({}^{-1}\) shows a higher intensity for the CNF\({}_{2}\) film compared with the CNF\({}_{1}\) film. Differences in surface chemistry could also influence the rheological behavior. CNF\({}_{2}\) gel has a strong network structure and consequently higher viscosity than CNF\({}_{1}\), requiring more stress to achieve the same strain rate.  
In the same figure, the graph on the right reports the viscosity of the CNF gel as a function of the shear rate. The viscosity of both gels decreases sharply with the increasing shear rate. At the beginning, when shear values are low, the viscosity of gels is very high due to the organized network of fibrils. As the shear stress increases, the network structure is slowly broken, and the viscosity progressively decreases due to the alignment of fibrils in the flow. The slightly higher viscosity of the CNF\({}_{2}\) gel could be associated with the fibril size and crystallinity. These results are also reported by other authors [22; 23].