## 3 Ice-Templating of Nanocellulose Suspensions
### Preparation of Nanocellulose Gels  
As mentioned in the introduction, CNCs and CNFs are extracted from cellulose fibres by mechanical and/or chemical processing routes in the form of aqueous colloidal suspensions. Because of its hydrogen bonding ability and polarity, water is a good dispersant for the CNCs and CNFs that exhibit many hydroxyl groups (-OH) at their surface. CNF suspensions are commonly manufactured at a nanofibre concentration of about 1-2 wt% [8; 20; 26]. Due to the high aspect ratio of CNFs (\(100<\,l/d<500\)), their suspensions tend to form highly viscous gels even at very low nanofibre content. In contrast, the less slender CNCs (\(l/d<100\)) can be produced at higher concentrations, typically from 5 to 15 wt% [34]. Depending on the extraction processes, the homogeneity and colloidal stability of nanocellulose suspensions are different [16; 26]:  
* CNF suspensions produced by mechanical disintegration with or without enzymatic pre-treatment [16; 86; 87] commonly exhibit heterogeneous mesostructures, characterised by the presence of mesoscale flocs or aggregates. At a finer scale, these suspensions contain both microscale elements such as partially fibrillated cellulose fibres but also nanoscale elements in the form of elementary fibrils or bundles of fibrils with a diameter that ranges between 5 and 100 nm and a length that ranges between 1 and 10 \(\upmu\)m [16; 88]. * In contrast, CNFs suspensions prepared using chemical pre-treatments of cellulose fibres such as carboxymethylation [23], carboxylation [17; 18], and quaternisation [89] prior to mechanical treatment are less polydisperse suspensions that contain a few fibril bundles and mostly individualised fibrils having a diameter ranging from 3 to 6 nm and a length from 1 to 2 \(\upmu\)m. The aforementioned chemical pre-treatments consist of a chemical modification of hydroxyl groups (-OH) at the surface of the fibrils to functional polar groups [26]. Therefore, at low ionic strength, i.e., without the addition of salts and/or additives, CNF aqueous suspensions are homogeneous, nearly transparent (Figure 1d), and electrostatically stabilised by the presence of negatively (or positively) charged groups at their surface [90; 91]. * Similarly, CNC suspensions that are mainly produced from the hydrolysis by sulphuric acid of cellulose fibres, are homogeneous and electrostatically stabilised colloidal systems [92]. Using specific and time-consuming drying techniques [93; 94], CNC and CNF suspensions can be concentrated up to 20 and 10 wt%, respectively [95; 96]. However, the most concentrated suspensions are usually extremely viscous and thus are difficult to use in forming processes. Nanocellulose suspensions are colloidal fibrous systems that exhibit complex rheology between that of soft solids and liquids [16]. The rheology of these systems is usually characterised using transient and steady-state shear experiments (Figure 4) [97; 98]. At low strains and/or low strain-rates, the rheology of these systems is characterised by a solid-like behaviour [16]. The dynamic oscillation curves (e.g., Figure 4a) that are performed at small prestrains for CNC and CNF suspensions, show typical features of gels and soft glassy materials [101; 102; 103] with a storage modulus G' much higher than the loss modulus G'. The solid-like properties of nanocellulose suspensions can also be observed from steady-state flow curves (e.g., Figure 4b). As shown in Figure 4b, CNF suspensions exhibit a macroscale yield stress \(\tau_{0}\) at low strain-rates [100; 104; 105; 106]. This regime is then followed by a shear-thinning behaviour, where the suspension viscosity decreases with increasing the strain-rates [107]. The rheological properties of the suspensions, namely the dynamic modulus G', the yield stress \(\tau_{0}\) and the viscosity \(\mu\) of the suspensions are power-law functions of the nanofibre concentration [104; 105; 108].