## 3 Ice-Templating of Nanocellulose Suspensions
### Forming Processes
#### 3.2.1 Freeze-Casting/Freeze Moulding Processes  
This forming process involves the preparation of a colloidal suspension that is poured into a mould, which is then frozen and subjected to sublimative drying of the suspending fluid under vacuum. This approach has first been developed as a near-net-shape manufacturing process to obtain complex-shaped (monolithic) dense ceramics parts with fine replicate of the mould details [59; 115]. However, in that case the formation and growth of ice crystals was unwelcome, as they turn into defects in the final materials. Later, it was realised that the crystal nucleation and growth could be a benefit if properly controlled to obtain materials with porous microstructures [116].  
Various strategies can be used to achieve complete freezing of the nanocellulose suspensions. A first strategy consists in progressively introducing the mould filled with the suspension in a cold liquid (Figure 5a) [40; 117; 118; 119]. Changing the temperature of the liquid bath enables the cooling rate of nanocellulose suspensions to be varied [56; 118]. Also, the establishment of the temperature gradient within the moulds can be tuned by using moulds with different thermal resistances. For instance, [PERSON] et al. [119] designed a mould composed of three parts: the top and bottom surfaces were made of high thermal conductivity aluminium, whereas the lateral edges were made of low thermal conductivity polymer. This mould design allowed a quasi-unidirectional temperature gradient to be applied to the nanocellulose suspensions. A second strategy is to deposit the mould on a cold surface, leading to the unidirectional freezing of the suspensions [41; 120; 121]. In addition, the freezing of the suspension can also be achieved by placing the mould inside a cold chamber [122; 123; 124]. However, this last way of freezing does not allow a good control of the temperature gradient inside the mould. After complete freezing of the suspensions, the dehydration process is commonly carried out by placing the frozen samples inside freeze-dryer apparatus.  
This forming process enables thick foam panels with densities \(\rho\) ranging from 1 to 200 kg m\({}^{-3}\) (i.e., a porosity ranging from 99% to 87%) to be produced (Figure 5). The density (or the porosity) of the final materials depends on the initial concentration \(\mathrm{C_{i}}\) of the nanocellulose suspension and volumetric shrinkage that is prone to occur during the sublimation of ice-crystals. [PERSON] et al. [56] observed that the foam shrinkage increased drastically when \(\mathrm{C_{i}}\) was lower than a critical concentration \(\mathrm{C^{*}}\). This critical concentration \(\mathrm{C^{*}}\) was found to depend on the nanofibre slenderness and colloidal interactions between the particles.  
At a finer scale, the foams produced using this process commonly exhibit a cellular-like architecture (Figure 5c,d). The morphology of the cells is different from that usually observed in most of the synthetic polymer cellular foams. Instead of the polyhedral cells typical of many foams [43], nanocellulose foams consist of irregular arrangements of thin plates (in the form of nanocellulose films [40; 125]), tens of microns in length that occasionally intersected with remaining partially fibres or dangling mass [56]. The plates themselves are curved and nanoporous [40; 56]. However, the pore structure of these materials is almost a direct replica of the ice-crystals formed during the freezing process, and therefore this structure ultimately depends on the freezing conditions (e.g., temperature gradient and cooling rate) [62]. This point will be discussed further in Section 4.