## 4 Materials And Methods
### Assembly Methods  
#### 4.2.1 Films Obtained by Convective Self-Assembly (CSA)  
Convective self-assembly (CSA) is a modified blade-coating technique where the withdrawing velocity of the substrate on which the coating will be laid (Figure 8) is so small that evaporation competes advantageously with the film drawing rate: both the meniscus and the wet film (deposited continuously due to the displacement of substrate with good wetting properties) act as suction pumps via evaporation that drag the nanoparticles in this confined zone to eventually build a dense film, which fully dries soon after [7; 8; 9]. The final thickness \(h\) of the film is controllable via the velocity \(v\), the evaporation rate, the bulk volume fraction \(\phi_{0}\) of the dispersion, shape of the meniscus, etc. From mass conservation and as a rough guide [6; 35]:  
\[h\sim\frac{\phi_{0}}{\phi_{c}-\phi_{0}}\frac{Q_{e}}{v}, \tag{3}\]  
where \(\phi_{0}\) is the volume fraction of the dispersion in the reservoir, \(\phi_{c}\) the one of the final deposit, and \(Q_{e}\) the volumetric evaporation rate per unit of length in the lateral direction, perpendicular to \(v\) ([\(Q_{e}\)] \(\equiv\) m\({}^{2}\).s\({}^{-1}\)). Interestingly, the coating thickness increases with a decreasing \(v\), which sustains the enhanced role of evaporation in order to pre-concentrate the dispersion at the level of the meniscus.  
Figure 7: Transmitted-electron microscopy. (TEM) of \(\mathrm{CeO}_{2}\) nanoparticles deposited on the TEM grid, with diameters in the range of \(4\pm 1\) nm (**left**) and \(40\pm 10\) nm (**right**).  
This regime differs from the so-called Landau-Levich (LL) regime where the withdrawing velocity \(v\) is fast enough that the deposited film remains liquid during deposition and dries afterwards. Here, the liquid film thickness increases with the velocity \(v\) due to enhanced viscous dissipation in the liquid film, and consequently, the thickness of the final deposit increases with \(v\).  
The transition between CSA and LL regimes as a function of velocity, concentration, evaporation rate, etc., lead to a V-shaped curve for \(h\) against \(v\)[15]: \(h\sim v^{-1}\) at low \(v\) and \(h\sim v^{2/3}\) at high \(v\). It has been largely explored both experimentally and theoretically but remains a debated issue subject to continuous improvements. In particular, there exist considerable differences for the drying mechanisms between molecular or polymeric solutions and colloidal dispersions; heterogeneous coatings are often observed but remain largely unexplained, e.g., stick-slip sparse coatings or continuous yet undulating coatings (Figure 2, top). Nevertheless, homogeneous coatings are also at stake (Figure 2, bottom).  
In practice, a small volume of a dispersion of nanoparticles (\(\sim 100\,\upmu\)L) is placed in between the static blade and the substrate which moves at a velocity \(v\) in the range of \(1-100\,\upmu\)m.s\({}^{-1}\). Here, both the blade and the substrate are \(3\times 2\) square-inches, \(1\) mm thick glass slides (Marienfield Ref. 11400420) which were super-cleaned via piranha treatment (20 min) followed by air plasma treatment (1 min), both performed in a clean room. Glass slides are exposed to air only at the time of setting up the CSA experiment.