*4.2. 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 [7–9]. The final thickness *h* of the film is controllable via the velocity *v*, the evaporation rate, the bulk volume fraction *φ*0 of the dispersion, shape of the meniscus, etc. From mass conservation and as a rough guide [6,35]:

$$h \sim \frac{\phi\_0}{\phi\_\varepsilon - \phi\_0} \frac{Q\_\varepsilon}{v} \,'\tag{3}$$

where *φ*0 is the volume fraction of the dispersion in the reservoir, *φc* the one of the final deposit, and *Qe* the volumetric evaporation rate per unit of length in the lateral direction, perpendicular to *v* ([ *Qe*] ≡ m2.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 8.** Sketch of the convective self-assembly (CSA) set-up: (**A**) A small volume of a dispersion of nanoparticles (the 'reservoir', in blue) is placed in-between a static blade and a substrate moving at controlled velocity *v*. (**B**) Side view of the geometry which leads to a deposit, in red, of controllable thickness *h* drying soon at the level of the meniscus [7,8]. The figures were reprinted with permission from *Langmuir* 2016, 32, 51, 13657–13668. Copyright 2016 American Chemical Society.

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* ∼ *v*<sup>−</sup><sup>1</sup> at low *v* and *h* ∼ *v*<sup>2</sup>/<sup>3</sup> 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 ye<sup>t</sup> 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 (∼ 100 μL) is placed in between the static blade and the substrate which moves at a velocity *v* in the range of 1 − 100 μm.s<sup>−</sup>1. Here, both the blade and the substrate are 3 × 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.

#### 4.2.2. Deposits Engineered with Microfluidic Pervaporation

Microfluidic Pervaporation combines the moulding in micro-capillary methods (MIMIC [14]) with the ability of the poly(dimethyl siloxane) (PDMS), to let solvents pervaporate (permeation followed by evaporation) across it, including water even though the PDMS is hydrophobic [12,36,37]. In the MIMIC method, the PDMS mould contains micro-channels with precise and sometimes complex morphologies engineered with soft-lithography (typical dimensions: thickness 0.5–50 μm, width 10–500 μm, length 1–10 mm) which are filled with a dispersion which then dries, mostly from the openings. It leads to a solid material made out of the dispersion which nicely replicates the shape of the micro-channels. Including thin membranes to the PDMS mould [12] (Figure 9) enhances tremendously the drying via permeation across the membrane, and permits us to build solidified materials out of (possibly ultra-) dilute dispersions [13,38–40]. Additionally, the versatility of soft-lithography leads to a rich variety of mould's topographies into which the growth of a material is made possible.

**Figure 9.** Sketch of the μ-pervaporation technique leading to micro-structured deposits: A poly(dimethyl siloxane) (PDMS) mould (**left**) is designed with channels initially filled with a dilute dispersion; solvent spontaneously permeates across the elastomer, concentrates the nanoparticles, and leads to a solid material out of the dispersion upon removal of the mould (**right**).

Indeed, we fabricated here two types of structured moulds: a series of long and thin parallel channels (width 100 μm, height 3 μm, length 30 mm), and a large structure consisting of a single channel (width 2.5 mm, height 3 μm, length 30 mm) with poles preventing the collapse of the PDMS mould. This structures are suggestive to the potential of the μ-pervaporation for engineering structures that go beyond simple coatings, e.g., gratings for instance.
