Although the evaporation process of the solvent is also an important step in other self-assembly processes such as air-liquid and liquid-liquid interfacial assembly, this subsection targets self-assembly processes where the evaporation of the solvent takes place on the substrate during the actual film formation. It is common knowledge that simple drop casting of nanoparticle colloids to form homogeneous films on horizontal substrates is limited by the so called "coffee ring effect" that spontaneous drives the particles to accumulate at the liquid-solid-air contact line during drying, Figure (a). The higher rate of solvent evaporation at the contact line induces replenishment by capillary flow from the bulk liquid of the droplet resulting in an outward flow that leads to the accumulation the particles at the contact line. As shown in Figure (b), slow droplet evaporation by the control of relative humidity and temperature leads to reduced Marangoni flow and hence to the formation of more uniform assemblies. As summarized by Mampalli et al., different strategies such as contact line pinning, disturbing of capillary flow, trapping solute and so on, have been adopted for the suppression of the coffee ring effect. Interestingly, Bigioni et al. could produce Au nanoparticle monolayers as shown in Figure (c) even with rapid solvent evaporation and attractive particle-particle interaction. However, the pinning of the contact line at the edge of substrate during the drying process was important in the process. An interesting strategy adopted to avoid the coffee ring effect during drop-let drying is by introducing the nanoparticles on the surface of the drying droplet so that the accumulation from the bulk can be avoided. For stable contact line pinning in such strategies, the substrate can be chemically pre-treated, for instance, with dopamine. Importantly, the coffee ring effect is a dynamic phenomenon particular to droplet drying, especially on a smooth horizontal surface. As shown by Dimitrov and Nagayama, drying of a colloid next to a vertical wall is a simple yet effect evaporative self-assembly process as the liquid-solid contact forms a thin liquid film for fast evaporation and replenishment. Thus, the nanoparticles in the fast-evaporating liquid film spontaneously form a film on the vertical surface while the nanoparticles continuously migrate to the contact line, Figure . Miyahara and co-workers have reported extensively on the different parameters governing the nanoparticle assembly from a drying liquid on an adjacent vertical substrate. They showed that only above a certain nanoparticle volume fraction (ϕ > 10 -3 ) in the colloid, continuous nanoparticle films form at the liquid-solid contact points, while at a lower concentration regime (ϕ < 10 -4 ), nanoparticles form horizontal strips. Interestingly, controlled evaporative assembly can be used to produce complex structures like binary assemblies as well. As shown in Figure (e), by mixing nanoparticles of two different sizes, complex binary assemblies of different configurations such as superlattices with AB, AB2, AB3, AB4, AB5, AB6, and AB13 stoichiometry, and cubic, hexagonal, tetragonal, and orthorhombic symmetries could be obtained. Further sophistication by including more nanoparticle sizes and shapes is a promising way forward. Another effective strategy is to create a thin film of the colloidal solution by capillary force so that the evaporation from the thin film region will lead to the formation of a closed packed nanoparticle film while the nanoparticles from the bulk will continuously replenish the evaporating region, Figure (f). This is similar to evaporative self-assembly on a vertical substrate as in this case also the solvent evaporate form the meniscus where the layer of the solvent is thin leaving behind the nanoparticles. The evaporation can also be controlled by microstructures on the substrate. For example, over a substrate with ~300 nm large dome-like structures, a 30 nm nanoparticle colloid dries in a way that the nanoparticles assemble around the domes in satellite-like structures, Figure (g). For comparison and overview, Table summarizes the main features of evaporative self-assembly techniques reported in important literature. Table . Summary of evaporative self-assembly procedures reported in selected literature.