2D colloidal crystalline assemblies on solid substrates by drying hanging drops Figure depicts a typical experiment: a 5 μL aqueous drop containing silica particles (diameter dp = 304 -2400 nm, concentration Cp = 0.04 -0.1 mg/mL) and minute amounts of dodecyltrimethylammonium bromide surfactant (DTAB, concentration Cs = 5-10 μM) was deposited on a polystyrene (PS) Petri dish, turned upside-down and left drying. The Petri dish was kept close to avoid air flows, while temperature and relative humidity were maintained at T = 20.5 ± 1 °C and RH = 30 ± 5 %. Due to their high density, particles were first sedimenting, getting adsorbed at the air/water (a/w) interface thanks to the surfactant added, and then accumulated as a monolayer toward the centre of the drop (Fig. ). Meanwhile, the volume of the drop was progressively reduced by evaporation (Fig. ,1b; Movies S1, S2). On a PS substrate, the absence of pinning led to an evaporation process at constant receding contact angle θc = 84° (Fig. ), thus progressively reducing the contact drop diameter Ddrop while maintaining a high curvature over the whole interface. In the case of 560 nm diameter particles, we observed after 60 min the appearance of bright and angle-dependent structural colours at the drop surface (Fig. ; Movie S3) indicating the formation of periodic assemblies of particles at the a/w interface. This crystallization process, starting from the centre of the drop and propagating towards its edges, is attributed to the 2D confinement of repulsive particles selforganizing in the curved interface. After 90 min, the interface pinned probably due to the presence of the particle assembly near the contact line. At that time, the contact angle started to decrease at fixed Ddrop until final evaporation and particle deposition (grey dashed line in Fig. ). The resulting disk-like deposits kept displaying structural colours on both glass (Fig. ) and polystyrene (Fig. , top) indicating successful transfer of ordered particle arrays on the substrate. Scanning electron microscopy (SEM) analyses of the deposits revealed a close-packed particle assembly with hexagonal order (Fig. , bottom). This particle organization allowed one to directly visualize the polycrystalline structure of the deposits regardless of particle size for dp ≥ 560 nm, through the structural colour distribution arising from juxtaposing domains with various orientations relative to the light source (Figs. and, top). For particles of smaller diameter (dp = 304 nm, Fig. , right), we could still observe the local hexagonal packing by SEM but the structural coloration of the deposit appeared homogenous, probably due to irradiation conditions mismatching the periodic grating. These results show that anionic particles sedimented in the upside-down drop and adsorbed at the curved a/w interface, where they self-confined and organized into polycrystalline arrays, a process that could be directly visualized with large particles (dp = 2.4 µm, Movie S4). The particle organization was maintained upon the evaporation-driven deposition leading to the direct and facile preparation of polycrystalline particle arrays on solid substrates. Interestingly, the resulting solid deposit remained firmly tethered to the substrate, and particles attached to each other by strong attractive surface forces after drying. This allows for easy manipulation of the self-assembled disks on their support, either for their direct use or for further implementation in materials of interest. For instance, as proof-of-concepts, we successfully adapted conventional microfabrication methods to embed or replicate the colloidal polycrystalline structures into various polymeric materials (photoresist, silicone rubber) while keeping the unique polycrystalline structural coloration signatures (Fig. ). Used as is or further combined with desired materials, the resulting assemblies could thus be useful in applications such as encryption or colloidal painting.