Versatile Deposition of Complex Colloidal Assemblies from the Evaporation of Hanging Drops

Existing strategies designed to produce ordered arrangements of colloidal particles on solid supports are of great interest for their wide range of applications, from colloidal lithography, plasmonic and biomimetic surfaces to tags for anti-counterfeiting, but they all share various degrees of complexity hampering their facile implementation. Here, a drastically simplified methodology is presented to achieve ordered particle deposition, consisting in adding micromolar amounts of cationic surfactant to a colloidal suspension drop and let it evaporate in an upside-down configuration. Confinement at the air/water interface enables particle assembly into monolayers, which are then transferred on the substrate producing highly ordered structures displaying vivid, orientation-dependent structural colors. The method is compatible with many particle types and substrates, while controlling system parameters allows tuning the deposit size and morphology, from monocrystals to polycrystalline disks and "irises", from single-component to crystal alloys with Moire patterns, demonstrating its practicality for a variety of processes. Simply drying a colloidal suspension drop containing micromolar amounts of surfactant in a hanging configuration enables the deposition of a variety of highly ordered particle arrays displaying vivid structural colors. This includes single crystals and polycrystalline disks (top right), phase-separated crystal monolayers of binary mixtures (bottom left), crystal alloys forming Moire patterns (bottom middle), and polycrystalline "irises" (bottom right).image