Control of monolayer sheet size and spatial order in colloidal assemblies by drying sessile drops of suspensions on oil layers

We investigate the assembly of monodisperse polymer-coated polystyrene colloids during the evaporation of sessile drops on oil-coated substrates. In our system, the particles rapidly adsorb at the air-water interface, and as evaporation proceeds, those left behind in the shrinking droplet, are immediately captured by the moving interface. In sufficiently dilute dispersions, nearly all particles adsorb well before the interface becomes saturated, leading to the formation of monolayer sheets. In contrast, at higher particle concentrations, the interface saturates while a significant fraction of particles remain in the bulk, preventing monolayer formation. At any particle concentration, the thickness of the oil layer relative to the initial drop size plays a key role in determining whether monolayers form. The thinnest oil layers provide the broadest range of particle concentrations for successful monolayer assembly. A minimal theoretical model, based on rapid adsorption and negligible influence of internal flows, predicts the boundary between monolayer and non-monolayer regimes with good agreement to experiment. This suggests that interface saturation-not capillary flow-is the controlling mechanism in these drop-on-oil systems. Finally, analysis of the hexatic order parameter reveals that crystalline domain size increases with particle number. These findings offer a pathway to tune monolayer structure and order through controlled drying protocols, with potential applications in 2D materials and functional coatings.