Influence of the drying configuration on the patterning of ellipsoids – concentric rings and concentric cracks

Evaporation of colloidal dispersions leading to patterning of particles is a simple and elegant route for controlling the self-assembly of particles on a solid surface. In this article, we demonstrate that the configuration in which a colloidal dispersion is dried greatly influences the patterning of particles on a solid surface after complete evaporation of the solvent. Evaporation experiments are carried out using well-characterized stable aqueous dispersions of hematite ellipsoids and polystyrene spheres. The drying of particle laden sessile drops always give a “coffee-ring” deposit irrespective of the particle concentration. At a particle concentration ≥0.3 wt% circular cracks appear in the annular region of the coffee-ring deposit owing to the ordered arrangement of ellipsoids. In stark contrast, the deposits formed by drying the dispersion of ellipsoids in the sphere-on-plate configuration show a transition from “concentric rings” to “concentric cracks” in the micro-structure of the particulate film with an increase in the concentration of particles. Further, our experimental findings reveal that long-range circular cracks and long-range assemblies of particles can be achieved by drying of the dispersion in the sphere-on-plate configuration. While the nature of patterns – that is – coffee-rings and concentric rings – is independent of the shape of the particles, a strikingly different crack morphology is shown to be dictated by the shape of the particles in the dispersion. The results presented show that the drying of colloidal dispersions in the sphere-on-plate configuration enables the fabrication of a long range ordered assembly of particles over an area as large as few square millimeters.