From thin “coffee rings” to thick colloidal crystals, through drop spreading inhibition by the substrate edge

“Coffee rings” structures formed by the evaporation of a nanosphere colloidal drop which rests on a substrate can be grown as thick, good quality colloidal crystals if the drop is settled onto the flat upper side of a pillar in such a way that its spreading is inhibited by the pillar edge. The colloidal crystal thus obtained has a torus shape of a 6 mm in diameter (~ 19 mm in length), with a semi-elliptical cross section (width ~ 0.7 mm, height ~ 1 mm) and has a shape factor (the ratio between its width and its height) around 1, much lower than that formed in a classical manner, onto a semi-infinite surface, which is usually very flat. These microsphere crystals can be detached from the substrate as free-standing, cut and manipulated without special precaution. Growing thick rings in only few hours, in a simple set-up, the method could be useful for the researchers which currently use these colloidal crystals as sacrificial templates for inverse-opals synthesis.