Credit: AM. CHEM. SOC.

Langmuir doi:10.1021/la902918m (2009)

How can the miniature arched structures pictured below be created? When droplets of dissolved or suspended matter dry on a surface, material often spreads to the edges of the droplet forming a 'coffee ring' stain. These rings can pile up into ridges that form cups or hollow domes.

Julian Evans and Lifeng Chen at University College London placed droplets containing an aluminium powder onto circular platforms about 1 millimetre apart. Powder ridges on adjacent edges of the platforms grew slowly owing to high local humidity, and ridges on the far edges grew faster in the lower relative humidity. The drops eventually bumped into one another and connected, forming arches.