Arched structures created by colloidal droplets as they dry.

When a sessile drop containing colloidal particles or solute dries, the well-known "coffee ring" forms at the periphery because of radial flows. Here we demonstrate for the first time that if such drops are in close proximity to each other they form arches as they dry. The interaction between adjacent drops causes spatial variations in humidity so that drying from neighboring regions slows down and particles pile up in places where drying is most rapid. This discovery shows that the deceptively simple process of droplet drying still has more to reveal, and the effect probably influences all droplet technologies in an environment of locally nonuniform humidity.