A drop of coffee usually leaves a ring-shaped stain when it dries on a surface, but researchers have now used light to control the shape of the deposit left behind.

In the coffee-ring effect, particles flow to a droplet's edge as the liquid evaporates, forming the ring. To change this, Damien Baigl and Manos Anyfantakis from the Ecole Normale Supérieure in Paris added surfactants to polystyrene nanoparticles suspended in water droplets. The polarity of surfactant molecules changed when exposed to ultraviolet light, sticking the nanoparticles to the top of the droplet. This stopped the particles moving to the edge as liquid evaporated, so that they formed a disc instead of a ring.

The technique could prove useful for controlling deposits when droplets that contain particles dry, such as in ink-jet printing.

Angew. Chem. Int. Ed. http://doi.org/f2vh3d (2014)