Coffee stains for better solar cells.

Prof. Dr. Nicolas Vogel and his team at Friedrich-Alexander-Universität Erlangen-Nürnberg at the chemistry and bioengineering department developed a strategy together with scientists from Edinburgh, to suppress the coffee-stain-effect and to reach an even drying pattern.

That means: When you spill coffee, the liquid evaporates faster at the edges of the droplet than in the center. Because the coffee contains small, solid particles from the brewing. And these have a different drying time. That’s why they are drawn to the edge of the droplet – which causes the coffee ring to form. (So far so clear?) The same applies to ink, red wine, and other liquids that contain particles.

The “researchers have discovered that the trick to achieving consistent drying patterns is modifying the surfaces of the particles with polymer chains so that they efficiently repel each other while being strongly attracted to the surface of the droplet. Spatial repulsion prevents particle accumulation at the edge of the droplet, while particle attachment to the droplet surface results in a homogeneous particle film. When they are combined both effects prevent the formation of a coffee ring.” And that again helps to optimize the coating processes of, for example, solar cells.