If you’ve ever spilled a drop of coffee, you may have noticed that the color concentrates at the edges when the coffee dries. This is known as the coffee ring effect — The drop edges are pinned to the surface, so the drop cannot shrink in circumference during evaporation. Instead it flattens out, pushing particles in the solution towards the edges where they are deposited as the drop fully evaporates.

Arjun Yodh and co-workers, University of Pennsylvania, USA, found the effect can be eliminated simply by changing the shape of the particle. Ellipsoid particles were shown to cause undulation of the air-water interface which induced strong attractions between the ellipsoids. Thus the ellipsoids remained on the surface and inhibited the flow toward the drop’s edges during evaporation. Suspensions of spheres mixed with a small number of ellipsoids also produce uniform deposition.
The coffee ring effect plays a key role in the drying of paints, inks and coatings and this discovery could result in a relatively simple way to produce uniform coatings.

• Suppression of the coffee-ring effect by shape-dependent capillary interactions
  P. J. Yunker, T. Still, M. A. Lohr, A. G. Yodh,
  Nature 2011, 476, 308—311.
  DOI: 10.1038/nature10344