MacGregor Campbell, consultant
If your office desk is constantly plagued by coffee rings, they could now be prevented thanks to physics. Previous research focused on changing the properties of the surface but Peter Yunker and colleagues at the University of Pennsylvania tackled the problem by examining the shape of particles in the liquid.
When a drop of coffee dries on a surface, its edges are held in place, preventing the drop from shrinking as it dries. Instead, it must flatten out while maintaining its original circumference. This flattening pushes water ? and the particles suspended in it ? to the edges so that by the time the drop dries, most of the particles have bunched up to form a ring that's annoyingly hard to remove.
Yunker and his team found that if the particles were spherical, they detached easily from the boundary of the drop and accumulated at the edges. However if they were torpedo-shaped, they would be distributed evenly through the drop as it dried. The discovery could be applied to drying ink or paint: adding elongated particles could help it dry more evenly.
You can read our news story about the discovery here.
For another surprising fluid video, see how a mixed fluid can be unmixed.
Journal reference: Nature, DOI: 10.1038/nature10344
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