When shape matters

Abstract

The well-known boundaries of coffee stains are caused by the outward flow of particles suspended in the liquid. Experiments show that ellipsoidal particles can prevent the formation of such boundaries. See Letter p.308 When a drop of coffee dries, a halo of particles accumulates at the drop's edge. This 'coffee-ring effect', first described formally in a Nature paper in 1997, is a common occurrence when a solution of suspended colloidal particles evaporates. Far from being just a household curiosity, it has turned out to have relevance for many applications in which a uniform particle deposition is required, such as inkjet printing, assembly of photonics components and manufacture of DNA chips. In this issue, Peter Yunker and colleagues show that ellipsoidal particles suppress the coffee-ring effect. Attractive interparticle interactions between ellipsoids are sufficiently strong to counteract the forces that drive spherical particles towards the drop's edge as the drop evaporates. The coffee-ring effect can be restored for ellipsoids in solution containing surfactant, and 'designed' mixtures of spheres and ellipsoids can lead to uniform deposition.