Suppression of the coffee-ring effect by shape-dependent capillary interactions

Coffee rings have hidden depths 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. When a drop of liquid dries on a solid surface, its suspended particulate matter is deposited in ring-like fashion. This phenomenon, known as the coffee-ring effect 1 , 2 , 3 , is familiar to anyone who has observed a drop of coffee dry. During the drying process, drop edges become pinned to the substrate, and capillary flow outward from the centre of the drop brings suspended particles to the edge as evaporation proceeds. After evaporation, suspended particles are left highly concentrated along the original drop edge. The coffee-ring effect is manifested in systems with diverse constituents, ranging from large colloids 1 , 4 , 5 to nanoparticles 6 and individual molecules 7 . In fact—despite the many practical applications for uniform coatings in printing 8 , biology 9 , 10 and complex assembly 11 —the ubiquitous nature of the effect has made it difficult to avoid 6 , 12 , 13 , 14 , 15 , 16 . Here we show experimentally that the shape of the suspended particles is important and can be used to eliminate the coffee-ring effect: ellipsoidal particles are deposited uniformly during evaporation. The anisotropic shape of the particles significantly deforms interfaces, producing strong interparticle capillary interactions 17 , 18 , 19 , 20 , 21 , 22 , 23 . Thus, after the ellipsoids are carried to the air–water interface by the same outward flow that causes the coffee-ring effect for spheres, strong long-ranged interparticle attractions between ellipsoids lead to the formation of loosely packed or ar