Self-assembled nanoparticle deposits formed at the contact line of evaporating micrometer-size droplets

We report on the formation of self-assembled rings of CoPt3 nanoparticles (ring diameter ranging from 0.6 to 1.5 mum, particle diameter 6 nm) formed in an evaporating thin film. The latter was achieved on the surface of water by spreading a binary mixture composed of two solutions: nitrocellulose dissolved in amyl acetate and CoPt3 particles stabilized by hexadecylamine dissolved in hexane. The self-assembly process of the nanometer-sized particles into micrometer-sized rings results from phase separation in a thin film of the mixed solutions, leading to a bilayer, and the subsequent decomposition during solvent evaporation of the top hexadecylamine-rich layer into droplets. Finally, the evaporation of the remaining solvent from these droplets gives rise to a retraction of their contact line. The CoPt3 particles located at the contact line follow its motion and self-assemble along this line.