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 0.60.5em0exto0.5em0ex1.50.3em0exμm, particle diameter 60.3em0exnm) 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.