Film climbing of particle-laden interfaces

Recent experiments have shown that coalescence of an oil/water/nanoparticle Pickering emulsion contained in a vial induces a nanoparticle-film to climb up the walls of the vial. Here we show that this phenomenon is highly general and can be induced by a variety of particle types, particle sizes ranging from a few nm to a few mu m, and different emulsion types. Many of the features of film growth described previously with nm-sized particles are found to remain true even with the far larger particles studied here. Binks et al. [B.P. Binks, J.H. Clint, RD.I. Fletcher, T.J.G. Lees, P. Taylor, Growth of gold nanoparticle films driven by the coalescence of particle-stabilized emulsion drops, Langmuir 22 (2006) 4100-4103] have postulated that the particle films that climb up the walls of a vial are actually comprised of one oil layer and one water layer, with particles adsorbed at the interface between them. We confirm this "sandwich" structure experimentally and also show that such interfacially adsorbed particles can easily exert the very modest surface pressure necessary to sustain the weight of the film. Our experiments highlight the importance of prewetting the walls of the vials; films do not climb unless the walls are prewetted. Finally, while some climbing films are tightly packed particle monolayers, tight packing is not essential; even sparsely populated monolayers can display film climbing. (c) 2007 Elsevier B.V. All rights reserved.