DISPERSION STABILITY DUE TO STRUCTURAL CONTRIBUTIONS TO THE PARTICLE INTERACTION AS PROBED BY THIN LIQUID-FILM DYNAMICS

Recent experiments on thinning of single liquid films formed from concentrated, monodispersed colloidal dispersions have shown that these films stratify, become thinner, in a stepwise fashion. Stratification has been observed in films formed with surfactant micellar solutions. The phenomenon is explained by us on the basis of the formation of periodic colloid structures induced by structural transition occurring inside the thinning film. We now show that this is a universal phenomenon and present new results for films formed from concentrated suspensions of silica particles as well as polystyrene latexes with a narrow size distribution and prevailing repulsive forces. Using both the reflected light microinterferometric and the transmitted multiple light scattering imaging techniques, the transition from the initial periodic colloid structure to the formation of local particle networks or percolation clusters due to particle bonding or sticking has been observed at high concentrations inside the film for the first time. The effects of particle concentration, size, polydispersity, and surfactant concentration on the oscillatory structure stability of films are reported. Calculations are presented for the structural contributions to the disjoining pressure, which arises from particle interactions when the concentration is increased, and this pressure is shown to balance the capillary pressure and to stop the film thinning process. Both the phase diagram for the order/disorder structural transition and the film stability as a function of particle concentration and film size for the silica hydrosol system are presented. The unique property of the film having ordered microstructures inside it is that the film does not stratify if its size is small. The dynamic process of stratification, or multilayer, microstructuring in submicrometer thin liquid films, is shown to be an important tool for probing the long-range structural interaction forces in concentrated colloidal dispersions. From a practical point of view, this phenomenon of the formation of a long-range ordered microstructure inside the films over distances of the order of 100 nm and this structural contribution to the particle or micellar interaction offer a new mechanism for the stabilization of foams, emulsions, and particle dispersions. New evidence for the similarity between the microstructure formation in single thin films and in foam lamellae in bulk dispersions is highlighted.