Novel approach for calculating the equilibrium foam nanofilm-meniscus contact angle and the film free energy.

The film meniscus is a capillary system that is part of everyday observed phenomena, such as in foams, emulsions, liquid suspensions of nanoparticles (nanofluids), and liquid-wetting solids. The capillarity of a microscopic free foam lamella with a meniscus is important for a fundamental understanding of the role of the surface forces vs. thickness and stability of dispersed systems. The film-meniscus transition region, known as the Gibbs-Plateau border, and macroscopic contact angle, defined by the extrapolated meniscus Laplace surfaces, are the characteristics of capillary systems that reveal how the surface forces contribute to the stability of the dispersed systems. The foam nanofilm formed from a nanofluid due to nanoparticle self-layering under the film surface confinement thins in a multiple regular stepwise manner (not like soap films) above the CMC. The equilibrium thickness of the nanofilm is governed by the film area rather than the capillary pressure, as was reported for common and Newtonian films. Our video clip shows that the nanofilm thins layer by layer as the film area decreases. Our observation reveals that the nanofilm with a small film area remains at the equilibrium thickness with several layers. An iterative method is proposed to locate the film meniscus contact line. The film-meniscus profile of the transition region is examined using the reflected light interferometry and by applying the two radii of curvature. The micro- and macroscopic contact angles between nanofilm and meniscuses are calculated. The foam nanofilm's structural free energy is calculated vs. the number of layers. The knowledge gained from this research will help to improve the understanding of the dispersion stability of foams, emulsions, and liquid suspensions of nanoparticles.