Colloidal particles with appropriate surface properties adsorb strongly at liquid/liquid and vapor/liquid interfaces; hence, they are used as stabilizers for emulsions and foams. Particle surface wettability can be tuned to entrap water in oil, or oil in water, for example, and even to switch between these two regimes. In the case of a vapor/liquid interface, such inversion behavior—the shift from air bubbles dispersed in water, as in a foam, to water droplets dispersed in air—has been explored only recently. Binks and Murakami stabilize a full range of air/water dispersions by adding silica particles 20 to 30 nm in diameter that vary in their wettability, which the authors reduce by lowering the concentration of surface silanol (SiOH) groups via hydrophobic capping. High SiOH content gives rise to stable aqueous dispersions, whereas intermediate particle hydrophobicity leads to air-in-water foams. At the lowest SiOH content, the particles drive a transitional inversion, coating discrete water droplets to stabilize a water-in-air powder. This powder releases water to the skin when sheared by rubbing, suggesting possible applications in cosmetics. The authors further show that varying the ratio of water to air at fixed SiOH content can also force an inversion (in this case formally termed “catastrophic”), giving rise to a soufflé-like material stickier and more highly aggregated than the water-in-air powder. — Marc. S. Lavine
