Powderization of Aqueous Foams

Abstract

Colloidal particles can be irreversibly adsorbed at fluid interfaces, such as oil-water and air-water interfaces. The particle adsorption leads to stabilization of dispersed systems of two immiscible fluids and particle-stabilized, that is, Pickering-type emulsions and foams can be prepared. These materials show some unique properties as a result of adsorption of the particles at the fluid-fluid interface. One of the striking phenomena is that liquid drops can be dispersed in air with the liquid-air surfaces coated by liquid-repellent particles. When the liquid is water, a water-in-air material, named dry water, is produced by aerating water in the presence of very hydrophobic particles. The dry water is a free-flowing powder which can contain significant quantities of water as micrometer-sized drops. Emulsions with water-continuous phase have been also powderized by following the same method. The prepared materials are regarded as oil-in-water-in-air (o/w/a) materials (powdered o/w emulsions). To prepare the powdered o/w emulsions efficiently, it has been shown that the extent of creaming of the oil droplets has to be suppressed. It is expected that, by applying the strategy used to stabilize the powdered o/w emulsions, air-in-water-in-air (a/w/a) materials (powdered aqueous foams) can be prepared. One of the difficulties to prepare the powdered aqueous foams is the significant density difference between air and water, which could enhance the extent of creaming of air bubbles during the preparation and leads to their disappearance due to coalescence. We have reduced the extent of creaming by increasing the viscosity of the water phase and enhanced the preparation of powdered aqueous foams.