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The influence of the surface tension of the liquid (g la ) on the material formed upon aerating mixtures of the liquid and fluoroethylene particles is investigated. For non-polar oils of relatively low tension, an oil dispersion of particles is formed as the liquid wets the particles. For polar liquids of relatively high tension like ethylene glycol or water which do not wet the particles, a powder-like material is formed composed of drops of the liquid coated with particles dispersed in air. However, for polar oils of intermediate tension (30-45 mN m À1 ) which partially wet these particles, stable air-in-oil foams can be prepared in which particles form a close-packed layer enveloping air bubbles. Using five commercial poly-or oligotetrafluoroethylene particle samples, we describe the effect of particle type, oil type and particle concentration on these foams and use freeze fracture electron microscopy to observe the arrangement of particles at the air-oil surface. Using contact angle (q) measurements on pressed tablets of the particles, the relation between g la and q is established: particle-stabilised oil foams form for q between ca. 40 and 90 .

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 (or Pickering) emulsions and foams can be prepared. [1][2][3][4] 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 (w/a) material, named dry water, is produced by aerating water in the presence of extremely hydrophobic silica particles. [5][6][7][8][9] The dry water is a free-flowing powder which can contain significant quantities of water as micron-sized drops. One may imagine that when particle-stabilised emulsions with water as the continuous phase, i.e., oilin-water (o/w) emulsions, are aerated with similar hydrophobic particles, the surfaces between the emulsion drops and air can be encased in the hydrophobic particles and oil-in-water-in-air (o/w/a) materials may be prepared, Figure 1a. However, the preparation requires precise tuning of both the properties of the o/w emulsions and the mixing conditions. We have stabilized o/w/a materials using colloidal particles alone and found that control of the coalescence of oil droplets in o/w emulsions on aeration is crucial in the ability to produce powdered emulsions successfully.Dispersed systems consisting of two immiscible fluids, such as emulsions and foams, are usually prepared using molecular or polymeric surfactants. Colloidal particles also act as emulsifiers and foam stabilizers by adsorbing at oil-water and airwater interfaces and forming armoured layers around drops or bubbles, respectively. [1-3] Such particle-stabilized fluid systems show outstanding stability against coalescence and disproportionation. [1,2,10] The driving force for the adsorption of particles to fluid interfaces is the free energy gain in losing an area of interface obliterated by the particles. The types of material formed depend significantly on the wettability of the particles at the interface, quantified by their three-phase contact angle, θ, measured into the water phase. [1,2] It has been shown that, for mixtures of air and water, relatively hydrophilic particles (θ < 90°) preferably form air-in-water (a/w) materials, that is aqueous foams, while relatively hydrophobic particles (θ > 90°) are required for stabilizing water-in-air (w/a) materials. [5][6][7] The w/a materials are represented by water marbles and dry water. The latter behaves like a free-flowing powder but contains a large amount of water (up to 95 wt.% relative to the total mass) as micron-sized droplets. Dry water can be used as a delivery vehicle in the cosmetics and pharmaceutical sectors but also a Ryo Murakami, * Hiroshi Moriyama, Masahiro Yamamoto, Bernard P. Binks,* and Anaïs Rocher Particle Stabilization of Oil-in-Water-in-Air M...

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We describe the stabilisation of liquid-air surfaces by microparticles of a low surface energy solid. By varying the surface tension of the liquid, various particle-stabilised materials from oil dispersions to air-in-oil foams to dry water can be prepared.