The influence of the surface tension of the liquid (γla) 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-1) 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 (θ) measurements on pressed tablets of the particles, the relation between γla and θ is established: particle-stabilised oil foams form for θ between ca. 40 and 90 degrees.
Binks, B. P., Rocher, A., & Kirkland, M. (2011). Oil foams stabilised solely by particles. Soft matter, 7(5), 1800-1808. https://doi.org/10.1039/c0sm01129k