Influence of the degree of fluorination on the behaviour of silica particles at air-oil surfaces

We study the behaviour of fumed silica particles coated to different extents with a perfluoro-alkoxysilane in mixtures of air and liquid, for liquids ranging from non-polar hydrocarbons and polar oils to glycerol and water. Comparisons are made with equivalent hydrophilic and very hydrophobic (hydrocarbon-coated) particles. The surface energy of the particles, calculated from contact angle data, decreases upon increasing the degree of fluorination. As a result, an oil dispersion of particles forms in liquids of low surface tension, e.g. silicone oil, with particles of low fluorine content (oleophilic). Particle-stabilised air-in-oil foams form both in systems of oils of higher tension (>32 mN m^-1, e.g. sunflower oil) and particles of intermediate fluorine content and with oils of lower tension (down to 27 mN m^-1, e.g. hexadecane) and particles of high fluorine content. The foamability of the oils increases with particle concentration and the majority of foams are stable to coalescence and disproportionation for at least 6 months. In many of the oil-particle combinations stabilising foams, oil-in-air liquid marbles can also be prepared, with particles of highest fluorine content encapsulating oils of lowest surface tension. Contact angles of the liquid in air on rough surfaces spin coated with the particles vary from ca. 80°to 150°for systems forming oil foams and oil liquid marbles. Omniphobic surfaces can be prepared relatively easily using such particles.