Preparation and attachment of liquid-infused porous supra-particles to liquid interfaces
We prepared model porous composite supra-particles and investigated the effect of the initial infused fluid phase on their attachment at the liquid–fluid interface. We used a simple method for fabrication of millimetre-sized spherical porous supra-particles from much smaller monodisperse latex microparticles as building blocks by evaporation of a polystyrene sulphate latex suspension on a hot super-hydrophobic surface. We annealed the dried supra-particles at the polymer's glass transition temperature to fuse partially their latex particle building blocks. Spherical porous supra-particles were produced above 40 wt% initial concentration of the latex particles in the suspension, which had a rough surface, with a porous and amorphous structure. We controlled the supra-particle size by varying the initial volume of the latex suspension drop, the latex particle concentration and the drop evaporation temperature. This preparation technique allowed limited control over the porosity of the supra-particles by varying the initial concentration of the latex particle suspension, the rate of evaporation and the annealing temperature. We characterised the surface morphology and the inner structure of supra-particles by SEM imaging. We report for the first time results of an MRI study of supra-particles attached to an air–water or an oil–water interface, which indicated that only the surface layer of the building block particles attaches to the liquid interface while the pore fluid was not displaced by the outer fluid. We observed that supra-particles infused with water had different wettability and attachment positions at the oil–water interface compared with the same particles infused with oil. Similarly, the infusion of the porous supra-particles with water led to a different attachment at the air–water interface compared to the attachment of the same supra-particle when dry. The fundamental importance of this result is that the porous particles (or colloid particle agglomerates) may give an oil-in-water or water-in-oil Pickering emulsion depending on whether they are initially impregnated with oil or water. The results of this study are relevant for particle-stabilised emulsions and foams in a range of pharmaceutical, food and cosmetic formulations as well as ore flotation.