Generation and manipulation of bubbles and foams stabilised by magnetic nanoparticles

Much interest is being given these days to the development of techniques, which render liquid foams not only super-stable, but also responsive to non-contact stimulation. Both of these goals can be met simultaneously by the application of magnetic particles as stabilising agents, which not only suppress the traditional ageing mechanisms of foams (such as coalescence and coarsening), but also provide means of non-contact manipulation and characterisation by the application of magnetic fields. In this spirit we provide here detailed investigations into the use of commercially available MAGSILICA® H8 nanoparticles, which consist of single-domain iron oxides surrounded by a silica shell, for the generation of super-stable bubbles and foams, and study their response to various types of magnetic fields. More specifically, we investigate the foaming behaviour of hydrophobic magnetic particles in water/ethanol mixtures for which we confirm that best foaming behaviour is achieved for intermediate wettabilities of the particles. We characterise the magnetisation of these foams and use magnetic field gradients to not only validate the super-stability of bubbles, but also to demonstrate the ease of their magnetic manipulation. Both kinds of measurements provide us with more detailed information on particle-detachment forces and the presence of a thick shell of aggregated particles at the gas/liquid interface. Last but not least, we demonstrate how such foams may conveniently be heated and kept at well-controlled temperatures through the application of oscillating magnetic fields.