ferrofluids are colloidal liquids made of nanoscale ferromagnetic particles suspended in a carrier fluid. surfactants in the fluid prevent the agglomeration of the nanoparticles because of their strong van der waals forces.
this particular arrangement was achieved by carefully positioning two rare-earth disc magnets, one above and one below a silicon wafer.
acoustic levitation - preparation
this is a pool of silicone oil in a dish that is glued onto a loudspeaker. the loudspeaker is sinusoidally vibrated at 15Hz with an peak-to-peak amplitude of roughly 3mm. the syringe is used to dispense silicone oil droplets that bounce up and down above the surface, at 15Hz. the wavelength of the perturbed surface, which is a function of the frequency and silicone oil depth, determines the optimal spacing between the bouncing droplets. interference between the droplet-induced waves locks the droplets into phase and keeps them at one wavelength apart. these droplets can bounce indefinitely, but game is over as soon as some impurity breaks the surface of the drop, which then disappears immediately.