Ferrofluid Droplets Falling and Impacting in a Non-Uniform Magnetic Field

Ferrofluids, i.e. fluids containing magnetic colloids, are most famous for forming spiky Rosensweig instabilities when placed in a magnetic field. The dynamics of their deformation within a magnetic field are less well studied. Here, high-speed photography experiments have been carried out in which drops of a ferrofluid fall vertically through the non-uniform magnetic field generated by a simple disc magnet, before impacting on a glass slide placed above the magnet. As the drop falls, it becomes elongated in the direction of motion. Following image analysis, the forces acting on the drops and their consequent deformation and trajectory may be modelled. The modelling has suggested that the surface component of the magnetic force should not be neglected. Following impact, instabilities can form as early as the initial spreading phase, apparently seeded by crown-rim instabilities. The dynamics of spreading affect the eventual distribution of instabilities. Unlike static drops, the largest peaks are nucleated at the edge of the drops. These experiments used a hydrocarbon and magnetite ferrofluid with magnetic susceptibility χ = 2.64, and a magnet with an on-axis B-field up to 22 mT. The droplets were typically 2 mm in diameter, and were released from heights up to 200 mm.