Experimental investigation on falling ferrofluid droplets in vertical magnetic fields

The dynamic behaviors of a water-based ferrofluid droplet falling in silicone oils subjected to vertical gradient magnetic fields were studied experimentally. The effects of magnetic field, droplet diameter and oil viscosity are investigated. It is found that the droplet undergoes significant deformation with its shape transforming from an initial oblate ellipsoid to a sphere, then to a prolate ellipsoid and finally to a teardrop. We observe more obvious deformation for a bigger droplet in a lower viscosity of the oil and a higher field gradient. Even the satellite droplet appears accompanying with the breakup of the droplet tail, which can be explained by the non-uniform field distribution. To predict the transient velocity of the droplet, a theoretical velocity model is presented. Both the experimental and theoretical results show that the velocity increases with either the field gradient or the droplet size, but decreases as the oil viscosity increases. The behaviors of two falling droplets with different initial diameters in the current magnetic fields were also studied. If the initial separation distance between them does not exceed a threshold, the droplets eventually come into contact to form a dimer moving along the field direction. When the field is suddenly applied in the middle of the journey of the droplet pair, they would be rearranged and aggregated aligning with the field due to the magnetophoretic effects.